Failure Points in Storage Configurations: Common Problems ......manage databases and database...

EMC Proven Professional Knowledge Sharing 2010

Failure Points in Storage Configurations: Common Problems and a Solution Charles Macdonald

Charles MacdonaldSenior Technology [email protected]

2010 EMC Proven Professional Knowledge Sharing 1

Table of Contents Introduction ....................................................................................................................... 1 Components of the Storage Environment ......................................................................... 2

Roles and Responsibilities ............................................................................................ 2 Table 1 – Typical Roles in a Storage Environment ................................................... 4

I/O System Components ................................................................................................... 6 Physical Components of the I/O System: Hosts, Connectivity, and Storage ................ 6

Figure 1 – Hosts, Connectivity, and Storage ............................................................. 7 Hosts: Physical Components .................................................................................... 8 Figure 2 – Host Physical Components in the I/O Path .............................................. 9 Connectivity: Physical Components ........................................................................ 10 Figure 3 – FC Network Physical Components in the I/O Path ................................ 12 Storage: Physical Components ............................................................................... 13 Figure 4 – Storage Physical Components in the I/O Path ....................................... 14 Figure 5 – Physical Components in the I/O Path and Roles ................................... 16

Logical Components of the I/O System ....................................................................... 17 Hosts: Logical Components .................................................................................... 17 Figure 6 - Host Logical Components in the I/O Path ............................................... 20 Connectivity: Logical Components .......................................................................... 21 Storage: Logical Components ................................................................................. 21 Figure 7 – Storage Logical Components in the I/O Path ......................................... 24 Figure 8 – Logical Components in the I/O Path ...................................................... 25

Configurable Items in the Storage Environment ......................................................... 26 Figure 9 – I/O Path for Solaris 10 with Leadville and DMX ..................................... 31

Defining the Problem ................................................................................................... 33 Solving the Problem .................................................................................................... 33

Figure 11 – Configuration Management Goal ......................................................... 36 Conclusion ...................................................................................................................... 37 Disclaimer: The views, processes or methodologies published in this compilation are those of the authors. They do not necessarily reflect EMC Corporation’s views, processes, or methodologies


Introduction In large organizations, responsibility for configuring items between the application and

the physical disks that may affect storage availability and performance often cross

several functional groups, e.g., storage operations, systems administration, database

administration, application support, and design/architecture. Configuration errors or

omissions at any level can contribute to performance degradation or decreased

availability. However, requirements are often not well understood by all the groups

involved. This article offers a generalized overview of the path an I/O operation takes

between the application and the storage to illustrate how the interaction of various

configurable items provides optimal performance and availability.

This article will provide Storage Administrators with a common language for potential

configuration issues with:

• Other Systems Administrators

• Database Administrators

• Application Support

• Design/Architecture

• Managers responsible for technical groups

An understanding between management and the technical groups ensures that they all

work collaboratively to deploy and maintain systems with appropriate configurations that

meet performance and availability requirements.


Components of the Storage Environment A storage environment includes three categories of components:

1. Physical components of the I/O system

2. Logical components of the I/O system

3. Human actors

Physical components include servers, storage arrays, and connectivity devices. Logical

components include databases, server operating systems, and storage system

microcode. Human Actors, i.e., the people associated with the storage environment,

include business users who create, view, and manipulate data within the storage

environment, as well as anyone involved in the architecture, design, deployment,

maintenance, and ongoing operation of the storage environment’s physical and logical

components.

The purpose of a storage environment is to support business functions and objectives at

the least possible cost. Business objectives that drive costs include not just the amount

of primary data that must be stored, but also multipliers to the amount of data, such as

the number of backups required, backup retention periods, archive retention, and

disaster recovery requirements. Associated cost drivers include performance

requirements for speed of access to the data, availability requirements that require

redundancy, and sufficiency of systems to meet recovery time and recovery point

objectives. The three components of a storage environment must be working in concert

to fulfill this purpose.

Roles and Responsibilities

The Business User is the most important person within the storage environment. He/she

is the reason for the existence of the storage environment. Others in the storage

environment perform the tasks required to design, deploy, and operate the storage

environment. These tasks are generally consistent regardless of the business setting,

but the way that tasks are allocated in different organizations may be quite different. For

example, the configuration of disk and file systems on a server may be a Systems

Administrator function in one organization, but the responsibility of the Storage

Administrator in another. Similarly, higher level functions, such as planning the


particulars of storage allocations, may be determined by a Design group in one

organization, but reside within a Storage Operations group in another. Different

structures may co-exist within silos in the same organization for a variety of reasons

such as legacy structures inherited with the acquisition of other corporations, or

independent management structures in different functional business groups or branches

of business.

