Post on 31-Jan-2016
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CS 519: Lecture 4
I/O, Disks, and File Systems
2Computer Science, Rutgers CS 519: Operating System Theory
I/O Devices
So far we have talked about how to abstract and manage CPU and memory
Computation “inside” computer is useful only if some results are communicated “outside” of the computer
I/O devices are the computer’s interface to the outside world (I/O Input/Output)
Example devices: display, keyboard, mouse, speakers, network interface, and disk
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Basic Computer Structure
CPU Memory
Bridge
Disk NIC
Memory Bus(System Bus)
I/O Bus
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OS: Abstractions and Access Calls
OS must virtualize a wide range of devices into a few simple abstractions:
StorageHard drives, tapes, CDROM
Networking
Ethernet, radio, serial line
Multimedia
DVD, camera, microphones
Operating system should provide consistent calls to access the abstractions
Otherwise, programming is too hard
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User/OS Interface
Same interface is used to access devices (like disks and network cards) and more abstract resources (like files)
4 main calls:open()
close()
read()
write()
Semantics depend on the type of the device (block, char, net)
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Unix I/O Calls
fileHandle = open(pathName, flags, mode)a file handle is a small integer, valid only within a single process, to operate on the device or file
pathname: a name in the file system. In Unix, devices are put under /dev. E.g. /dev/ttya is the first serial port, /dev/sda the first SCSI drive
flags: blocking or non-blocking …
mode: read only, read/write, append …
errorCode = close(fileHandle)Kernel will free the data structures associated with the device
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Unix I/O Calls
byteCount = read(fileHandle, buf, count)read at most count bytes from the device and put them in the byte buffer buf. Bytes placed from 0th byte.
Kernel can give the process fewer bytes, user process must check the byteCount to see how many were actually returned.
A negative byteCount signals an error (value is the error type)
byteCount = write(fileHandle, buf, count)write at most count bytes from the buffer buf
actual number written returned in byteCount
a negative byteCount signals an error
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I/O Semantics
From this basic interface, two different dimensions to how I/O is processed:
blocking vs. non-blocking vs. asynchronous
buffered vs. unbuffered
The OS tries to support as many of these dimensions as possible for each device
The semantics are specified in the open() system call
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Blocking vs. Non-blocking vs. Asynchronous I/O
Blocking – process is blocked until all bytes in the count field are read or written
E.g., for a network device, if the user wrote 1000 bytes, then the OS would only unblock the process after the write() call completes.
+ Easy to use and understand
- If the device just can’t perform the operation (e.g. you unplug the cable), what to do? Give up an return the successful number of bytes.
Non-blocking – the OS only reads or writes as many bytes as is possible without blocking the process
+ Returns quickly
- More work for the programmer (but really good for robust programs)
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Blocking vs. Non-blocking vs. Asynchronous I/O
Asynchronous – similar to non-blocking I/O. The I/O call returns immediately, without waiting for the operation to complete. I/O subsystem signals the process when I/O is done. Same advantages and disadvantages of non-blocking I/O.
Difference between non-blocking and asynchronous I/O: a non-blocking read() returns immediately with whatever data available; an asynchronous read() requests a transfer that will be performed in its entirety, but that will complete at some future time.
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Buffered vs. Unbuffered I/O
Sometimes we want the ease of programming of blocking I/O without the long waits if the buffers on the device are small.
Buffered I/O allows the kernel to make a copy of the data and adjust to different device speeds.
write(): allows the process to write bytes and continue processing
read(): as device signals data is ready, kernel places data in the buffer. When process calls read(), the kernel just makes a copy.
Why not use buffered I/O?
