Basic filesystem concepts. Tuesday, November 22, 2011

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1 Physical Page 1 Basic filesystem concepts Tuesday, November 22, :04 PM Review of basic filesystem concepts A filesystem is a structure on disk that Allows one to store and retrieve files. Keeps track of the names of the files. Keeps track of file attributes such as protection and owner. A linux filesystem consists of three kinds of entities: blocks: contain the actual file contents. inodes: contain information on where the file is. super-blocks: describe where inodes and blocks are located on disk

2 Physical Page 2 What's important to filesystems Tuesday, November 20, :30 PM What's important Filesystems are based upon the existence of the paging subsystem. Heavily use the principle of locality: If we're messing around with one file or region of disk, we're likely to continue. Utilize very efficient binary flag structures. (An array is any function f that locates an object f(n) in memory (n = integer) in O(1).) This is all about representing sets and sequences. A set of items has no order. A sequence of items has an order. "a file is a sequence of blocks"

3 Physical Page 3 What is an inode? Tuesday, November 22, :06 PM What is an inode? "Identity node". Contains all attributes of a file or directory: owner, group, protection, where its blocks are located. Does not contain the name. Found by number, not name.

4 Physical Page 4 What is a block? Tuesday, November 22, :08 PM What is a block? Just a big bag of bits. Typically, 8192 bytes, but this can vary.

5 Physical Page 5 Inode/block striping Tuesday, November 22, :12 PM Typically, inodes and blocks are striped into alternating stripes on the disk:

6 Physical Page 6 What is a file? Tuesday, November 22, :10 PM What is a file? A pair: <inode, sequence of blocks> Inode identified as a number. inode is an offset into a descriptor table; descriptor tells where the blocks are and how to find them. Better to say: "array" of blocks. O(1) random access.

7 Physical Page 7 What is a directory? Tuesday, November 22, :10 PM What is a directory? A special kind of file. Contains a mapping from names to inodes. Covers one part of a file path.

8 Physical Page 8 What is a super-block? Tuesday, November 22, :09 PM What is a super-block? A descriptor with pointers to All groups of inodes All block groups What directory is root ("/") Duplicated all over the disk. If it is lost, disk data becomes meaningless!

9 Logical structure of a linux filesystem Thursday, December 03, :11 PM Physical Page 9

10 Physical Page 10 Some really difficult things to understand Tuesday, November 22, :49 PM Some really difficult things to understand The identity of a file is a number. Its name is constructed as a sequence of directories. One file can have multiple names, but only one number.

11 Physical Page 11 Constructing a name from a number Tuesday, November 22, :50 PM Constructing a filename from a number: Start at file. Find the directory it's in. Go up to its parent using '..'. Find the entry of the parent directory that points to this one. That name is a component of the filename. Repeat until the number for. == the number for.. This means you are at the root.

12 Example: constructing a name from a number Tuesday, November 22, :55 PM Physical Page 12

13 Some very counter-intuitive facts Tuesday, November 22, :55 PM Some very counter-intuitive facts One of the most expensive system calls is getcwd(), which returns the current working directory for a process. Why? It stores that directory in the PCB as a number. If a filesystem has a loop, then doing a getcwd inside the loop will crash the OS. Why? Preventing an infinite loop would make getcwd even slower than it already is! For this reason, hard links to directories may only be created by root. Physical Page 13

14 Physical Page 14 So far Thursday, December 02, :33 PM So far, Our naïve models of storage have been based upon "free" and "used" lists. Logically, they're sets, in the sense that they're unordered. In a realistic filesystem, there might be "free" or "used" arrays. The reason that this is reasonable is the principle of locality: that, on average, processes reference stuff in a local region. So, the used array can be cached, and tends to stay in cache. A realistic file system takes advantage of both locality and the fact that there is a block cache!

15 Logical superstructure Thursday, December 03, :21 PM Physical Page 15

16 From logical to physical Thursday, December 03, :29 PM From logical to physical So far, we are using "sequence" in a loose way. A file is a "sequence of blocks". The physical way that sequence is defined depends upon the recording medium and a number of design tradeoffs. Rock and hard place Rock: need to be space efficient. Hard place: need to be able to get to any file block in O(1) Physical Page 16

17 Physical Page 17 Magnetic disks Monday, November 23, :51 PM Unlimited writes -- no degradation. Minimum time before failure (MBTF) does not depend upon # of writes. Can write or erase a block at a time.

