m13res01

Copyright 2013 EMC Corporation. All rights reserved

This module focuses on an overview of VNX Snapshot, as well as managing VNX Snapshots.

VNX Snapshot 1


This lesson covers the purpose, requirements, managed objects and theory of operations of VNX snapshots.

VNX Snapshot 2


VNX Snapshot is a storage system-based software application that allows the user to create snapshots of pool-based LUNs. In fact, VNX Snapshots can only be used with pool LUNs. A snapshot is a virtual point-in-time copy of a LUN and takes only seconds to create. VNX Snapshots use a very different internal mechanism to that used by SnapView snapshots, though both are pointer-based. VNX Snapshot data may be in the original Primary LUN space or may have been written to a different location in the Pool. As a result of the Relocate on First Write (ROW) technology used, VNX Snapshots use appreciably less additional space than a full copy would use. A VNX Snapshot will use appreciably less space than that occupied by its Primary LUN, and will make more efficient use of space than SnapView Snapshots.

An enabler gives the user access to VNX Snapshots, while a separate enabler allows the use of SnapView Snapshot and Clone technology. These two methods of making point-in-time copies are independent, and have limits which are independent of each other. They can coexist on the same storage system, and even on the same Pool LUNs. Note that VNX Snapshots cannot be used on Classic LUNs.

Management of VNX Snapshots is performed through Unisphere or Navisphere Secure CLI. A host-based utility, SnapCLI, can perform a subset of the VNX Snapshot management operations, and will be discussed later.

3 VNX Snapshot


The table above lists the hardware and software requirements for VNX Snapshots.

The SnapCLI program runs on host system in conjunction with VNX Snapshot running on the EMC VNX storage processors (SPs), and allows the user to create and delete snapshots, and expose them to host systems. All SnapCLI commands are sent to the storage system through the Fibre Channel or iSCSI connection.

4 VNX Snapshot


The Primary LUN is the production LUN that is replicated. This is the LUN that is in use by the application (and the production host) and it is not visible to secondary hosts. When a snapshot is attached to a snapshot mount point, it is made available to a secondary host.

A Snapshot is the VNX Snapshot equivalent of the SnapView session.

A Snapshot Mount Point is the VNX Snapshots equivalent of the SnapView Snapshot a virtual LUN that is used to make the replica visible to a secondary host. The SMP is associated with the primary LUN, and can be used for snapshots of that LUN only.

Consistency Groups allow primary LUNs or Snapshot Mount Points to be grouped together persistently. Operations can be performed on the group as a single object.

5 VNX Snapshot


VNX Snapshots address limitations of copy on first write (COFW) SnapView Snapshots. The VNX Snapshot technology is redirect on write (or ROW). VNX Snapshots are limited to Pool-based LUNs (i.e. not Classic LUNs). Up to 256 writeable VNX Snapshots can be associated with any Primary LUN, though only 255 are user visible. Because the VNX Snapshot uses pointers rather than a full copy of the LUN, it is space-efficient, and can be created almost instantaneously. The ROW mechanism does not use a read from the Primary LUN as part of its operation, and thus eliminates the most costly (in performance terms) part of the process.

A Reserved LUN Pool is not required for VNX Snapshots - VNX Snapshots use space from the same Pool as their Primary LUN. Management options allow limits to be placed on the amount of space used for VNX Snapshots in a Pool.

VNX Snapshots allow replicas of replicas; this includes Snapshots of VNX Snapshots, Snapshots of attached VNX Snapshot Mount Points, and Snapshots of VNX Snapshot Consistency Groups. VNX Snapshots can coexist with SnapView snapshots and clones, and are supported by RecoverPoint.

If all VNX Snapshots are removed from a Thick LUN, the driver will detect this and begin the defragmentation process. This converts Thick LUN slices back to contiguous 256 MB addresses. The process runs in the background and can take a significant amount of time. The user can not disable this conversion process directly, however, it can be prevented by keeping at least one VNX Snapshot of the Thick LUN.

Note: while a delete process is running, the Snapshot name remains used. So, if one needs to create a new Snapshot with the same name, it is advisable to rename the Snapshot prior to deleting it.

VNX Snapshot 6


A VNX Snapshot Mount Point (SMP) is a container that holds SCSI attributes

WWN

Name

Storage Group LUN ID, etc.