-- Extra copy overhead
-- Delays sending data
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Getting Back to Device Types
Most OSs have three device types (in terms of transfer modes):
Character devicesUsed for serial-line types of devices (e.g., USB port)
Block devices Used for mass-storage (e.g., disks and CDROM)
Network devices Used for network interfaces (e.g., Ethernet card)
What you can expect from the read/write calls changes with each device type
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Character Devices
Device is represented by the OS as an ordered stream of bytes
bytes sent out to the device by the write system call
bytes read from the device by the read system call
Byte stream has no “start”, just open and start reading/writing
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Block Devices
OS presents device as a large array of blocks
Each block has a fixed size (1KB - 8KB is typical)
User can read/write only in fixed-size blocks
Unlike other devices, block devices support random access
We can read or write anywhere in the device without having to ‘read all the bytes first’
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Network Devices
Like block-based I/O devices, but each write call either sends the entire block (packet), up to some maximum fixed size, or none.
On the receiver, the read call returns all the bytes in the block, or none.
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Random Access: The File Pointer
For random access in block devices, OS adds a concept called the file pointer
A file pointer is associated with each open file or device, if the device is a block device
The next read or write operates at the position in the device pointed to by the file pointer
The file pointer points to bytes, not blocks
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The Seek Call
To set the file pointer:absoluteOffset = lseek(fileHandle, offset, from);
from specifies if the offset is absolute, from byte 0, or relative to the current file pointer position
The absolute offset is returned; negative numbers signal error codes
For devices, the offset should be a integral number of bytes.
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Block Device Example
You want to read the 10th block of a disk
Each disk block is 4096 bytes longfh = open(/dev/sda, , , );
pos = lseek(fh, 4096*9, 0);
if (pos < 0) error;
bytesRead = read(fh, buf, 4096);
if (bytesRead < 0) error;
…
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Getting and Setting Device-Specific Info
Unix has an I/O control system call: ErrorCode = ioctl(fileHandle, request, object);
request is a numeric command to the device
Can also pass an optional, arbitrary object to a device
The meaning of the command and the type of the object are device-specific
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Programmed I/O vs. DMA
Programmed I/O is ok for sending commands, receiving status, and communication of a small amount of data
Inefficient for large amount of dataKeeps CPU busy during the transfer
Programmed I/O memory operations slow
Direct Memory AccessDevice read/write directly from/to memory
Memory device typically initiated from CPU
Device memory can be initiated by either the device or the CPU
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Direct Memory Access
Used to avoid programmed I/O for large data movement
Requires DMA controller
Bypasses CPU to transfer data directly between I/O device and memory
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Programmed I/O vs. DMA
CPU Memory
Disk
Interconnect
CPU Memory
Disk
Interconnect
CPU Memory
Disk
Interconnect
ProgrammedI/O
DMA DMADevice Memory
Problems?
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Six Steps to Perform DMA Transfer
Source: SGG
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Life Cycle of a Blocking I/O Request
driver
Naïve processing of blocking request:device driver executed by a dedicated kernel thread; only one I/O can be processed at a time.
More sophisticated approach wouldnot block the device driver and wouldnot require a dedicated kernel thread.
Source: SGG
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Life Cycle of a Blocking I/O Request
Source: SGG
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Performance
I/O a major factor in system performanceDemands CPU to execute device driver, kernel I/O code
State save/restore due to interrupts
Data copying
Disk I/O is extremely slow
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Improving Performance
Reduce number of context switches
Reduce data copying
Reduce interrupts by using large transfers, smart controllers, polling
Use DMA
Balance CPU, memory, bus, and I/O performance for highest throughput
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Device Driver
OS module controlling an I/O device
Hides the device specifics from the above layers in the kernelSupporting a common API
UNIX: block or character deviceBlock: device communicates with the CPU/memory in fixed-
size blocks
Character/Stream: stream of bytes
Translates logical I/O into device I/OE.g., logical disk blocks into {head, track, sector}
Performs data buffering and scheduling of I/O operations
StructureSeveral synchronous entry points: device initialization,
queue I/O requests, state control, read/write
An asynchronous entry point to handle interrupts
29Computer Science, Rutgers CS 519: Operating System Theory
Some Common Entry Points for UNIX Device Drivers
Attach: attach a new device to the system.