Organizational roles and responsibilities should be well defined and well understood by

all of the groups and individuals involved, as should methods of engagement and

communication between the functional groups. Insufficient collaboration, coordination,

and communication are likely to increase the frequency of preventable business function

disruptions by introducing failure points in the storage environment.

In this article, the roles of the typical actors in a storage environment are summarized in

Table 1. These definitions are for illustration only; they are not intended to define a

standard that would be suitable for all organizations. The role descriptions use the term

Service Level Agreement (SLA) that implies that some level of service has been well

defined. That may not be true in many organizations.


Table 1 – Typical Roles in a Storage Environment

Actor Role

Business Users store, retrieve, and manipulate data

within the storage environment to carry out business

functions. Business User requirements* provide the

justification for the cost of deploying and maintaining a

storage environment, and drive the creation of SLAs for

items such as performance, availability, recovery point

objectives, recovery time objectives, data retention

periods, and disaster recovery strategies.

*Note: Business Users might not be very good at defining

requirements, e.g., the desire “we want 100% uptime and

never delete anything” usually does not become a

requirement when the business is presented with a price

tag to achieve it.

Architects set general and specific technology directions

at a high level, having broad impact on strategic

technology decisions. Architects create standards and

blueprints for the technology that will be available within

an organization, including vendor selection.

Designers create specific technology solutions for

defined business requirements. Designers produce

designs within the constraints set by Architects on

technology and standards.

Application Support Analysts deploy, configure, and

manage applications. Application Support Analysts

maintain applications to meet defined SLAs, including

performance and availability requirements.


Database Administrators deploy, configure, and

manage databases and database related software, such

as Oracle ASM, in accordance with designs provided by

Designers. Database Administrators maintain the

database environment to meet SLAs, including

performance and availability requirements.

Systems Administrators deploy, configure, and manage

server hardware and operating systems, in accordance

with designs provided by Designers. Systems

Administrator responsibilities include configuring server

components relating to storage, such as HBAs, path

management software, volume managers, and file

systems. Systems Administrators maintain the server

environment to meet SLAs, including performance and

availability requirements.

Storage Administrators deploy, configure, and manage

storage arrays and dedicated storage networks, in

accordance with designs provided by Designers. Storage

Administrators maintain the storage arrays and networks

to meet SLAs, including performance and availability

requirements.

Backup Administrators deploy, configure, and manage

backup applications, and dedicated hardware that

supports the application, such as tape libraries. Backup

Administrators maintain the backup environment to meet

SLAs, including performance and availability

requirements.


I/O System Components The following sections provide a generic description of the I/O System in terms of

physical components, logical components, and their relationships. Most of the examples

are for hosts attached to block storage over a fibre channel network, but the general

discussion applies to other storage networks as well.

Physical Components of the I/O System: Hosts, Connectivity,

and Storage

The physical components of a storage environment are segregated into host,

connectivity, and storage devices. Hosts are the computers that run the applications that

users interact with to store, retrieve, and manipulate data. Hosts can be anything from a

notebook PC, to large mid-range systems, and may also refer to virtual hardware, such

as a VMware server.

Storage refers to any storage medium that is external to the host, such as magnetic

tape, optical disk drives, and solid state disk drives. This article is only concerned with

intelligent storage arrays that do not merely provide access to disk, but also provide a

feature rich environment that mediates the access to disk.

Connectivity refers to the physical components that provide the medium for

communication between hosts and storage, such as optical cables, twisted pair cables,

or copper cables. In the case of networks, it also includes hubs, routers, and switches.

The physical components of the storage environment are discussed in more detail in the

following sections. Below, Figure 1 illustrates the high level relationship between hosts,

connectivity, and storage, for both block based storage on an FC SAN, and network

attached storage.


Figure 1 – Hosts, Connectivity, and Storage


Hosts: Physical Components

To illustrate data flow within a computer, the physical components can be generalized to

the Central Processing Unit (CPU), Internal Storage, I/O Devices, and Internal

Connectivity. The CPU consists of the physical components that perform the instructions

contained in the programs. The CPU also contains some amount of high speed storage

(registers and cache) to facilitate CPU operations. Internal Storage consists of memory,

e.g., RAM and ROM, and larger storage devices, such as disk drives, tape drives, and

CD/DVD drives. I/O Devices include devices that handle user to host communications,

such as the keyboard, monitor, and mouse, and devices such as Network Interface

Cards (NICs) and Host Bus Adapters (HBAs) that enable host to host or host to external

storage communications. The CPU, internal storage, and I/O devices communicate

within a computer over buses and bridges that form the internal connectivity.

