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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) - PowerPoint PPT Presentation
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ANATOMY OF LINUX JOURNALING FILE SYSTEMS
M. Tim JonesEmulex
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)
• Silicon Graphics' XFS– Released in 1994– Ported into Linux in 1994
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
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
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
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
Writeback mode issues
Committed i-node
Block BDue to a crash,block B was neverwritten to disk
Data mode issues
• Most reliable• Slowest:
– All data must be written twice
JFS2
• Supports– Ordered journaling– Extent-based allocation:
• Allocates contiguous sets of blocks–Better read and write performance–Metadata are only updated for the
extent
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
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
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
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
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, …
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
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
Without tail packing
File A
File B
File C Too much wasted space
With tail packing
File A
File B
File C Shares last block of file A
Now occupies a single block
Reiser 4
• Was designed from scratch• Was to use
– Wandering logs– Delayed allocation of extents
• As in XFS
Ext4fs (I)
• Evolution from ext3fs– Can mount an ext4fs partition as ext3fs or
an 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)
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
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
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