Close: note the device is not in use.
Halt: prepare for system shutdown.
Init: initialize driver globals at load or boot time.
Intr: handle device interrupt.
Ioctl: implement control operations.
Mmap: implement memory-mapping.
Open: connect a process to a device.
Read: character-mode input.
Size: return logical size of block device.
Start: initialize driver at load or boot time.
Write: character-mode output.
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User to Driver Control Flow
user
kernel
read, write, ioctl
special file ordinary file
file system
buffer cache
blockdevice
characterdevice
character queue
driver driver
31Computer Science, Rutgers CS 519: Operating System Theory
Buffer Cache
When an I/O request is made for a block, the buffer cache is checked first
If block is missing from the cache, it is read into the buffer cache from the device
Exploits locality of reference as any other cache
Replacement policies similar to those for VM, but LRU is feasible
UNIX
Historically, UNIX has a buffer cache for the disk which does not share buffers with character/stream devices
Adds overhead in a path that has become increasingly common: disk NIC
32Computer Science, Rutgers CS 519: Operating System Theory
Disks
Seek time: time to move the disk head to the desired track
Rotational delay: time to reach desired sector once head is over the desired track
Transfer rate: rate data read/write to disk
Some typical parameters:Seek: ~2-10ms
Rotational delay: ~3ms for 10000 rpm
Transfer rate: 200 MB/s
Sectors
Tracks
33Computer Science, Rutgers CS 519: Operating System Theory
Disk Scheduling
Disks are at least four orders of magnitude slower than main memory
The performance of disk I/O is vital for the performance of the computer system as a whole
Access time (seek time + rotational delay) >> transfer time for a sector
Therefore the order in which sectors are read matters a lot
Disk schedulingUsually based on the position of the requested sector rather than according to the process priority
Possibly reorder stream of read/write request to improve performance
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Disk Scheduling (Cont.)
Several algorithms exist to schedule the servicing of disk I/O requests.
We illustrate them with a request queue (tracks 0-199).
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
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FCFS
Illustration shows total head movement of 640 cylinders.
Source: SGG
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SSTF
Selects the request with the minimum seek time from the current head position.
SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests.
Illustration shows total head movement of 236 cylinders.
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SSTF (Cont.)
Source: SGG
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SCAN
The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues.
Sometimes called the elevator algorithm.
Illustration shows total head movement of 208 cylinders.
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SCAN (Cont.)
Source: SGG
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C-SCAN
Provides a more uniform wait time than SCAN.
The head moves from one end of the disk to the other, servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip.
Treats the cylinders as a circular list that wraps around from the last cylinder to the first one.
41Computer Science, Rutgers CS 519: Operating System Theory
C-SCAN (Cont.)
Source: SGG
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C-LOOK
Version of C-SCAN
Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk.
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C-LOOK (Cont.)
Source: SGG
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Disk Scheduling Policies
Shortest-service-time-first (SSTF): pick the request that requires the least movement of the head
SCAN (back and forth over disk): good service distribution
C-SCAN (one way with fast return): lower service variability
Problem with SSTF, SCAN, and C-SCAN: arm may not move for long time (due to rapid-fire accesses to same track)
N-step SCAN: scan of N records at a time by breaking the request queue in segments of size at most N and cycling through them
FSCAN: uses two sub-queues, during a scan one queue is consumed while the other one is produced
45Computer Science, Rutgers CS 519: Operating System Theory
Disk Management
Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write.
To use a disk to hold files, the operating system still needs to record its own data structures on the disk.
Partition the disk into one or more groups of cylinders.
Logical formatting or “making a file system”.
Boot block initializes system.
The bootstrap is stored in ROM.
Bootstrap loader program.
Methods such as sector sparing used to handle bad blocks.
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Swap Space Management
Virtual memory uses disk space as an extension of main memory.
Swap space is necessary for pages that have been written and then replaced from memory.
