CS140 Project 4 Due Thursday March 10th Slides adapted from Samir Selman’s Kiyoshi Shikuma

Overview Things to know before you start Project Requirements: –Buffer Cache –Indexed and Extensible Files –Subdirectories –Synchronization

Things to know before you start May build on top of Project 2 OR 3 Up to 5% extra credit if built on Project 3 Open Ended Design –Many ways to achieve functionality but you should think through the design first Good News: Easier than Project 3 Bad News: Longest project in terms of lines of code Read Design Doc and FAQ before you start, will help you guide your design.

Project Requirements (In suggested order of implementation): –Buffer Cache – Caches reads from disk much like paging and virtual memory –Indexed and Extensible Files – Indexed files avoid external fragmentation, Extensible allows files to grow –Subdirectories – Hierarchical namespace along with associated syscalls –Synchronization** – Remove global filesys lock **Should think about this throughout project –Miscellaneous other things

Buffer Cache Suggested that you implement this early Similar to VM –Keep cache of file blocks in memory –Read/Write checks cache first before going to disk Cache limited to 64 disk sectors –You can keep a copy of free map in memory, doesn’t count against this limit Implement an eviction policy that approximates LRU and is at least as good as clock algorithm (may give preference to metadata) FileInode Buffer Cache Disk

Buffer Cache cont’d Write Behind –Don’t write dirty blocks out to disk immediately –Write to disk when block is evicted –Write all dirty blocks to disk periodically and in filesys_done –This is a good application for timer sleep Read Ahead –Whenever a block is read in, automatically read in the next block –Do this asynchronously (helper thread)

Project 2 (or Project 3 if you are extending) tests should still pass!

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. */ };

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 Session9 Data Blocks Indirect Block Double Indirect Indirect Block Inode

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 that 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 Session10

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 Session11

File growth tests should pass!

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 Session13 “/”“/a/” “/a/b/” File1.txt y.c hello.c a b

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) – bool isdir (int fd) – int inumber (int fd) 02/27/09cs140 Review Session14

Subdirectory tests should pass!

Synchronization NEED TO THINK ABOUT THIS DURING ALL PARTS 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 Session16

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 different directories should be concurrent. – Operations on same directory can be serialized. 02/27/09cs140 Review Session17

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 Session18

Questions?

Good Luck!