File Systems Part 2. Operating Systems In Depth XV 1 Copyright 2018 Thomas W. Doeppner. All rights reserved.

Transcription

1 File Systems Part 2 Operating Systems In Depth XV 1 Copyright 2018 Thomas W. Doeppner. All rights reserved.

2 Extents runlist length offset length offset length offset length offset Operating Systems In Depth XV 2 Copyright 2018 Thomas W. Doeppner. All rights reserved.

3 Problems with Extents Could result in highly fragmented disk space lots of small areas of free space solution: use a defragmenter Random access linear search through a long list of extents solution: multiple levels Operating Systems In Depth XV 3 Copyright 2018 Thomas W. Doeppner. All rights reserved.

4 Extents in NTFS Top-level run list length offset length offset length offset length offset Run list length offset length offset length offset length offset Operating Systems In Depth XV 4 Copyright 2018 Thomas W. Doeppner. All rights reserved.

5 Good File Layout file8 file6 file7 file4 file5 file1 file2 file3 Operating Systems In Depth XV 5 Copyright 2018 Thomas W. Doeppner. All rights reserved.

6 Quiz 1 So far we've been attempting to optimize logical locality in file systems: bytes that are logically near one another within a file are physically near one another on disk. a) This is of marginal importance on singleuser systems b) This is important on both single-user systems and busy (multi-client) file servers c) This is important on single-user systems, but of marginal importance on busy (multiclient) file servers Operating Systems In Depth XV 6 Copyright 2018 Thomas W. Doeppner. All rights reserved.

7 A Different Approach We have lots of primary memory enough to cache all commonly used files Read time from disk doesn t matter Time for writes does matter Operating Systems In Depth XV 7 Copyright 2018 Thomas W. Doeppner. All rights reserved.

8 File Servers Servicing many unrelated clients successive requests to file system come from different clients how can we optimize the use of the disk? Operating Systems In Depth XV 8 Copyright 2018 Thomas W. Doeppner. All rights reserved.

9 Log-Structured File Systems file1 file3 file2 Operating Systems In Depth XV 9 Copyright 2018 Thomas W. Doeppner. All rights reserved.

10 Example We create two single-block files FFS dir1/file1 dir2/file2 allocate and initialize inode for file1 and write it to disk update dir1 to refer to it (and update dir1 inode) write data to file1 - allocate disk block - fill it with data and write to disk - update inode six writes, plus six more for the other file - seek and rotational delays Operating Systems In Depth XV 10 Copyright 2018 Thomas W. Doeppner. All rights reserved.

11 FFS Picture file2 inode dir2 inode file2 data dir2 data dir1 inode file1 data file1 inode dir1 data Operating Systems In Depth XV 11 Copyright 2018 Thomas W. Doeppner. All rights reserved.

12 Example (Continued) Sprite (a log-structured file system) one single, long write does everything Operating Systems In Depth XV 12 Copyright 2018 Thomas W. Doeppner. All rights reserved.

13 Sprite Picture file1 data file1 inode dir1 data dir1 inode file2 data file2 inode dir2 data dir2 inode inode map Operating Systems In Depth XV 13 Copyright 2018 Thomas W. Doeppner. All rights reserved.

14 Some Details Inode map cached in primary memory indexed by inode number points to inode on disk written out to disk in pieces as updated checkpoint file contains locations of pieces - written to disk occasionally - two copies: current and previous Commonly/recently used inodes and other disk blocks cached in primary memory Operating Systems In Depth XV 14 Copyright 2018 Thomas W. Doeppner. All rights reserved.

15 More Details What happens when end of log reaches end of disk? before: threading: end of log end of log compacting: end of log Operating Systems In Depth XV 15 Copyright 2018 Thomas W. Doeppner. All rights reserved.

16 Better Approach Compact into largish segments before: threading with compacting: end of log end of log Operating Systems In Depth XV 16 Copyright 2018 Thomas W. Doeppner. All rights reserved.

17 LFS Sprite was a proof of concept developed by Mendel Rosenblum and John Ousterhout LFS (log-structured file system) extended Sprite developed by Margo Seltzer, Keith Bostic, Kirk McKusick, and Carl Staelin added extents better integrated into Unix Operating Systems In Depth XV 17 Copyright 2018 Thomas W. Doeppner. All rights reserved.