Swap space can be carved out of the normal file system, or, more commonly, it can be in a separate disk partition.
Swap space management
4.3BSD allocates swap space when process starts; holds text segment (the program) and data segment. (Swap and heap pages are created in main memory first.)
Kernel uses swap maps to track swap space use.
Solaris 2 allocates swap space only when a page is forced out of physical memory, not when the virtual memory page is first created.
47Computer Science, Rutgers CS 519: Operating System Theory
Disk Reliability
Several improvements in disk-use techniques involve the use of multiple disks working cooperatively.
RAID is one important technique currently in common use.
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RAID
Redundant Array of Inexpensive Disks (RAID)A set of physical disk drives viewed by the OS as a single logical drive
Replace large-capacity disks with multiple smaller-capacity drives to improve the I/O performance (at lower price)
Data are distributed across physical drives in a way that enables simultaneous access to data from multiple drives
Redundant disk capacity is used to compensate for the increase in the probability of failure due to multiple drives
Improve availability because no single point of failure
Six levels of RAID representing different design alternatives
49Computer Science, Rutgers CS 519: Operating System Theory
RAID Level 0
Does not include redundancy
Data is stripped across the available disksTotal storage space across all disks is divided into strips
Strips are mapped round-robin to consecutive disks
A set of consecutive strips that maps exactly one strip to each disk in the array is called a stripe
Can you see how this improves the disk I/O bandwidth?
What access pattern gives the best performance?
strip 0 strip 3strip 2strip 1
strip 7strip 6strip 5strip 4
...
stripe 0
50Computer Science, Rutgers CS 519: Operating System Theory
RAID Level 1
Redundancy achieved by duplicating all the data
Every disk has a mirror disk that stores exactly the same data
A read can be serviced by either of the two disks which contains the requested data (improved performance over RAID 0 if reads dominate)
A write request must be done on both disks but can be done in parallel
Recovery is simple but cost is high
strip 0 strip 8strip 0 strip 8
strip 9strip 1 strip 9strip 1
...
51Computer Science, Rutgers CS 519: Operating System Theory
RAID Levels 2 and 3
b0 b1 b2 P(b) X2(i) = P(i) X1(i) X0(i)
Parallel access: all disks participate in every I/O request
Small strips (1 bit) since size of each read/write = # of disks * strip size
RAID 2: 1-bit strips and error-correcting code. ECC is calculated across corresponding bits on data disks and stored on O(log(# data disks)) ECC disks
Hamming code: can correct single-bit errors and detect double-bit errors
Example configurations data disks/ECC disks: 4/3, 10/4, 32/7
Less expensive than RAID 1 but still high overhead – not needed in most environments
RAID 3: 1-bit strips and a single redundant disk for parity bits
P(i) = X2(i) X1(i) X0(i)
On a failure, data can be reconstructed. Only tolerates one failure at a time
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RAID Levels 4 and 5
RAID 4Large strips with a parity strip like RAID 3
Independent access - each disk operates independently, so multiple I/O request can be satisfied in parallel
Independent access small write = 2 reads + 2 writes
Example: if write performed only on strip 0:P’(i) = X2(i) X1(i) X0’(i)
= X2(i) X1(i) X0’(i) X0(i) X0(i)
= P(i) X0’(i) X0(i)
Parity disk can become bottleneck
RAID 5Like RAID 4 but parity strips are distributed across all disks
strip 0 P(0-2)
P(3-5)strip 3
strip 2strip 1
strip 5strip 4
53Computer Science, Rutgers CS 519: Operating System Theory
Cost of Disk Storage
Main memory is much more expensive than disk storage
The cost/MB of hard disk storage is competitive with magnetic tape if only one tape is used per drive
The cheapest tape drives and the cheapest disk drives have had about the same storage capacity over the years
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Cost of DRAM
Source: SGG
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Cost of Disks
Source: SGG
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Cost of Tapes
Source: SGG
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File System
File system is an abstraction of the disk
File Tracks/sectors
File Control Block stores mapping info (+ protection, timestamps, size, etc)
To a user process
A file looks like a contiguous block of bytes (Unix)
A file system provides a coherent view of a group of files
A file system provides protection
API: create, open, delete, read, write files
Performance: throughput vs. response time
Reliability: minimize the potential for lost or destroyed data
E.g., RAID could be implemented in the OS (disk device driver)
58Computer Science, Rutgers CS 519: Operating System Theory
File API
To read or write, need to openopen() returns a handle to the opened file
OS associates a (per-process) data structure with the handle
This data structure maintains current “cursor” position in the stream of bytes in the file
Read and write takes place from the current position
Can specify a different location explicitly
When done, should close the file
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In-Memory File System Structures
Source: SGG
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Files vs. Disk
Files Disk
???