Figure 2 is a high level diagram of the physical components within a host, and their I/O

connections, for a host with connectivity to block based storage via an FC SAN. Note

that the host is depicted with two HBAs to provide physical redundancy for the I/O path

to the storage.


Figure 2 – Host Physical Components in the I/O Path


The physical components of the host must be sized appropriately for the expected

workload, and may require redundancy to meet availability requirements. You must also

consider exception scenarios when determining the necessary level of redundancy. If

business functions are particularly sensitive to performance degradation, and not just

downtime, for example, a host with two separate HBAs attached to a pair of redundant

SAN fabrics, it will still have access to disk in the case of a failure that interrupts the I/O

path for one HBA (e.g., HBA optic failure, switch port failure), but will also have a 50%

reduction in theoretical bandwidth to the storage. Additional HBAs may be required in

the host to provide redundancy for performance if the potential performance degradation

in this routine failure scenario is not acceptable to the business user.

Connectivity: Physical Components

There are a variety of connectivity options to support various storage network protocols,

such as Infiniband, Fibre Channel (FC), iSCSI, Fibre Channel over Ethernet (FCoE),

NFS, and CIFS. This article will only consider two types of storage networks: IP

networks and FC networks.

IP networks provide connectivity for host and storage for either block based or file based

I/O. Block based I/O over IP networks use protocols such as iSCSI, or FCoE. File I/O

protocols Network File System (NFS) and Common Internet File System (CIFS) that are

used to access Network Attached Storage (NAS) devices are more widely used. IP

networks are generally well understood, so are not described in this article. However,

troubleshooting problems with NAS is often complicated by having an additional

organizational unit involved (the IP Network Administrators) and a more complex

topology, particularly where the IP connectivity is not a dedicated storage network.

FC networks, also called fabrics, provide connectivity between hosts and storage for

block based I/O. The connectivity devices in FC networks can consist of routers, hubs,

and switches. FC switches are the prevalent interconnect device, so it is the only device

considered here; a generalized description of FC switches is given below.


FC switches consist of three key components: ports, internal connectivity, and control

processor (CP) units. Ports contain a transceiver (either a gigabit interface converter

(GBIC), or a small form-factor pluggable (SFP), which transmit and receive signals to

and from the node (host, storage, or switch) that is attached to the port. Cables used to

connect nodes to the FC switch are generally fibre optic cables, but copper may be used

in some circumstances. Port to port communication in the switch takes place via a bus or

backplane, which also allows provides the communication path for the controller units.

FC switches generally include a number of hot swappable components, such as GBICs,

fans, and power supplies. Director class switches may also have hot swappable

controller cards.

FC networks are typically deployed in pairs of redundant fabrics, so that a failure in one

fabric degrades connectivity, rather than interrupting it. Figure 3 illustrates the

components of a FC network.


Figure 3 – FC Network Physical Components in the I/O Path


Storage: Physical Components

The physical makeup of a storage array is described by grouping components into four

broad categories: front end, cache, back end, and physical disks.

The front end consists of the storage front adapters and their controllers. Front adapters

provide the interface between the hosts and the storage array, either through direct

connections, or a storage network.

Cache consists of a number of cards of volatile memory that mediates the transfer of

data between hosts and physical disks. I/O operations to cache are much faster than I/O

operations to disk. Cache increases the speed of write operations from a host

perspective, as the storage array acknowledges the write I/O as committed when it is

written to cache, then writes it to disk or destages it later.

Read ahead algorithms attempt to prefetch data into cache before hosts request it,

speeding up read I/O operations by facilitating some portion of the read I/O from the host

perspective to be performed at cache rather than disk speeds. Cache mirroring protects

uncommitted writes from cache board failures; writes are duplicated on two separate

cache boards. During a power failure, battery power dumps cache to vault disks.

The back end consists of storage back adapters and their controllers. The back adapters

connect to physical disks using SCSI or FC. Back adapters are configured to provide

multiple paths to the disks, so the back adapters are not a single point of failure in the

storage array. The back end controllers also contain some small amount of memory to

help facilitate and optimize data transfer between the cache and the physical disks.

The physical disks in the storage array are persistent data stores, i.e. data that is written

to disk is not dependent on power to maintain its state. One storage array may contain

different types of disks, such as FC, Serial ATA (SATA), or Solid State Drives (SSD).

Different disk sizes may also reside within the same storage array.

The internal connectivity in the storage array varies between storage vendors and

models of arrays, but all contain some type of redundancy. Figure 4 illustrates the

physical components of a storage array.


