Jeff's Filesystem Papers Review Part I. Review of "Design and Implementation of The Second Extended Filesystem"
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.
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.
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.
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.
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
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)
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
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)
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.
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
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.
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.
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.
Performance Block IO - Better than FFS, Character IO - Worse than FFS, Generally better than Xiafs and FFS.