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Files vs. Disk
Files Disk
What’s the problem with this mapping function?What’s the potential benefit of this mapping function?
ContiguousLayout
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Files vs. Disk
Files Disk
What’s the problem with this mapping function?
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UNIX File
i-nodes
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UNIX File Control Block (I-Node)
Source: SGG
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De-fragmentation
Want index-based organization of disk blocks of a file for efficient random access and no fragmentation
Want sequential layout of disk blocks for efficient sequential access
How to reconcile?
66Computer Science, Rutgers CS 519: Operating System Theory
De-fragmentation (cont’d)
Base structure is index-based
Optimize for sequential accessDe-fragmentation: move the blocks around to simulate actual sequential layout of files
Group allocation of blocks: group tracks together (cylinders). Try to allocate all blocks of a file from a single cylinder group so that they are close together. This style of grouped allocation was first proposed for the BSD Fast File System and later incorporated in ext2 (Linux).
Extents: on each write that extends a file, allocate a chunk of consecutive blocks. Some modern systems use extents, e.g. VERITAS (supported in many systems like Linux and Solaris), the first commercial journaling file system. Ext4 can use them also (extents are not the default option, though).
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Free Space Management
No policy issues here – just mechanism
Bitmap: one bit for each block on the diskGood to find a contiguous group of free blocks
Files are often accessed sequentially
For 1TB disk and 4KB blocks, 32MB for the bitmap
Chained free portions: pointer to the next oneNot so good for sequential access (hard to find sequential blocks of appropriate size)
Index: treats free space as a file
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File System
OK, we have files
How can we name them?
How can we organize them?
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Tree-Structured Directories
Source: SGG
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File Naming
Each file has an associated human-readable nameE.g., usr, bin, mid-term.pdf, design.pdf
File name must be globally uniqueOtherwise how would the system know which file we are referring to?
OS must maintain a mapping between a file name and the set of blocks belonging to the file
Mappings are kept in directories
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Unix File System
Ordinary files (uninterpreted)
Directories
Directory is differentiated from ordinary file by bit in i-node
File of files: consists of records (directory entries), each of which contains info about a file and a pointer to its i-node
Organized as a rooted tree
Pathnames (relative and absolute)
Contains links to parent, itself
Multiple links to files can exist: hard (points to the actual file data) or symbolic (symbolic path to a hard link). Both types of links can be created with the ln utility. Removing a symbolic link does not affect the file data, whereas removing the last hard link to a file will remove the data.