Figure 4 – Storage Physical Components in the I/O Path


Figure 5 illustrates an end-to-end look at the physical components in the I/O path, and

identifies the actors that are responsible for tasks associated with the physical

components. The roles and responsibilities for architecture, design, and operation of the

physical infrastructure are generally easy to define, and obvious failure points in the

infrastructure are relatively easy to avoid if you take care to introduce architectural

standards and solution designs that incorporate hardware redundancy, such as dual

HBAs and FC fabrics. The greater danger lies in the configuration of the storage

environment’s logical components.


Figure 5 – Physical Components in the I/O Path and Roles


Logical Components of the I/O System

The logical components of the I/O system control user interactions with the physical

components and interactions between the physical components and include:

• host applications

• databases

• host operating systems

• storage operating systems

• FC fabric operating systems

• logical volume managers

• file systems

• device drivers

The logical components contain a large number of configurable items that multiply to

create a vast number of configuration combinations. The availability and performance of

the storage environment is highly dependent on correctly configuring the logical

components to work together. Logical components may reach beyond the physical

divides between hosts, connectivity, and storage, but the physical divides are still a

useful model for organizing the discussion of the logical components.

Hosts: Logical Components

Operating Systems (OS) provide the logical container that the rest of the logical

components on a host work within. The OS manages the interactions and interfaces

between the users, the logical components, and the physical components on the host.

Examples of operating systems include Microsoft Windows, and the various flavours of

UNIX, such as Solaris, AIX, Linux, and HPUX.

Device drivers enable the OS to communicate with specific hardware devices. Device

drivers may be embedded with the OS, or may be installed separately, depending on the

OS and the specific hardware. Device drivers interact with the programmed instructions

called firmware or microcode that reside on the hardware devices. Some hardware

devices, such as HBAs, support user microcode upgrades. The OS and drivers provide

all of the basic services used for I/O on the host.


Multipath Managers provide a layer of virtualization above what the OS sees as physical

disks. Devices are presented over multiple physical and logical paths to provide

redundant access to storage, and to increase throughput. Path managers recognize that

the multiple paths point to a single storage device, and mediate access to the disk.

Multipath managers may also provide performance enhancement functionality by

incorporating algorithms to spread I/O over multiple paths. Multipath managers may be

integrated in the OS, like Solaris MPxIO, or may be provided by a third party, e.g., EMC

Powerpath™, HDS HDLM, or Veritas DMP.

Logical Volume Managers (LVM) provide a virtualization layer above what the OS sees

as physical disks. LVMs group a disk or disks into a volume group, and then create

logical volumes that are again presented to the OS as physical disks. Logical volumes

can be a partition, i.e., a portion of a physical disk, or may span multiple physical disks.

Other LVM functionality may include snapshots, software RAID implementations, and

the ability to move data around on physical disks without any disruption to data access.

LVMs may be integrated with the OS, such as AIX LVM and Windows Logical Disk

Manager, or may be a third party product, such as Veritas Volume Manager.

File Systems provide a method to store and organize data in collections of files. The file

system maps user data to logical units of storage called file system blocks. These are

then mapped to LVM extents if an LVM is present, and then to the OS disk physical

extents, which in the case of external storage are also a virtual entity. For example, the

OS disk physical events are mapped to actual physical disks by the storage subsystem.

Applications directly provide services to users. They contain the functionality and logic to

allow users to perform groups of related tasks. Examples of application classes include

word processors, web browsers, and video games. Applications reside in a logical layer

above the file system. Database applications are specialized for organizing logically

related data to facilitate data analysis, storage, and retrieval. Oracle and Microsoft SQL

are examples of database applications. Applications and databases both rely on the file

system to manage data storage and retrieval. Oracle Automatic Storage Management

(ASM) provides a file system and LVM-like functionality specifically for Oracle database

applications. From an OS perspective, ASM uses raw disk that does not pass through

any other LVM or file system present on the host.


Since the logical components in the I/O path are layered, the logical path of an I/O

passes through several layers.

• I/O begins in the application/database layer

• is passed to the file system

• then the LVM

• then to the SCSI target drivers

• followed by the multipath driver

• then the SCSI command is encapsulated by the driver or drivers that handle the

fibre channel protocol (FCP)

• then passes on to the HBA driver

• and out to the SAN

This layering is depicted in Figure 6 below.


Figure 6 - Host Logical Components in the I/O Path


Connectivity: Logical Components

Logical components in the connectivity environment direct traffic between the source

and destination devices on the network. For FC fabrics, the principal logical component

of interest from a configuration perspective is FC zoning that restricts communications

between the nodes that are logged into the FC fabric. A zone set is a collection of zones

that identifies which nodes are visible to each other on the fabric. Zoning decreases

interference between nodes that do not need to communicate with each other, provides

security by restricting communications between nodes, and simplifies management.

