Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14CS140 Review SessionProject 4 – File SystemsEric Stratmann02/27/09 cs140 Review SessionGeneral Points•May build project 4 on top of Project 2 or Project 3•Up to 5% Extra Credit if built on top of Project 3•Good News: “Easier than Project 3”•Not so good news: Probably the biggest assignment in terms of code lines.•Read the Design Document before starting. It will help you design your project better.•Open ended design–Like project 3 you have to figure out the design to get the required functionality.02/27/09 cs140 Review Session 2Requirements•Indexed and Extensible Files•Subdirectories•Buffer Cache•Synchronization at a finer level02/27/09 cs140 Review Session 3Indexed and Extensible Files•Current file system is an extent based file system.•The size of file is specified at file creation time.•Continuous sectors allocated on disk for the file.•It suffers from external fragmentation. struct inode_disk { disk_sector_t start; /* First data sector. */ off_t length; /* File size in bytes. */ unsigned magic; /* Magic number. */ uint32_t unused[125]; /* Not used. */ };02/27/09 cs140 Review Session 4Indexed and Extensible Files (cont..)•Modify the on-disk inode structure, to remove external fragmentation. •Generally some indexed structure of direct, indirect and double indirect blocks is used.struct inode_disk { disk_sector_t start; /* First data sector. */ off_t length; /* File size in bytes. */ unsigned magic; /* Magic number. */ uint32_t unused[125]; /* Not used. */ };02/27/09 cs140 Review Session 5DataBlocks IndirectBlock IndirectBlock Double Indirect IndirectBlockInodeIndexed and Extensible Files (cont..)•Size of the on disk inode structure exactly equal to DISK_SECTOR_SIZE.•Size of each block is 512B.–Each block can store 512B/4B = 128 block addresses.•Assume that disk will not be larger than 8MB(minus metadata).•Must support files are large as the disk.•Don’t keep any upper limit on the number of files which can reside in the FS.02/27/09 cs140 Review Session 6Indexed and Extensible Files(cont..)•Implement File Growth.•Writing past the EOF should be possible and should extend the file to that position.–Might need to get additional data blocks and update the inode data structure.•A read from a position past the EOF should return zero bytes.•Handle race between a reader reading past EOF and the writer writing past EOF02/27/09 cs140 Review Session 7Subdirectories•In current FS, all files live in a single directory.•Directories are files, but have a different layout.–Consist of an array of directory entries. •Ensure directories can expand just like any other file.•Extend the directory structure in directory.c so that directory entries can be both files and other directories.02/27/09 cs140 Review Session 8“/” “/a/”“/a/b/”File1.txty.chello.cabSubdirectories (cont…)•Update existing system calls, to allow relative or absolute path wherever a file name is required.–Each process should have a separate cwd.–May use strtok_r() to parse path names•Implement new system calls–bool chdir (const char *dir) –bool mkdir (const char *dir) –bool readdir (int fd, char *name)–…02/27/09 cs140 Review Session 9Buffer Cache•Integrate buffer cache early into your design.•Similar to VM concept–Keep a cache of file blocks in main memory.–Read/Write call must first check the cache and then go to disk if not present in cache.•Cache is limited to 64 sectors in size.•Implement a cache replacement policy as good as “clock” algorithm ( may give preference to metadata).•Allowed to keep a copy of the free map in memory (doesn’t count against cache usage)02/27/09 cs140 Review Session 10File InodeBuffer CacheDiskBuffer Cache (cont…)•Write-behind policy: –Don’t write the dirty block immediately to disk. –Flush the dirty blocks when they are evicted.–Flush the entire cache at a periodical interval and also in filesys_done().–Can use timer_sleep from Project 1 for periodic flushing.•Read ahead policy:–When one block is read, automatically read the next block from disk.–Should be done asynchronously (can use a helper thread).02/27/09 cs140 Review Session 11Synchronization•Possibly the trickiest part of the assignment.•Remove the single file system lock you are currently using.–Multiple readers can read the same file simultaneously.–Multiple writers can write to same file simultaneously.•Their data may be interleaved.02/27/09 cs140 Review Session 12Synchronization (Cont …)–Extending a file should be atomic. If A and B trying to write past the EOF, only one should be allowed. –Operations on difference directories should be concurrent. –Operations on same directory can be serialized.02/27/09 cs140 Review Session 13Suggested Order of Implementation•Add buffer cache to existing file system–All tests from projects 2 and 3 should still pass•Implement extensible files– After this, you should pass file growth tests•Implement subdirectories–After this, you should pass subdirectory tests•Everything else* Remember to think about synchronization during all 4 steps02/27/09 cs140 Review Session
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