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Storage Organization
Info stored on the SB: size of the file system, number of free blocks,list of free blocks, index to the next free block, size of the I-node list,number of free I-nodes, list of free I-nodes, index to the next freeI-node, locks for free block and free I-node lists, and flag to indicatea modification to the SB
I-node contains: owner, type (directory, file, device), last modified time, last accessed time, last I-node modified time, access permissions, number of links to the file, size, and block pointers
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Unix File System (Cont’d)
Tree-structured file hierarchies
Mounted on existing space by using mount
No hard links between different file systems
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File Naming
Each file has a unique name
User visible (external) name must be symbolic
In a hierarchical file system, unique external names are given as pathnames (path from the root to the file)
Internal names: i-node in UNIX - an index into an array of file descriptors/headers for a volume
Directory: translation from external to internal name
May have more than one external name for a single internal name
Information about file is split between the directory and the file descriptor: name, type, size, location on disk, owner, permissions, date created, date last modified, date last access, link count
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Name Space
In UNIX, “devices are files”E.g., /dev/cdrom, /dev/tape
User process accesses devices by accessing corresponding file
/
usr A B
C D
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File System Buffer Cache
application: read/write files
OS: translate file to disk blocks
...buffer cache ...maintains
controls disk accesses: read/write blocks
hardware:
Any problems?
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File System Buffer Cache
Disks are “stable” while memory is volatileWhat happens if you buffer a write and the machine crashes before the write has been saved to disk?
Can use write-through but write performance will suffer
In UNIX
Use un-buffered I/O when writing i-nodes or pointer blocks
Use buffered I/O for other writes and force sync every 30 seconds
Will talk more about this in a few slides
What about replacement?
How can we further improve performance?
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Application-Controlled Caching
application: read/write files replacement policy
OS: translate file to disk blocks
...buffer cache ...maintains
controls disk accesses: read/write blocks
hardware:
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Application-Controlled File Caching
Two-level block replacement: responsibility is split between kernel and user level
A global allocation policy performed by the kernel which decides which process will give up a block
A block replacement policy decided by the user:Kernel provides the candidate block as a hint to the process
The process can overrule the kernel’s choice by suggesting an alternative block
The suggested block is replaced by the kernel
Examples of alternative replacement policy: most-recently used (MRU)
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Sound Kernel-User Cooperation
Oblivious processes should do no worse than under LRU
Foolish processes should not hurt other processes
Smart processes should perform better than LRU whenever possible and they should never perform worse
If kernel selects block A and user chooses B instead, the kernel swaps the position of A and B in the LRU list, and creates a “placeholder” tagged with B to point to A (kernel’s choice)
If the user process misses on B (i.e. it made a bad choice), and B is found in the placeholder, then the block pointed to by the placeholder is chosen (prevents hurting other processes)
Source: P. Cao et al. “Implementation and performance of integrated application-controlled file caching, prefetching, and disk scheduling”. ACM TOCS, 1996.
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File Sharing
Can multiple processes open the same file at the same time?
What happens if two or more processes write to the same file?
What happens if two or more processes try to create the same file at the same time?
What happens if a process deletes a file when another has it opened?
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File Sharing (Cont’d)
Several possibilities for file sharing semantics
Unix semantics: file associated with single physical imageWrites by one user are seen immediately by others who also have the file open.
One sharing mode allows file pointer to be shared.
Session semantics: file may be associated temporarily with several images at the same time
Writes by one user are not immediately seen by others who also have the file open.
Once a file is closed, the changes made to it are visible only in sessions starting later.
Immutable-file semantics: file declared as shared cannot be written.
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File System Consistency on Crashes
File system almost always uses a buffer/disk cache for performance reasons
Two copies of a disk block (buffer cache, disk) consistency problem if the system crashes before all the modified blocks are written back to disk
This problem is critical especially for the blocks that contain control information: i-node, free-list, directory blocks
Example: if the directory block (contains pointer to i-node) is written back before the i-node of new file and the system crashes, the directory structure will be inconsistent
Similar case when free list is updated before i-node and the system crashes, free list will be incorrect
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File System MetadataConsistency Problem: Examples
Example 1: create a new file
Two updates: (1) allocate a free I-node; (2) create an entry in the directory
(1) and (2) must be write-through (expensive) or (1) must be written-back before (2)
If (2) is written back first and a crash occurs before (1) is written back the directory structure is inconsistent and cannot be recovered
Example 2: write a new block to a file
Two updates: (1) allocate a free block; (2) update the address table of the I-node
(1) and (2) must be write-through or (1) must be written-back before (2)
If (2) is written back first and a crash occurs before (1) is written back the I-node structure is inconsistent and cannot be recovered
85Computer Science, Rutgers CS 519: Operating System Theory
More on File System Consistency
Utility programs for checking block and directory consistency
One approach to reduce inconsistency: Write critical blocks from the buffer cache to disk immediately. Data blocks can be written to disk periodically: sync
A more elaborate solution: use logs of metadata operations (“journaling”) to implement transactions. The idea is to write all metadata operations associated with a system call to a log. After these operations are logged, they are considered “committed” and the call can return. Meanwhile, the log is replayed across the actual file system structures. As changes are made, a pointer is updated to indicate which actions have been completed. When an entire transaction is completed, it is removed from the log. If the system crashes, it knows how to recover by looking at the log.