FC switches run their own operating systems that are also referred to as microcode. The

microcode provides all of the FC services and management components such as

command line interfaces (CLI), application interfaces (API), web interfaces, and alerting

via Simple Mail Transfer Protocol (SMTP).

This article does not provide a detailed discussion of the I/O path within the FC fabric, as

the logical configuration of FC switches is generally not the source of many systemic

problems in the storage environment.

Storage: Logical Components

The storage array operating system, often referred to as microcode, may contain a wide

range of features including support for multiple RAID levels, logical device cloning,

logical device snapshots, remote replication capabilities, and external storage

virtualization. The storage operating systems also manage performance optimization

algorithms. These include command queuing that optimizes I/O by re-ordering

commands to reduce seek time and rotational latency on the physical disks, and read

ahead algorithms that recognize sequential read I/O, then pre-fetch data from disk to

cache in anticipation of a host request to increase read hits. Read hits decrease I/O

response time by serving reads at cache rather than disk speeds.


Storage arrays partition physical disks into logical devices (ldevs) that are then

presented to hosts as SCSI target devices, commonly referred to as LUNs. The storage

array must adapt its SCSI emulation to accommodate variations in host operating

system requirements. Active/passive arrays also need to accommodate different

behaviours for the various host multipath manager solutions.

LUN security, also known as LUN masking, allows multiple hosts to access LUNs

through shared front end ports on the storage array without seeing LUNs that belong to

other hosts.

The precise path an I/O takes through the storage array varies somewhat depending

whether the I/O is a write or a read, and on its interaction with the cache.

As mentioned earlier, storage arrays increase write performance by acknowledging write

I/O to the host once data has been written to mirrored cache, and then committing the

data to disk later. This is called a write hit, and describes the majority of write I/O. Some

storage arrays, such as the EMC CLARiiON®, allow the storage administrator to set write

aside limits that direct large I/Os directly to disk. This slows down the write response

time for the host, but prevents large I/Os from consuming too much cache. If a write I/O

is forced to wait for a cache slot to become free because the cache is globally stressed,

or a cache limit has been reached for the logical device, a delayed fast write has

occurred. The term write miss may also be applied to delayed fast writes, or to scenarios

where data is being written directly to disk for reasons other than write aside, such as

cache failure.

Read hits occur when a read I/O is serviced from data already resident in cache. As

discussed above, read-ahead algorithms increase the number of read hits by pre-

fetching data to cache in anticipation of host requests. A read miss occurs when a read

request has to wait for data to be read from the physical disks before it can be returned

to the host. Read hits service I/O at cache speeds, so have a significantly lower

response time to the host than read misses, which service I/O at disk speeds.


The sequence of events for a write hit on the storage array begins with FC frames being

received on the front end ports, after which the SCSI payload is extracted by the front

end controllers. The I/O is written to mirrored cache, and then the front end controller

creates a SCSI write acknowledgement, encapsulates it in an FC frame, and sends it to

the host via the front end port. Later, the data is destaged from cache to the back end

controllers, which then commit the data to disk. Note that not all writes are necessarily

committed to disk; if a host overwrites the data while it still resides in cache (a write

cache rehit), then the original write is never committed to disk.

From a logical perspective, the I/O passes through layers that deal with FCP, then SCSI,

then map logical devices to physical devices, then service the I/O to/from the physical

devices, as illustrated in Figure 7.


Figure 7 – Storage Logical Components in the I/O Path


Figure 8 – Logical Components in the I/O Path


Configurable Items in the Storage Environment

The logical layers in the storage environment have configurable components that affect

performance and availability. Incorrect configurations at any level can defeat physical

and logical designs for high availability. Correct configurations require end-to-end

compatibility for the configuration of the logical components. However, that end-to-end

view can be difficult to accomplish as the various actors often do not understand the

end-to-end configuration requirements. Most default configurations will work when

deployed, concealing issues that will only become apparent when an exception scenario

occurs with the storage environment. Storage environments should be able to withstand

many types of physical failures and brief connectivity interruptions without causing

significant disruption to applications and databases.

Common failures and events that should not cause disruption to service include:

• Single disk failure within a RAID group

• HBA failure in a multipath environment

• Switch or switch port failure in a multipath environment

• Storage port failure in a multipath environment

• Redundant power supply failure on a storage array

• LUN trespassing in active/passive storage arrays

Some of the above may cause performance degradation in busy systems. For example,

the loss of one of two paths as the result of a switch port failure decreases theoretical

throughput by 50%, and the loss of a disk within a RAID group causes an increase in

disk activity when the RAID group rebuilds to a new disk or a hot spare.

