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The ex4, or fourth extended file system, is a journailing file system for Linux which evolved from ext3. It is marked stable in Linux kernel 2.6.28.

ext4 adds many notable features particularly extents - a single descriptor for a range of contiguous blocks, as opposed to the traditional block mapping scheme used by ext2 and ext3. This represents an efficient way to represent large files, as well as utilising CPU more efficiently, and with fewer metadata IOs. This also makes for quicker file system checks, which becomes more beneficial as the file system grows in size.


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Support Sites:
Wiki, FAQ, Kernel Newbies

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Features include:

  • Extended attribute blocks and large inodes 
  • Extent-based disk format
  • Online Defragmentation
  • Online filesystem resize reservations
  • Hash-indexed directories
  • Journal file/device present
  • File type in directory entries
  • Journal recovery required
  • Files allocated with extent format
  • Support for more than 2^32 filesystem blocks
  • Flexible block group metadata location
  • Reduced block group backups
  • Reduced superblock backups
  • Files larger than 2GiB in size
  • Group descriptor checksums and sparse inode table
  • Over 32000 subdirectories
  • Inode-features
    • Larger inodes
    • Inode reservation
    • Nanosecond timestamps and creation time 
  • Files larger than 2TiB in size
  • Journal checksumming - checksums the journal data to know if the journal blocks are failing or corrupted
  • Option to turn off journaling
  • Persistent preallocation
  • Backward compatible with ext2 and ext3
  • Allocation schemes:
    • Persistent pre-allocation
    • Delayed allocation
    • Multi-block allocation
    • Stripe-aware allocation
  • Extended attributes
  • Quota journaling
  • Implements discard/TRIM

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Last Updated Monday, May 05 2014 @ 03:10 AM EDT

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