18 S5FS Layouts Data Region I-list Superblock Boot block Operating Systems In Depth XV 18 Copyright 2018 Thomas W. Doeppner. All rights reserved.

19 FFS Layout cg n-1 cg i data inodes cg block super block data data cg 1 cg 0 cg summary inodes cg block super block boot block Operating Systems In Depth XV 19 Copyright 2018 Thomas W. Doeppner. All rights reserved.

20 NTFS Master File Table MFT MFT Mirror Log Volume Info Attribute Definitions Root Directory Free-Space Bitmap Boot File Bad-Cluster File Quota Info Expansion entries User File 0 User File 1... Operating Systems In Depth XV 20 Copyright 2018 Thomas W. Doeppner. All rights reserved.

21 Quiz 2 Your C drive is nearing capacity. So you buy a new, larger drive to replace it. You copy your old C drive to the new one. What else has to be done to take advantage of the larger disk? a) Nothing b) Modify the free-space bitmap c) Modify the free-space bitmap and adjust the extent lists for all files d) All of the above, plus more Operating Systems In Depth XV 21 Copyright 2018 Thomas W. Doeppner. All rights reserved.

22 The Buffer Cache Buffer User Process Buffer Cache Operating Systems In Depth XV 22 Copyright 2018 Thomas W. Doeppner. All rights reserved.

23 Multi-Buffered I/O Process read( ) i-1 i i+1 previous block current block probable next block Operating Systems In Depth XV 23 Copyright 2018 Thomas W. Doeppner. All rights reserved.

24 Maintaining the Cache buffer requests Aged probably free buffers oldest returns of no-longeractive buffers LRU probably active buffers youngest returns of active buffers Operating Systems In Depth XV 24 Copyright 2018 Thomas W. Doeppner. All rights reserved.

25 In the Event of a Crash Most recent updates did not make it to disk is this a big problem? - equivalent to crash happening slightly earlier but you may have received (and believed) a message: file successfully updated homework successfully handed in stock successfully purchased - there s worse Operating Systems In Depth XV 25 Copyright 2018 Thomas W. Doeppner. All rights reserved.

26 File-System Consistency (1) New Node New Node Operating Systems In Depth XV 26 Copyright 2018 Thomas W. Doeppner. All rights reserved.

27 File-System Consistency (2) CRASH!!! New Node New Node Not on disk Not on disk Operating Systems In Depth XV 27 Copyright 2018 Thomas W. Doeppner. All rights reserved.

28 How to Cope Don t crash Perform multi-step disk updates in an order such that disk is always consistent the consistency-preserving approach Perform multi-step disk updates as transactions implemented so that either all steps take effect or none do Operating Systems In Depth XV 28 Copyright 2018 Thomas W. Doeppner. All rights reserved.

29 Maintaining Consistency 2) Then write this asynchronously via the cache 1) Write this synchronously to disk New Node Operating Systems In Depth XV 29 Copyright 2018 Thomas W. Doeppner. All rights reserved.

30 Soft Updates An implementation of the consistencypreserving approach should be simple: - update cache in an order that maintains consistency - write cache contents to disk in same order in which cache was updated isn t - (assuming speed is important) Operating Systems In Depth XV 30 Copyright 2018 Thomas W. Doeppner. All rights reserved.

31 Which Order? directory inode file inode data block containing directory entries Operating Systems In Depth XV 31 Copyright 2018 Thomas W. Doeppner. All rights reserved.

32 However directory inode file inode data block containing directory entries Operating Systems In Depth XV 32 Copyright 2018 Thomas W. Doeppner. All rights reserved.

33 Soft Updates directory old inodedirectory inode file inode data block containing directory entries This is written to disk Operating Systems In Depth XV 33 Copyright 2018 Thomas W. Doeppner. All rights reserved.

34 Soft Updates in Practice Implemented for FFS in 1994 Used in FreeBSD s FFS improves performance (over FFS with synchronous writes) disk updates may be many seconds behind cache updates Operating Systems In Depth XV 34 Copyright 2018 Thomas W. Doeppner. All rights reserved.