COMP3891

EXT2 File System

Overview

Features:

• Block size (1024, 2048, and 4096) configured at FS creation.
• inode-based FS.
• Performance optimisations to improve locality.

Main problem:

• Unclean unmount requires e2fsck.

Structure of Inodes

|Field name|Description| |-|-| |mode|Type: regular file or directory. Access mode: read/write etc…| |uid|User id.| |gid|Group id.| |mtime|Last modified time.| |size|Offset of the highest byte written.| |block_count|Number of disk blocks used by the file. Note: can be much less expected given the file size if the file is sparsely populated.| |reference_count|Needed because files with different names may be linked to the same inode.| |direct_blocks|Block numbers of first 12 blocks in the file.| |single_indirect_blocks|Block number of a block containing block numbers| |double_indirect_blocks|Block number of a block containing block numbers of blocks containing block numbers| |triple_indirect_blocks|As above, but another layer.|

Where/How are Inodes Stored

The hard disk is partitioned into block groups.

Layout of Block Group

|Region|Size|Description| |-|-|-| |Super Block|1 block|Contains information about the overall file system. Contains overall free inode and block counters. Contains replicated information about the file system health. (Replicated for redundancy)| |Group Descriptors|n blocks|Location of the bitmaps. Counter for free blocks and inodes in the group. Number of directories in the group.| |Data Block Bitmap|1 block|| |Inode Bitmap|1 block|| |Inode Table|m blocks|| |Data Blocks|k blocks|Proximity of inode table and data blocks reduces seek time.|

Directory Implementation

Directories are just special files.

• Directories translate names to inode numbers.
• Directory entires are of variable length.
• Entries can be deleted in place.

Performance Considerations

• Block groups cluster related inodes and data blocks.
• Up to 8 blocks are preallocated on writes.
  • Aim for better contiguity when there are concurrent writes.
• Aim to store files within a directory in the same group.

Reliability Problems

Disk writes are buffered in RAM.

• OS crash => lost data.
• Commit writes to disk periodically.
• Use sync command to force a FS flush.

FS operations are non-atomic.

• Incomplete transaction can leave the FS in an incomplete state.