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22. File System Recovery & Defag

05/12/22

File System Consistency​

Checking Consistency​

  • Journaling heavily reduces the probability of having inconsistencies in a file system. The log stores what operations were not run.
  • However, it can still be possible to get some inconsistencies (data blocks weren't flushed to the drive)
  • Can be problematic, especially for structural blocks such as i-nodes, directories and free lists
  • System utilities are available to restore file systems
  • Two main consistency checks; blocks and directory

Block Consistency​

  • Checks whether blocks are assigned/used the correct way
  • Block consistency is checked by building two tables:
    1. How often a block is present in a file (based on the i-nodes)
    2. Counts how often a block is present in the free list
  • Consistent file system has a 1 in either of the tables for each block
  • Typically, this is a very slow process, can take hours

Restore Block Consistency​

  • A missing block - does not exist in any of the tables -> add it to the free list
  • A block is double counted in the free list ("disaster" waiting to happen) -> re-build the free list
  • A block is present in two or more files
    • Removing one file results in the adding the block to the free list
    • Remove both files will result in a double entry in the free list
    • Solution: use new free block and copy the content (file is still likely to be damaged 😞)
FSCK Algorith:
1. Iterate through all the i-nodes
    - retrieve the blocks
    - increment the counters
2. Iterate through the free list
    - incremenet counters for free blocks

Restore I-node consistency​

  • Checking the directory system
  • I-node counter is higher than the number of directories containing the file
    • Removing the file will reduce the i-node counter by 1
    • Since the counter will remain larger than 1, the i-node/disk space will not be released to future use
  • I-node counter is less than the number of directories containing the file
    • Removing the file will (eventually) set the i-node counter to 0 whilst the file is still referenced
    • The file/i-node will be released, even though the file was still in use
- Recurse through the directory hierarchy
    - Check file specific counters
    - I.e. each file is associated with one counter
- One file may appear in multiple directories
    - Compare the file counters and i-node counters
    - Correct if necessary

File System Defragmentation​

  • At the start all free disk space is in a single contiguous unit
  • Creating and removing files, a disk may end up badly fragmented
  • Defrag utilities make file blocks contiguous (very slow operation), and free space in one or more large contiguous regions on the disk
  • Defragmentating SSD is counter-productive

Linux File System​

  • Minix File System - Max: 64MB, file names limited to 14 char
  • Extended File System (extfs) - file names were 255 char, max file size 2GB
  • ext2 - larger files, names, better performance
  • ext3-4 - journaling

Directory Entries​

  • Superblock contains file system information
  • Group descriptor contains bitmap locations, number of free blocks, i-nodes and directories
  • Data block bitmap and i-node bitmap, used to keep track of free disk blocks and i-nodes
  • Table of i-nodes containing file and disk block information
  • Data blocks containing file and directory blocks

EXT2​

  • Partition is split into several block groups
    • Reduce fragmentation by storing i-nodes and files, and parent directories and files in the same block group is possible
    • Reduce seek time and improve performance
  • All groups have the same size and are stored sequentially