86Computer Science, Rutgers CS 519: Operating System Theory
Protection Mechanisms
Files are OS objects: unique names and a finite set of operations that processes can perform on them
Protection domain defines a set of {object,rights} where right is the permission to perform one of the operations
At every instant in time, each process runs in some protection domain
In Unix, a protection domain is {uid, gid}
Protection domain in Unix is switched when running a program with SETUID/SETGID set or when the process enters the kernel mode by issuing a system call
How to store info about all the protection domains?
87Computer Science, Rutgers CS 519: Operating System Theory
Protection Mechanisms (cont’d)
Access Control List (ACL): associate with each object a list of all the protection domains that may access the object and how
In Unix, an ACL defines three protection domains: owner, group and others
Capability List (C-list): associate with each process a list of objects that may be accessed along with the operations
C-list implementation issues: where/how to store them (hardware, kernel, encrypted in user space) and how to revoke them
88Computer Science, Rutgers CS 519: Operating System Theory
Protection Mechanisms (cont’d)
Most systems use a combination of access control lists and capabilities. Example: In Unix, an access list is checked when first opening a file. After that, system relies on kernel information (per-process file table) that is established during the open call. This obviates the need for further protection checks.
89Computer Science, Rutgers CS 519: Operating System Theory
Log-Structured File System (LFS)
As memory gets larger, buffer cache size increases increases the fraction of read requests that are satisfied from the buffer cache with no disk access
In the future, most disk accesses will be writes
But writes are usually done in small chunks in most file systems (control data, for instance) which makes the file system highly inefficient
LFS idea: structure the entire disk as a log
Periodically, or when required, all the pending writes being buffered in memory are collected and written as a single contiguous segment at the end of the log
Source: M. Rosenblum and J. Ousterhout. “The design and implementation of a log-structured file system”. ACM TOCS, 1992.
90Computer Science, Rutgers CS 519: Operating System Theory
LFS segment
Contain i-nodes, directory blocks and data blocks, all mixed together
Each segment starts with a segment summary
Segment size: 512KB - 1MB
Two key issues: How to retrieve information from the log?
How to manage the free space on disk?
91Computer Science, Rutgers CS 519: Operating System Theory
File Location in LFS
The i-node contains the disk addresses of the file blocks as in standard UNIX
But there is no fixed location for the i-node
An i-node map is used to maintain the current location of each i-node
i-node map blocks can also be scattered but a fixed checkpoint region on the disk identifies the location of all the i-node map blocks
Usually i-node map blocks are cached in main memory most of the time, thus disk accesses for them are rare
92Computer Science, Rutgers CS 519: Operating System Theory
Segment Cleaning in LFS
LFS disk is divided into segments that are written sequentially
Live data must be copied out of a segment before the segment can be re-written
The process of copying data out of a segment: cleaning
A separate cleaner thread moves along the log, removes old segments from the end and puts live data into memory for rewriting in the next segment
As a result, an LFS disk appears like a big circular buffer with the writer thread adding new segments to the front and the cleaner thread removing old segments from the end
Bookkeeping is not trivial: i-node must be updated when blocks are moved to the current segment