1. Jeff's Filesystem Papers Review Part I. Review of "Design and Implementation of The Second Extended Filesystem"

2. The Design and Implementation of The Second Extended Filesystem  By Remy Card, Theodore Ts'o and Stephen Tweedie  Pascal Institut, MIT, and Edinburgh  Very Linux-oriented. This presentation is an academic review, the ideas presented are either quotes or paraphrases of the reviewed document.

3. History  VFS Virtual File System  Developed to ease addition of new FS's into Linux  EFS Extended File System  increased max filesystem size and max filename size but used linked lists to keep track of inodes and no timestamping, and bad perfromance and fragemtnation  Xia Extension of old Minix FS  Ext2FS  similar funtionality to Xia, but based on EFS code.

4. Basic Concepts  Inode  File Type, access rights, owners, timestamps, size, pointers to datablocks.  Directories  hierarchical tree, can contain files and subdirs  Implemented as a special type of file  contains list of entries ƒ Each entry is inode and filename.

5. Basic Concepts - continued  Links  Multiple names associated w/ an inode.  Hardlinks only for files in same FS. ƒ Not dirs and not cross FS.  Symlinks a file that contains a filename, can be used for dirs and for cross FS files.  Device Special Files  an access point for the device driver.  Char mode/ block mode  Doc contains some specifics on how to access.

6. VFS  System calls use the VFS so you can have any type of FS underneath.  VFS has a set of funcs that every FS must implement  abstracting the interface...  Good coding maybe an example of bridge pattern if you feel like being SE450 oriented

7. ext2fs standard features  Supports std UNIX filetypes:  regular files  dirs  dev files  symlinks.  4TB limit on FS size.  Long filenames  Reserve 5% of blocks for root user to prevent procs from filling up FS (of course this doesn't work if you are doing something stupid like running a daemon as root)

8. ext2fs advanced features  File attribs on a directory level and directory inheretance for new files.  Can force metadata to be written synch to maintain consistency or can be done asynch  Users can choose logical block size to trade off between seek time and disk wastage.  Fast Symlinks  store targetfile name in inode rather than in datablock  Tradeoff, filename must be <60 chars

9. ext2fs advanced features (continued)  Clean/not clean flag in superblock  forced checks after certain number of mounts  Source deletion of files  random data overwrite of file when deleted  can be enabled or disabled  Immutable files  Append-only files (for logs)

10. ext2fs physical structure  Block Groups  a block is a contiguous chunk of disk  Superblock data (for that block) is replicated in that block group. ƒ recovery from failure can occur when superblock is corrupt ƒ reduces seek time to update control structure data for that block is proximate to actual data.

11. ext2fs directory implementation  Directory is just a file containing  Inode identifier  entry length ƒ entry length is variable to save space in directory entry  name length  filename

12. ext2fs performance optimizations  Buffer cache managment  readaheads...reads several contiguous blocks into buffer cache when one is requested.  Allocation  tries to put both inode and data into same block.  Preallocates more proximate blocks when allocating a new block.

13. ext2fs library  Routines for programs to bypass VFS and access ext2fs directly  Open and close FS  read and write bitmaps  create new FS  check bad blocks.  Create and expand directories,  add and remove dir entries  path inode resolution.  Scan Inode table read and write Inodes,  allocate and dealloc blocks, etc.

14. ext2fs tools  tune2fs  Tuning, and repair, modifies fs configuration.  e2fsck  scans disk and checks it for bad inodes  compiles new bitmaps  fixes datablocks claimed by multiple inodes  directory validity check  link count for inodes  debugfs  an interactive interface to the ext2fs library.

15. Performance  Block IO - Better than FFS,  Character IO - Worse than FFS,  Generally better than Xiafs and FFS.