18 What is a sequence? Thursday, December 03, :34 PM Design question 1: what is a sequence of blocks? Physical Page 18

19 Design question 2: how to keep track of unused blocks? Thursday, December 03, :39 PM Design question 2: how to keep track of unused blocks. Physical Page 19

20 Case study: ext2fs Thursday, December 03, :44 PM Case study: ext2 Primitive linux filesystem. No fancy features (until ext3!). Desires: robustness, simplicity. Medium properties: unlimited writes: a write does not degrade the medium (it's degraded by time spent running). Physical Page 20

21 Ext2 design decisions Thursday, December 03, :46 PM Ext2 design decisions Free and used blocks and inodes tracked via arrays of T/F bits Sequences of blocks represented via array indirection that uses whole blocks as arrays of pointers to blocks. Physical Page 21

22 Ext2 inode Thursday, December 03, :48 PM Ext2 inode contains: owner, group, protections access count of references from directories 12 block pointers for first twelve blocks of file. 1 pointer to a block used for single-level indirection 1 pointer for double-level indirection 1 pointer for triple-level indirection These are not memory pointers like you're used to. They are offsets into the block descriptor arrays. Each one is an integer defining a block on disk. Physical Page 22

23 Picture of an ext2 inode Thursday, December 03, :50 PM Picture of an ext2 inode: Physical Page 23

24 Physical Page 24 Why indirection? Thursday, December 03, :08 PM Why indirection? Statistically, most files are small! The inode contains enough information for files up to 1024*12 bytes. After this, we allocate a block that can represent indirection to 256*1024 more bytes.... Reason it's O(1): You remember the boundaries first 1024*12 bytes: in inode next 256*1024 bytes: in single indirection next 256*256*1024 bytes: in double indirection. Algorithm for indirection: determine which indirection kind, subtract off size of previous block divide by block size. Suppose you want byte number 50,000 In first indirection block Base of indirection block is 50,000-12*1024 Offset of block = (50,000-12*1024)/1024 Offset of byte in block (50,000-12*1024)% 1024

25 Physical Page 25 Ext2 block handling Thursday, December 03, :59 PM Typical ext2 block sizes (early version) 1024-byte blocks 64-byte inodes, 1024/64 = 2 10 /2 6 =2 4 = 16 inodes per inode block 4-byte block pointers, 1024/4 = 256 block pointers per indirection block

26 Big point: Tuesday, November 20, :15 PM Big point: Pager is completely unaware of the nature of pages. But the pager is absolutely essential to the timely function of this algorithm, because all disk objects become -- for a time -- memory objects! Physical Page 26

27 The super-block Thursday, December 03, :14 PM The super-block contains "magic" code for kind of filesystem size of a block number of free and total data blocks number of free and total inodes location of inode for root of filesystem locations of all "block groups" It's on the disk. It's accessed all the time. It's always cached! Physical Page 27

28 Block groups Thursday, December 03, :10 PM Block groups In ext2, the disk is partitioned into block groups, each with its own inode table its own data blocks a copy of the super-block bitmaps determining used inodes and used data blocks. Physical Page 28

29 Physical Page 29 Three things in concert Thursday, December 02, :50 PM a) b) c) reuse freed blocks as fast as possible => same block of the used bitarray will be used for the previous free and next allocation. => the bit allocation array will stay in the page cache!

30 A really slimy trick Monday, November 23, :09 PM Keep track of the last block allocated. To allocate another, look forward in the bit array for a 0. On average, this is in the same cache page! it's a memory operation! Physical Page 30

31 Array access Thursday, December 03, :18 PM Array access Reason that filesystem is organized this way: quick array indirection. Determining whether block i is used: check bit i%32 of block bitarray element i/32, assuming 32-bit words in the bitarray. inode checking works likewise. Formula: (word[bit/32] & (1<<(bit%32)))!=0 Physical Page 31

32 Array access continued Thursday, December 03, :21 PM Array access continued How to find block b of a file: Suppose inode block pointers start at p. if b<12, block number is p[b] if b<268, block number is p[12][b-12] (treating block as array) Similar pattern for multiple-indirection: for two levels: p[13][(b-268)/32][(b-268)%32] Same issue for block groups. If we want block n on a disk and there are m blocks in a group n/m is the group number (starting at 0) and n%m is the block in the group. Physical Page 32

33 Crazy like a fox! Thursday, December 03, :29 PM Crazy like a fox! Q: Why do I have no worries about using a bit array for determining used blocks and inodes? A: because block paging is still in effect! Disk version of principle of locality: If data we need is localized in a small number of blocks, then on average, that data will remain paged in, and we'll be interacting with memory rather than disk! Examples: Superblock is always paged in, always written to. If I sequentially access a file, its block descriptors are read in one at a time with a minimum number of reads. Physical Page 33

34 Basic principles of filesystem design Thursday, December 03, :27 PM Why were these choices made? use bit-arrays for determining used blocks and inodes because on average, these bit arrays will remain in memory! use indirection for finding blocks in large files because on average, files are small! Physical Page 34

35 Physical Page 35 The cosmic fact Monday, November 18, :00 PM This all works because two subsystems obey contractual obligations to one another with no detailed knowledge of the function of the other subsystem. There is no explicit interface that does this; the contractual obligations suffice!

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