CSE380 - Operating Systems

Transcription

1 CSE380 - Operating Systems Notes for Lecture 17-11/10/05 Matt Blaze, Micah Sherr (some examples by Insup Lee)

2 Implementing File Systems We ve looked at the user view of file systems names, directory structure, attributes, operations on files We ve looked at some of the internal data structures directories, lined lists, FATs, i-nodes Now we ll look at file system algorithms, performance and tools

3 Links May need more than one name for a file maybe in a different directory Hard link - early binding pointer to i-node added to the directory entries link counter in the i-node linked to incremented i-node can only be deleted (data addresses cleared) if counter goes down to 0 (why?) then space can be reclaimed Soft link (symbolic) - late binding new special file created of type LINK, which contains just the path name new file also has its own i-node created

4 Linked files File system containing a linked file

5 Linked Files i-node (a) Prior to linking (b) After the link is created (c) After the original owner removes the file

6 Disk Space Management Some similarity between disk management and memory management keeping track of free blocks segmentation caching and paging Natural first question: how big should block size be? smaller block size leads to less waste within a block but larger block size yields better data transfer rate (block assess time is dominated by seek time and rotational delay) experiments lead to estimate of median file size at 2KB Unix systems commonly use 1KB or 4KB probably not optimal; choice was made a long time ago MS-Dos has a limit on number of blocks, making block size proportional to disk size

7 Block size Block size Dark line (left hand scale) gives data rate of a disk Dotted line (right hand scale) gives disk space efficiency In this experiment, all files are exactly 2KB

8 Managing Free Disk Space Familiar problem: keeping track of unallocated blocks Linked Lists a list of blocks containing addresses of free blocks e.g., each block address is 32 bits (4 Bytes) a 1KB block used can hold addresses of 255 free blocks and an address of the next block in the list for free space management note: list can be stored in free blocks themselves Bitmap method: an array of bits, one entry per block, indicating whether that block is free or not 16-GB disk has 16 million 1 KB blocks storing the bitmap requires 16 million bits, or 2048 blocks How do the two methods compare in terms of space requirements?

9 Free Blocks (a) linked list free list (b) bit map free list

10 File System Backup Disks aren t very reliable Backup purpose recover from (natural) disaster recover from error recover from attack Backup tools full dump incremental dump physical dump logical dump

11 Logical Dump File that has not changed A file system to be dumped squares are directories, circles are files shaded items, modified since last dump each directory & file labeled by i-node number

12 Logical Dump Bitmaps (a) all directories and modified files (b) all modified files, their parent directories and modified directories (c) all directories to be dumped (d) all files to be dumped

13 Reliability by Redundancy Mirroring (or Shadowing): duplicate disks Write writes every block to both of the disks Read can be performed from either disk this can be exploited to allow parallelism and speeding up of reads mainly for error correction: if first read fails, try the other Crash recovery: If one disk crashes, the other can restore data If failures are independent, this dramatically increases mean-time to failure Mirroring can be managed by device controller hardware (if it can handle multiple disks) or by the OS software Disadvantage: duplicate disks are expensive (twice the cost) and can be slow if not mirrored by hardware

14 RAID: Redundant Array of Independent Disks Basic idea: spread work over several physical disks RAID controller talks to OS just like a device driver and manages a suite of disks to improve reliability and performance Striping: Organize logically consecutive chunks of data across different disks to improve performance Striping can be at bit-level, byte-level, or sector-level Incoming request is split into all relevant disks by RAID controller, and their outputs are put together Redundancy through duplication: Multiple requests can be load-balanced Error recovery on faults

15 RAID Level 0 Array of disks with block-level striping Stripe = k sectors (for suitably chosen k) First k sectors are on first stripe of first disk, next k sectors are on first strip of second disk, and so on Advantages: A large request spanning multiple stripes can be parallelized easily Not much overhead Used in high-performance applications Disadvantages No redundancy Mean-time to failure is, in fact, less than standard disks (why?) No parallelism for small requests

16 RAID Level

17 RAID Level 1 Like mirroring, but with striping of level 0 Use same number of backup disks with same striping Same benefits/issues as in mirroring

18 RAID Level 2 Called memory-style error correcting code organization Uses bit-level striping Some of the bits used for parity or other error correction Sample: 4-bits of data encoded by additional 3-bits of ECC, and resulting 7 bits stored on 7 different disks CM-2 machine: 32-bits of data with 7-bits of ECC spanning over array of 39 disks Advantages Great parallelism for read as well as write Even if one disk crashes, all data can be recovered (due to ECC) Disadvantages Drives have to synchronized Overhead in putting together reads from different disks Crash recovery much slower (compared to RAID-1)

