1. CS140 Review Session Project 4 – File Systems Samir Selman 02/27/09cs140 Review Session Due Thursday March,11

2. General 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. 02/27/09cs140 Review Session2

3. Requirements Indexed and Extensible Files Subdirectories Buffer Cache Synchronization at a finer level 02/27/09cs140 Review Session3

4. Indexed 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. */ }; 02/27/09cs140 Review Session4

5. Indexed 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. */ }; 02/27/09cs140 Review Session5 Data Blocks Indirect Block Double Indirect Indirect Block Inode

6. Indexed 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. 02/27/09cs140 Review Session6

7. Indexed 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 EOF 02/27/09cs140 Review Session7

8. Subdirectories 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. 02/27/09cs140 Review Session8 “/”“/a/” “/a/b/” File1.txt y.c hello.c a b

9. Subdirectories (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) – … 02/27/09cs140 Review Session9

10. Buffer 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) 02/27/09cs140 Review Session10 FileInode Buffer Cache Disk

11. Buffer 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). 02/27/09cs140 Review Session11

12. Synchronization 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. 02/27/09cs140 Review Session12

13. Synchronization (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. 02/27/09cs140 Review Session13

14. Synchronization (Cont …) – Two Writers pointing to EOF and writing 10 bytes concurrently. How to Synchronize? – A Writer extending file past EOF and a reader wanting to read past EOF. How to Synchronize? – Its ok to grab a coarse lock to update couple of variables. But you should not grab a coarse lock and do I/O 02/27/09cs140 Review Session14

15. Suggested 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 steps 02/27/09cs140 Review Session15