Common maintenance and deployment activities that should not cause disruption to

service include:

• Activating zone sets on the SAN fabric

• Creating LUNs on storage arrays

• Allocating LUNs to hosts

• Microcode upgrades on switches

• Microcode upgrades on storage arrays


Some of these activities may cause some performance degradation in busy systems.

Additional care needs to be taken with active/passive storage arrays such as the EMC

CLARiiON if the activity requires a controller failover, e.g., a CLARiiON microcode

upgrades requires the service processors (SP) to reboot, resulting in many multipath

events on connected hosts as LUNs trespass back and forth between the two SP.

From an availability perspective, storage administrators’ tasks are fairly straightforward,

with only minor variations in configuration to suit the OS of the attached host. For

example, the following Storage Administrator activities are OS agnostic:

• LUN masking (unless LUN masking also includes SCSI emulation)

• LUN creation

• MetaLUN configuration (e.g., striped vs. concatenated)

• Fabric Zoning

Storage Administrator activities that are not OS agnostic are due to OS variations in

SCSI implementation and fail over requirements. This affects activities like:

• SCSI LUN numbering (e.g., Volume Set Addressing for HPUX)

• SCSI emulation and failover configurations (e.g., port flag settings on DMX, HBA

registration on CX)

Storage Administrators face greater challenges with configuration for performance

considerations. They affect decisions about items such as:

• RAID type (e.g., RAID 5 vs. RAID 10)

• Disk tier (e.g., SSD vs. FC vs. SATA, 15000 RPM vs. 10000 RPM)

• MetaLUN configuration (striped vs. concatenated)

• Fan out ratios


Most storage availability issues related to configuration reside in the logical components

at the host level, due to a number of factors:

• the large number of configurable items

• the large number of product choices available at the various logical levels (e.g.,

ZFS vs. VxFS, native multipath vs. EMC PowerPath® vs. VxDMP)

• the responsibility for configurable items affecting availability and performance

resides in different functional groups (e.g., Systems Administrators, Database

Administrators, and Application Support Analysts).

Storage vendors provide guidance on host configurations that have been tested for

availability, and are suitable general performance requirements. This guidance widely

comes in the form of an interoperability matrix that contains information on a wide range

of host-connectivity-storage combinations, for both logical and physical components.

These documents are updated regularly to cover new products, and include required

OS, microcode, and driver patches and upgrades. Detailed connectivity guides for a

particular OS and storage vendor combination is usually available from the storage

vendor. These detailed guides provide an in-depth look at configuration options, and

may include guidelines for parameter tuning beyond the guidelines published in the

interoperability matrix.

EMC also provides the web-based Host Environment Analysis Tool (HEAT) to assist

Systems Administrators to validate the configuration of hosts that are attached to EMC

Symmetrix® or CLARiiON arrays via FC. The System Administrator runs a data collection

tool to capture the configuration of a host (EMCGrab for UNIX, EMCReports for

Windows), then uploads the compressed output file to the HEAT website. HEAT then

compares the host configuration against the current interoperability matrix, and provides

a report on the configuration highlighting any discovered deficiencies.

Each logical layer of the I/O path has some mechanism for dealing with imperfect I/O

operations. As a result, there are multiple mechanisms for retrying failed I/O operations,

and timeout settings for reporting I/O operation failures up to the next logical layer.

Incompatible configurations between the layers can lead to service disruptions if any

layer is not able to appropriately handle I/O operation exceptions. Several examples of

failures due to configuration or compatibility issues are discussed below.


There must be some mechanism at the application level to deal with I/O interruptions

that are considered to be normal operations in the storage environment. Whether or not

there is a configurable item at the application layer depends on the application, so in

some cases problems are introduced at the design stage, when applications are

introduced that are not able to tolerate normal operations in the storage environment.

For example, WebLogic 8.1 JMS queues have internal timers that may cause JMS to

shut down if I/O hangs during the ‘non-disruptive’ failover of the NAS head during a

microcode upgrade, even though NFS on the host recovers gracefully.

With Oracle ASM, Database Administrators have taken on some additional configuration

responsibilities, as they now have more control over the configuration of logical devices

on the host. DBAs can make many configuration changes that affect performance, but

they can also impact availability. For example, adding new devices into an ASM disk

group triggers a rebalancing operation that can have a significant performance impact.

DBAs also choose how to allocate raw devices into the ASM disk groups; as a result,

poor communication between the DBA, Systems Administrator, and Storage

Administrator could lead to devices with dissimilar performance characteristics (e.g., FC

vs. SATAII) being added to the same ASM disk group.

