Update Documentation/filesystems/ext4.txt
Fix paragraph with recommendations on how to tune ext4 for benchmarks. Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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@ -58,13 +58,22 @@ Note: More extensive information for getting started with ext4 can be
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# mount -t ext4 /dev/hda1 /wherever
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- When comparing performance with other filesystems, remember that
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ext3/4 by default offers higher data integrity guarantees than most.
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So when comparing with a metadata-only journalling filesystem, such
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as ext3, use `mount -o data=writeback'. And you might as well use
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`mount -o nobh' too along with it. Making the journal larger than
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the mke2fs default often helps performance with metadata-intensive
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workloads.
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- When comparing performance with other filesystems, it's always
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important to try multiple workloads; very often a subtle change in a
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workload parameter can completely change the ranking of which
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filesystems do well compared to others. When comparing versus ext3,
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note that ext4 enables write barriers by default, while ext3 does
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not enable write barriers by default. So it is useful to use
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explicitly specify whether barriers are enabled or not when via the
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'-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
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for a fair comparison. When tuning ext3 for best benchmark numbers,
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it is often worthwhile to try changing the data journaling mode; '-o
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data=writeback,nobh' can be faster for some workloads. (Note
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however that running mounted with data=writeback can potentially
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leave stale data exposed in recently written files in case of an
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unclean shutdown, which could be a security exposure in some
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situations.) Configuring the filesystem with a large journal can
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also be helpful for metadata-intensive workloads.
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2. Features
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===========
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@ -74,7 +83,7 @@ Note: More extensive information for getting started with ext4 can be
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* ability to use filesystems > 16TB (e2fsprogs support not available yet)
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* extent format reduces metadata overhead (RAM, IO for access, transactions)
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* extent format more robust in face of on-disk corruption due to magics,
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* internal redunancy in tree
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* internal redundancy in tree
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* improved file allocation (multi-block alloc)
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* fix 32000 subdirectory limit
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* nsec timestamps for mtime, atime, ctime, create time
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@ -116,6 +125,12 @@ grouping of bitmaps and inode tables. Some test results available here:
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When mounting an ext4 filesystem, the following option are accepted:
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(*) == default
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ro Mount filesystem read only. Note that ext4 will
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replay the journal (and thus write to the
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partition) even when mounted "read only". The
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mount options "ro,noload" can be used to prevent
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writes to the filesystem.
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extents (*) ext4 will use extents to address file data. The
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file system will no longer be mountable by ext3.
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@ -144,7 +159,11 @@ journal_dev=devnum When the external journal device's major/minor numbers
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identified through its new major/minor numbers encoded
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in devnum.
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noload Don't load the journal on mounting.
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noload Don't load the journal on mounting. Note that
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if the filesystem was not unmounted cleanly,
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skipping the journal replay will lead to the
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filesystem containing inconsistencies that can
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lead to any number of problems.
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data=journal All data are committed into the journal prior to being
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written into the main file system.
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@ -219,9 +238,12 @@ minixdf Make 'df' act like Minix.
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debug Extra debugging information is sent to syslog.
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errors=remount-ro(*) Remount the filesystem read-only on an error.
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errors=remount-ro Remount the filesystem read-only on an error.
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errors=continue Keep going on a filesystem error.
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errors=panic Panic and halt the machine if an error occurs.
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(These mount options override the errors behavior
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specified in the superblock, which can be configured
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using tune2fs)
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data_err=ignore(*) Just print an error message if an error occurs
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in a file data buffer in ordered mode.
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