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ANATOMY OF LINUX JOURNALING FILE SYSTEMS

ANATOMY OF LINUX JOURNALING FILE SYSTEMS. M. Tim Jones Emulex. Overview. Paper surveys past and current Linux JFS Presents three modes of operation Writeback mode Ordered mode Data mode Discusses tail packing. History. IBM's JFS First released in 1990 Updated since then (JSF2)

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ANATOMY OF LINUX JOURNALING FILE SYSTEMS

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  1. ANATOMY OF LINUX JOURNALING FILE SYSTEMS M. Tim JonesEmulex

  2. Overview • Paper surveys past and current Linux JFS • Presents three modes of operation • Writeback mode • Ordered mode • Data mode • Discusses tail packing

  3. History • IBM's JFS • First released in 1990 • Updated since then (JSF2) • Silicon Graphics' XFS • Released in 1994 • Ported into Linux in 1994

  4. History • Smart FS • Developed for the Amiga • Supported by Linux until 2005 • ext3fs • Most commonly used • Extension of ext2 with journaling • Supported by Linux since 2001

  5. History • Reiser File System • Introduced many new features • Author now serves 15 years to life sentence for second-degree homicide • Killed his estranged wife • Plea-bargained his first-degree homicide conviction

  6. Variation on Journaling • Writeback mode: • Only journals metadata • Makes no guarantee that data updates will be written to disk before associated metadata are marked as committed • Ordered mode: • Makes that guarantee • Data mode: • Journals metadata and data updates

  7. Writeback mode issues • Metadata can be marked as committed before the data they point to are written to disk • File system can be corrupted if the system crashes • After some metadata are marked as committed • Before the data they point to are written to disk

  8. Writeback mode issues Committed i-node Due to a crash, block B was never written to disk Block B

  9. Data mode issues • Most reliable • Slowest: • All data must be written twice

  10. JFS2 • Supports • Ordered journaling • Extent-based allocation: • Allocates contiguous sets of blocks • Better read and write performance • Metadata are only updated for the extent

  11. JFS2 • Uses B+ trees for • Fast directory lookups • Managing extent descriptors • Has no internal journal commit policy • Relies on timeouts of kupdate daemon • Daemon that periodically writes modified buffers to disk • Typically every five seconds

  12. XFS • Supports full 64-bit addressing • Uses B+ trees for both directories and file allocation • Uses extent-based allocation with variable block size support (512 B to 64 KB) • Uses delayed allocationfor extents • Extent is not allocated until blocks are ready to be written on disk

  13. Extent-based allocation • When a process creates a file, the file system allocates a set of contiguous physical blocks to the file • Improves access times for large files • Reduces file fragmentation • Large files can occupy multiple extents • ext4 extents can go up to 128 MB with a 4 KB block size

  14. Ext3fs • Compatible with non-journaling ext2 FS • Supports • Writeback • Ordered • Journal data journaling modes • Does not support extents • Not as fast as JFS, XFS and Reiser FS

  15. A parenthesis: Ext2 • Essentially analogous to the UNIX fast file system we have discussed • Fifteen block addresses per i-node • Cylinder groups are called block groups • Major differences include • Larger maximum file size: 16 GB to 2 TB • Various extensions • Online compression, full ACLs, …

  16. ReiserFS • Introduced in 2001 • Now dead • Default mode is ordered • Includes tail packing • Uses empty space at the end of large files • Reduces internal fragmentation

  17. Tail packing • Also known as tail merging • Tail here refers to the last block of a file • Rarely full • Tail packing stores in the same block • Tails of several files • Very small files • Reduces internal fragmentation • Adds complexity

  18. File A File B File C Without tail packing Too much wasted space

  19. File A File B File C With tail packing Now occupies a single block Shares last block of file A

  20. Reiser 4 • Was designed from scratch • Was to use • Wandering logs • Delayed allocation of extents • As in XFS

  21. Ext4fs (I) • Evolution from ext3fs • Can mount an ext4fs partition as ext3fs oran ext3fs partition as ext4fs • 64-bit file system • 48-bit block addresses • Can support very large volumes • One exabyte, that is, 230 gigabytes! • Very large files (16 terabytes)

  22. Ext4fs (II) • Can support extents • Becomes then incompatible with ext3fs • Uses delayed extent allocation • Reduces file fragmentation • Especially when file grows • Checksums contents of journal • More reliable

  23. Ext4fs (III) • Uses H-trees instead of B+ or B* trees for indexes • Includes an online defragmenting tool • e4defrag • Can defragment individual files or entire file systems • Minimum timestamp resolution is one ns

  24. Conclusions • Journaling file systems • Protect data against computer crashes and power failures • Allow faster file system recovery after a crash • No need to fschk the whole file system • Have become the new standard

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