ANATOMY OF LINUX JOURNALING FILE SYSTEMS

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ANATOMY OF LINUX JOURNALING FILE SYSTEMS

• M. Tim JonesEmulex

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Overview

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

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History

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

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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

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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

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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

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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

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

• Most reliable
• Slowest
• All data must be written twice

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JFS2

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

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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

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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 allocation for extents
• Extent is not allocated until blocks are ready to
  be written on disk

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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

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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

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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,

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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

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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

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Without tail packing
Too much wasted space
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With tail packing
Now occupies a single block
Shares last block of file A
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Reiser 4

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

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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)

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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

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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

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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