19 RAID Levels 3-5 Raid levels 3 through 5 Backup and parity drives are shaded

20 File System Consistency Disk I/O is buffered (delayed write) there may be a crash before modified blocks in memory are written back to the disk File system might become inconsistent fsck scandisk block consistency a block is either used (listed in i-node) by a file or is free (listed in free list) file consistency a file has the same number of directory entries (i.e. the same number of hard links) as the link count in its i-node

21 FS Consistency Check (a) consistent (b) missing block (c) duplicate block in free list (d) duplicate data block

22 Caching Since disk I/O is slow, a part of memory is reserved to hold disk blocks that are likely to be accessed repeatedly Basic data structure: Maintain a doubly linked list of blocks Use hashing so that a block can be quickly accessed from its address Efficient LRU implementation possible: every time a block is accessed move it to the back of the list (no special hardware needed)

23 Cache Writing Strategies If there is a crash, modified blocks in cache will be lost especially bad for blocks containing i-nodes or FAT and LRU guarantees loss of the exactly the worst blocks! MS-DOS: write-through cache every time a block is modified, a request to write it to disk is issued loop with putchar() generates busy disk I/O reads can still be fast Classify blocks and treat them separately (Unix) i-node blocks indirect blocks with addresses of file blocks free space management blocks data blocks of files Critical blocks (first 3 kinds) written upon modification data blocks just use standard LRU

24 Case Study: The CD-ROM ISO 9660 adopted in 1988 block size 2048 begins with 16 blocks whose functions are undefined can be used as boot blocks primary volume descriptor root and directories Directory filename 8+3 depth of nesting limited to 8

25 CD-ROM Directory Entry

26 And another: MS-DOS Originally intended as a method for storing data on floppy disks, first version has single directory Later used by MS-DOS with supports for a hierarchical directory structure, and fixed disks as well as floppy disks directory entries are fixed 32 bytes different versions support FAT-12, FAT-16 or FAT-32

27 Block Sizes and FAT Sizes Disk Size FAT-12 FAT-16 FAT-32 FAT entry Max # of blocks Block size 12 bits 4086 ~ KB to 4KB 16 bits ~2 16 2KB to 32KB 28 bits ~2 28 4KB to 32KB FAT size 8KB 128KB 1GB

28 Block and Max Partition Sizes Internally sector sizes are 512 bytes Empty boxes represent invalid combinations

29 Windows 95 and 98 Long file names supported by Win95 V2 and Win98 In order to be backwards compatible to MS-DOS, each file has two names - an 8 character MS DOS name plus a long one Long names in Windows98

30 The Classic Unix File System Each disk partition has 4 Regions block 0: boot block block 1: super block. Contains the size of the disk and the boundaries of the other regions i-nodes: list of i-nodes, each is a 64-byte structure free storage blocks for the contents of files. Each i-node contains owner, protection bits, size, directory/file and 13 disk addresses. The first 10 of these addresses point directly at the first 10 blocks of a file. If a file is larger than 10 blocks (5,120 bytes), the 11th points at a block for secondary indexing (single indirect block)

31 Classic Unix File System (Cont) Second-level index block contains the addresses of the next 128 blocks of the file (70,656 bytes) Two levels of indirection:12th entry (double indirect block) points at up to 128 blocks, each pointing to 128 blocks of the file (8,459,264 bytes) Three levels of indirection: 13th address (triple indirect block) is for a three layered indexing of 1,082,201,087 bytes. A directory is accessed exactly as an ordinary file. It contains 16 byte entries consisting of a 14-byte name and an i-number (index or ID of an i-node). The root of the file system hierarchy is at a known i-number (2).

32 Unix i-node

33 Directory Entry: Unix V7 Each entry has file name and an I-node number I-node contains all the attributes Restriction: File name size is bounded (14 chars); newer versions use a heap to allow longer namers

34 Traversing Directory Structure Suppose we want to read the file /usr/ast/mbox Location of a i-node, given i-number, can be computed easily i-number of the root directory known (usually 2) Read in i-node 2 from the disk into memory Find out the location of the root directory file on disk, and read the directory block in memory If directory spans multiple blocks, then read blocks until usr found Find out the i-number of directory usr, which is 6 Read in i-node 6 from disk Find out the location of the directory file usr on disk, and read in the block containing this directory Find out the i-number of directory ast (26), and repeat

35 UNIX V7 Directory Lookup The steps in looking up /usr/ast/mbox