An SMP is similar to a Snapshot LUN in the SnapView Snapshot environment. It is independent of the VNX Snapshot (though it is tied to the Primary LUN), and can therefore exist without a VNX Snapshot attached to it. Because it behaves like a LUN, it can be migrated to another host and retain its WWN. In order for the host to see the point in time data, the SMP must have a VNX Snapshot attached to it. Once the Snapshot is attached, the host will see the LUN as online and accessible. If the Snapshot is detached, and then another Snapshot is attached, the host will see the new point in time data without the need for a rescan of the bus.

VNX Snapshot 7


The VNX Snapshot Consistency Group allows Snapshots to be taken at the same point in time on multiple Primary LUNs. If individual Snapshots were made of the Primary LUNs, it is possible that updates to one or more Primary LUNs could take place between the time of the Snapshot on the first Primary LUN and the time of the Snapshot on the last Primary LUN. This causes inconsistency in the Snapshot data for the set of LUNs. The user can ensure consistency by quiescing the application but this is unacceptable in many environments.

A Consistency Group can have a Snapshot taken of it, and can have members added or removed. Restore operations can only be performed on Groups that have the same members as the Snapshot. This may require modifying Group membership prior to a restore.

When a Snapshot is made of a Group, updates to all members are held until the operation completes. This has the same effect as a quiesce of the I/O to the members, but is performed on the storage system rather than on the host.

VNX Snapshot Set a group of all Snapshots from all LUNs in a Consistency Group. For simplifications, is referred to as CG Snap throughout the material. VNX Snapshot Family a group of Snaps from the same Primary LUN

VNX Snapshot 8


This slide summarizes the differences between SnapView Snapshot and VNX Snapshot terms.

VNX Snapshot 9

Copyright 2012 EMC

Corporation. Do not Copy -

All Rights Reserved.


This slide, and the following side, compares the two VNX snapshot technologies.

In this slide, the processes involved in a new host write to the source LUN (primary LUN) are compared. In the familiar SnapView Snapshot environment, the COFW process reads the original 64 KB data chunk from the source LUN, writes that chunk to the Reserved LUN, and updates the pointers in the Reserved LUN map area. Once these steps complete, the host write to the Source LUN is allowed to proceed, and the host will receive an acknowledgement that the write is complete. If a SnapView Snapshot is deleted, data in the RLP is simply removed, and no processing takes place on the Source LUN.

In the case of a VNX Snapshot, a new host write is simply written to a new location (redirected) inside the Pool. The original data remains where it is, and is untouched by the ROW process. The granularity of Thin LUNs is 8 KB, and this is the granularity used for VNX Snapshots. New data written to a Thick LUN with a VNX Snapshot is mapped in the same way as the data for a Thin LUN, and it is expected that this will reduce Thick LUN performance. If a VNX Snapshot is removed from a Thin LUN, data will be consolidated into 256 MB slices to minimize wasted space. If the last VNX Snapshot is removed from a Thick LUN, the defragmentation process moves the new data to the original locations on disk, and freed space is returned to the Pool.

VNX Snapshot 10


In this slide, the processes involved in a secondary host read of a Snapshot are compared. In the familiar SnapView Snapshot environment, data which has not yet been modified is read from the source LUN, while data that has been modified since the start of the SnapView Session is read from the Reserved LUN. SnapView always needs to perform a lookup to determine whether data is on the Source LUN or Reserved LUN, which causes Snapshot reads to be slower than Source LUN reads.

In the case of a VNX Snapshot, the original data remains where it is, and is therefore read from the original location on the Primary LUN. That location will be discovered by a lookup which is no different to that performed on a Thin LUN which does not have a VNX Snapshot, so the performance is largely unchanged.

VNX Snapshot 11


VNX Snapshots support the following operations:

Create a Snapshot

Create a Snapshot Mount Point

Attach a Snapshot

Copy a Snapshot

Snap a Snapshot Mount Point (Cascading Snapshots)

Detach a Snapshot Mount Point

Destroy a Snapshot

Attach a Consistency Snapshot

These operations will be covered on the following slides.