Path management software may require SCSI target driver timeout settings that are

different than the OS default settings. For example, failing to set the ssd:ssd_io_time on

a Solaris 10 host running the Leadville stack can cause the host to panic or offline all

disk during regular SAN events such as LUN trespasses, switch microcode upgrades, or

storage array microcode upgrades.

Host SCSI queue depth settings that control the maximum number of outstanding SCSI

commands can be queued against a device. They are often thought to be only related to

performance, but queue depth can also be implicated in availability. For example,

inappropriately high queue depth settings can lead to unusually high I/O response times

on a busy storage array, which can trigger write timeout values within Oracle ASM. This

causes ASM to take disk groups offline.


FC protocol parameter tuning may also be recommended on some systems, affecting

items such as the number of I/O retries at the FC frame level, or the length of time the

HBA waits before it takes a port offline due to loss of light.

Storage vendor qualified HBA drivers and firmware may contain default settings that are

different from the OEM installation, again leading to potential availability issues for

common SAN events.

Figure 9 relates the generic logical layers presented in this article to a specific

configuration example: a Solaris 10 host using the Leadville stack, attached via dual FC

fabrics to an EMC DMX class array. Also included are some comments about

configurable items at each logical layer, mapped to the actor in the organization most

likely to perform the configuration.


Figure 9 – I/O Path for Solaris 10 with Leadville and DMX

fcp

fp

HOST

Applications Databases

File System Oracle Automatic Storage

Management (ASM)*

* if present

Logical Volume Manager (LVM)

SCSI target drivers

FC protocol drivers

HBA drivers

HBA HBA

Multipath drivers

Solaris 10 – Leadville

Oracle 10g Application

ZFS ASM

ssd

scsi_vhci (mpxio)

emlxs

I/O timeout I/O size

Block size

Diskgroups External Redundancy Allocation Unit Size Striping Stripe size

zpools RAID Striping Cache Quotas Snapshots Clones Block size Compression

/etc/system entries: I/O timeout: ssd_io_time Queue Depth: ssd_max_throttle SCSI retry: ssd ua retry count

/kernel/drv/fp.conf /kernel/drv/mpt.conf

/etc/system entries: Automatic LUN discovery: fcp:ssfcp_enble_auto_configuration Fail port: fp_offline_ticker FC Frame reties: fp retry count

/kernel/drv/emlxs.conf entries: Fail port: fp_offline_ticker FC Frame reties: fp_retry_count

Configurable Items


CONNECTIVITY

FC Zoning

Port

Port

FC Zoning

Port

Port

STORAGE

FC Protocol

SCSI Emulation

Logical Devices

Disk Controllers

Port Port

Port Port

D i s k

RAID

LUN Security

Cache Cache

Single initiator

zoning

Single initiator

zoning

Fibre and SCSI Port Flag Settings

LUN masking

MetaDevices

LUN mapping

RAID 5 (7+1)

Zoning Port speed

Port Flag settings: Common Serial Number Disable Queue Reset on Unit Attention Fan Out ratios SCSI LUN numbers LUN masking MetaDevice striping/concatenation

Disk Tier (disk type, speed, size) RAID level Share Everything, Share Nothing Hot Spares


Defining the Problem

On occasion, storage goes awry. It is easy to imagine nightmarish storage scenarios, all

you have to do is flip through the “Fixed Problems” section of the microcode patches you

haven’t applied yet to get some ideas. However, most storage related problems occur

because people associated with the storage environment deploy flawed configurations

that appear to be healthy, but may not withstand storage environment exception

scenarios, such as microcode upgrades and redundant hardware failures. In other

cases, incorrect configurations may manifest as problems on a future reboot, or disk

discovery. As a general statement, it is safe to state:

Most storage related failures are directly attributable to improper configurations

either as the result of initial deployment errors, or failure to maintain the

environment by pro-actively applying patches and upgrades.

As a corollary to the above, it follows that:

Most storage related failures are preventable.

Solving the Problem

Configuration of a storage environment is an iterative process. Configuration changes

resulting from new deployments into the environment, and sustainment activities arising

from problem, performance, and capacity management processes. Each of these

processes needs to be tied to a rigorous configuration management discipline to

maintain a storage environment that approaches optimal performance and availability

within the constraints of the environment’s physical capabilities.

The purpose of configuration management is to identify and track the characteristics of

the physical and logical components, with the amount of detail necessary to support

decision making regarding the potential impact configuration changes may have within

the storage environment. Configuration management begins with identifying the

components in the storage environment. Often, this information will be recorded in a

database referred to as the configuration management database (CMDB). The

configuration management discipline should also contain processes that control the


configuration (i.e., place controls on changes to the configuration), provide the ability to

report on the configuration, and require configuration audits. Configuration management

processes should be deeply integrated with problem management and change

management processes, and will assist in providing answers to questions such as:

• What business services, applications, databases, servers, etc. may be impacted

by a planned configuration change?

