The EXT2 Filesystem Oracle Tips by Mike Ault

The, Ext2fs is based on the Extfs code with many reorganizations and many improvements. It has been designed with evolution in mind and contains space for future improvements. The Second Extended File System has been designed and implemented to fix some problems present in the first Extended File System.

·         The Ext2fs supports standard Unix file types: regular files, directories, device special files and symbolic links.

·         Ext2fs is able to manage filesystems created on really big partitions. While the original kernel code restricted the maximal filesystem size to 2 GB, recent work in the VFS layer have raised this limit to 4 TB. Thus, it is now possible to use big disks without the need of creating many partitions.

·         Ext2fs provides long file names. It uses variable length directory entries. The maximal file name size is 255 characters. This limit could be extended to 1012 if needed.

·         Ext2fs reserves some blocks for the super user (root). Normally, 5% of the blocks are reserved. This allows the administrator to recover easily from situations where user processes fill up filesystems.

”Advanced” Ext2fs features

In addition to the standard Unix features, Ext2fs supports some extensions which are not usually present in Unix filesystems.

·         File attributes allow the users to modify the kernel behavior when acting on a set of files. One can set attributes on a file or on a directory. In the later case, new files created in the directory inherit these attributes.

·         BSD or System V Release 4 semantics can be selected at mount time.

·         BSD-like synchronous updates can be used in Ext2fs.

·         Ext2fs allows the administrator to choose the logical block size when creating the filesystem. Block sizes can typically be 1024, 2048 and 4096 bytes. Using big block sizes can speed up I/O since fewer I/O requests, and thus fewer disk head seeks, need to be done to access a file.

·         Ext2fs implements fast symbolic links.

·         Ext2fs keeps track of the filesystem state.

·         Always skipping filesystem checks may sometimes be dangerous, so Ext2fs provides two ways to force checks at regular intervals. A mount counter is maintained in the superblock. Each time the filesystem is mounted in read/write mode, this counter is incremented. When it reaches a maximal value (also recorded in the superblock), the filesystem checker forces the check even if the filesystem is ``Clean''.

·         Mount options can also be used to change the kernel error behavior.

·         An attribute allows the users to request secure deletion on files. When such a file is deleted, random data is written in the disk blocks previously allocated to the file. This prevents malicious people from gaining access to the previous content of the file by using a disk editor.

·         Last, new types of files inspired from the 4.4 BSD filesystem have recently been added to Ext2fs. Immutable files can only be read: nobody can write or delete them. This can be used to protect sensitive configuration files. Append-only files can be opened in write mode but data is always appended at the end of the file. Like immutable files, they cannot be deleted or renamed. This is especially useful for log files which can only grow.

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