VNX Snapshot 12


Creating a snapshot does not consume any pool space. The space starts being used when new writes to the primary LUN or to the snapshot itself arrive. Snapshots have a granularity of 8 KB, and theis blocks are tracked just like the blocks in thin LUNs. Every snapshot must have a primary LUN, and that property never changes.

A primary LUN cannot be deleted while it has snapshots.

VNX Snapshot 13


Creating a snapshot does not require any space from the pool. Each SMP is dedicated to a specific primary LUN. It is not possible to attach snapshots from two different primary LUNs to a single SMP. Therefore, a backup server that is backing up four different LUNs must have four different SMPs provisioned to back up the snapshots of those LUNs.

VNX Snapshot 14


Attaching is an asynchronous operation during which the SMP remains available, but the I/O is queued. This means that the host does not have to rescan the SCSI bus to view the snapshot. The rescan is required only to discover the SMP when it is first presented to the host.

VNX Snapshot 15


A VNX Snapshot can be copied to another snapshot. The resulting snapshot is a copy of the property is set to on the copy. The

snapshot copy retains the source LUN properties and resides within the same pool as the original snapshot. So, copying a snapshot increases the snapshot count for a given production LUN by one.

VNX Snapshot 16


A snapshot of a SMP still has the same Primary LUN property as the Mount point (and as the attached snapshot). The primary LUN properties of snapshots and the mount points that they are attached to will never be different. It is technically possible to attach a snapshot to a SMP that is not a part of a storage group. Therefore, it is possible to create a snapshot of such a SMP. The resulting snapshot will be slightly different from a regular snapshot copy. The source of this snapshot and the creation times will not be the same as the snapshot attached to the SMP.

VNX Snapshot 17


Detaching a snapshot from a SMP modifies these properties: Last Modified Date and Last Modified By. Detaching a snapshot from a SMP does not destroy the SMP by default, and the SMP remains provisioned to the host. Detaching is an asynchronous operation during which the SMP remains available, but the host I/O is queued at the array. After the detach operation completes, all queued I/O requests return a fail status to the host.

VNX Snapshot 18


Destroying (deleting) a snapshot reclaims space for reuse in the storage pool. Reclaim is not instant, and is done by an internal process. This internal process is throttled for better performance of the array and is not sequential, meaning that more than one snapshot can be destroyed at a time. Multiple snapshot destructions start on a first-come-first-served basis. VNX is tuned to destroy up to 16 snapshots simultaneously on each Storage Processor (SP). Additional destruction requests are queued until a destruction thread becomes available.

VNX Snapshot 19


Mounting an entire Consistency Snapshot requires the same number of SMPs as there are members in the CG.

VNX Snapshot 20


This lesson covered the purpose, requirements, managed objects and theory of operations of VNX snapshots.

VNX Snapshot 21


This lesson covers the management of SnapView snapshots and sessions.

VNX Snapshot 22


VNX Snapshot management is performed from the Data Protection tab in the top navigation bar. An option under Wizards is the Snapshot Mount Point Configuration Wizard, while the Consistency Group area has the Create Snapshot Consistency Group link.

The VNX Snapshot is visible under the Snapshots tab of the Data Protection> Snapshots dialog. VNX Snapshots may be taken of individual Primary LUNs, VNX Snapshots, or Consistency Groups. A VNX Snapshot of a Consistency Group implies that VNX Snapshots have been taken of the member Primary LUNs at the same point in time. This can be seen in the Creation Time column for the VNX Snapshots.

23 VNX Snapshots


Right-clicking the VNX Snapshot and choosing Properties launches a new dialog. The General tab displays much of the relevant information for the Snapshot, including the name, creation time, and expiration time. Note that several VNX Snapshot properties may be modified on the fly. When a snapshot is first created, it is write-protected.

Advanced section include allowing read/write mode for the snapshot, enabling automatic deletions based on the configuration of the hosting Pool, and configuring an expiration time. Default expiration time is 7 days; the selection ranges from 1 hour to 10 years. The system polls for expired Snapshots every hour, and deletes Snapshots once their expiration time is reached.

VNX Snapshots will not be deleted if they are in use attached, or involved in a restore - when their expiration date is reached; they will be removed when detached, or when the restore completes. The user will be warned before Snapshots are deleted.

24 VNX Snapshots

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