• What business services, applications, databases, servers, etc. may be impacted

by a problem that has been identified? (e.g., what servers are connected to a

failed switch, or what servers require proactive maintenance to apply an OS

patch?)

• What configuration changes were introduced in the storage environment that

may be linked to the introduction of a current problem?

Configuration management in the storage environment is complicated by the number of

organizational teams that may be involved in the design, deployment, and maintenance

of the configurable items in the I/O path. A successful configuration management

implementation must define, record, and standardize the configuration requirements for

all of the logical components in the I/O path. This will be an ongoing process, as it must

capture new technologies as they are introduced, and fixes and releases for existing

components (e.g., new driver versions, OS and microcode patches). Each of the

affected organizational groups should have a highly skilled resource involved in the

configuration management process since the establishment of deployment standards

requires specialized knowledge within each logical layer of the storage environment.

Some organizations may choose to create a separate organizational unit for

configuration management, while others may choose to create a committee containing

representation from several organizational groups. In any case, configuration standards

must be effectively communicated to the operational groups that apply the

configurations, and must be effectively integrated into their deployment and maintenance

procedures. Documentation of the standards should include a description of the

functionality of each configuration item, as well as the reason for the particular value it

should be set to, and, if appropriate, a reference to the vendor documentation that

specifies the setting. This higher level information will increase compliance by providing


operational staff with a context for configuration decisions, and generally increases their

level of awareness and comfort with the storage environment.

Audit configurations to ensure that they comply with the defined configuration standards.

Audits should be conducted for each deployment, and also periodically to measure the

degree of compliance with configuration standards and uncover any latent threats. In

large environments, periodic audits can be limited to some representative subset of the

environment to allow the overall compliance of the storage environment to be

extrapolated. Audits also allow compliance with configuration standards to be measured

and rated, which can be useful as an organizational performance metric, or as part of a

continuous improvement process.

Figure 11 illustrates the desired outcome of implementing a configuration management

process for the storage environment.


Figure 11 – Configuration Management Goal


Conclusion The purpose of a storage environment is to support business functions and objectives at

the least possible cost. To fulfill this purpose, the logical, physical, and human

components of a storage environment must be working in concert. However, storage

configurations often fail, with causes that are directly attributable to improper

configurations or poor maintenance practices. Thus, most storage related failures are

preventable.

Most storage configuration issues occur in the logical components at the host level, due

to a number of factors, such as:

• The large number of configurable items at the host level

• The large number of product choices available

• The responsibility for configurable items resides in several different functional

groups

When an organization first deploys a storage environment, the Storage Administrator

and the Systems Administrator roles are likely to reside within the same functional

group. As the storage environment grows larger, these roles are often separated into

different functional groups. As a result, storage related skills within the Systems

Administrator groups are likely to decline over time, as new Systems Administrators

have generally had very limited exposure to external storage. Since Systems

Administrators are responsible for a large number of configurable items related to

storage, this declining skill set presents challenges in maintaining the health of the

storage environment. Configuration and maintenance issues at the Systems

Administrator level can compound very quickly, such as when configuration errors are

embedded in deployment tools, or if server configurations and patch levels delay or

prevent microcode upgrades being deployed on storage arrays and switches.

We can reduce storage related failures by implementing a rigorous configuration

management discipline. To be successful, the configuration management processes

must include representation from all of the functional groups involved in configuring the

I/O path, and be integrated into their documentation and procedures. As well as tracking

the status of all of the configurations within the storage environment, the configuration


management process must include regular audit procedures to provide a measurable

verification of the level of compliance to standards.

Configuration management faces fewer challenges if the number of unique

configurations can be reduced by:

• Limiting the number of manual tasks in server deployments by using standard

images during deployment and tools such as Jumpstart for Solaris, Kickstart for

Red Hat Linux, or NIM for AIX.

• Decreasing SAN complexity by limiting the number of storage vendors within the

same class of storage, i.e., having different vendors for monolithic arrays,

modular arrays, and NAS is manageable, but having two vendors for modular

storage increases complexity unnecessarily.

Storage vendors provide guidance on host configurations that have been tested for

availability and are suitable general performance requirements. EMC also provides tools

that allow Systems Administrators to quickly check a host configuration against the

current EMC support matrix. In general, storage vendors are strongly motivated to help

keep your storage environment healthy. Their expertise and experience in other

customer sites should be leveraged when establishing a configuration management

process and in its ongoing maintenance.

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