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Commit 8e1a4857 authored by Theodore Ts'o's avatar Theodore Ts'o
Browse files

Update Documentation/filesystems/ext4.txt 



Fix paragraph with recommendations on how to tune ext4 for benchmarks.

Signed-off-by: default avatar"Theodore Ts'o" <tytso@mit.edu>
parent 59e315b4

Documentation/filesystems/ext4.txt

+32 −10
Original line number Original line Diff line number Diff line
@@ -58,13 +58,22 @@ Note: More extensive information for getting started with ext4 can be 




	# mount -t ext4 /dev/hda1 /wherever

	# mount -t ext4 /dev/hda1 /wherever





  - When comparing performance with other filesystems, remember that

  - When comparing performance with other filesystems, it's always

    ext3/4 by default offers higher data integrity guarantees than most.

    important to try multiple workloads; very often a subtle change in a

    So when comparing with a metadata-only journalling filesystem, such

    workload parameter can completely change the ranking of which

    as ext3, use `mount -o data=writeback'.  And you might as well use

    filesystems do well compared to others.  When comparing versus ext3,

    `mount -o nobh' too along with it.  Making the journal larger than

    note that ext4 enables write barriers by default, while ext3 does

    the mke2fs default often helps performance with metadata-intensive

    not enable write barriers by default.  So it is useful to use

    workloads.

    explicitly specify whether barriers are enabled or not when via the

    '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems

    for a fair comparison.  When tuning ext3 for best benchmark numbers,

    it is often worthwhile to try changing the data journaling mode; '-o

    data=writeback,nobh' can be faster for some workloads.  (Note

    however that running mounted with data=writeback can potentially

    leave stale data exposed in recently written files in case of an

    unclean shutdown, which could be a security exposure in some

    situations.)  Configuring the filesystem with a large journal can

    also be helpful for metadata-intensive workloads.





2. Features

2. Features

===========

===========
@@ -74,7 +83,7 @@ Note: More extensive information for getting started with ext4 can be 
* ability to use filesystems > 16TB (e2fsprogs support not available yet)

* ability to use filesystems > 16TB (e2fsprogs support not available yet)

* extent format reduces metadata overhead (RAM, IO for access, transactions)

* extent format reduces metadata overhead (RAM, IO for access, transactions)

* extent format more robust in face of on-disk corruption due to magics,

* extent format more robust in face of on-disk corruption due to magics,

* internal redunancy in tree

* internal redundancy in tree

* improved file allocation (multi-block alloc)

* improved file allocation (multi-block alloc)

* fix 32000 subdirectory limit

* fix 32000 subdirectory limit

* nsec timestamps for mtime, atime, ctime, create time

* nsec timestamps for mtime, atime, ctime, create time
@@ -116,6 +125,12 @@ grouping of bitmaps and inode tables. Some test results available here: 
When mounting an ext4 filesystem, the following option are accepted:

When mounting an ext4 filesystem, the following option are accepted:

(*) == default

(*) == default





ro                   	Mount filesystem read only. Note that ext4 will

                     	replay the journal (and thus write to the

                     	partition) even when mounted "read only". The

                     	mount options "ro,noload" can be used to prevent

		     	writes to the filesystem.



extents		(*)	ext4 will use extents to address file data.  The

extents		(*)	ext4 will use extents to address file data.  The

			file system will no longer be mountable by ext3.

			file system will no longer be mountable by ext3.
@@ -144,7 +159,11 @@ journal_dev=devnum When the external journal device's major/minor numbers 
			identified through its new major/minor numbers encoded

			identified through its new major/minor numbers encoded

			in devnum.

			in devnum.





noload			Don't load the journal on mounting.

noload			Don't load the journal on mounting.  Note that

                     	if the filesystem was not unmounted cleanly,

                     	skipping the journal replay will lead to the

                     	filesystem containing inconsistencies that can

                     	lead to any number of problems.





data=journal		All data are committed into the journal prior to being

data=journal		All data are committed into the journal prior to being

			written into the main file system.

			written into the main file system.
@@ -219,9 +238,12 @@ minixdf Make 'df' act like Minix. 




debug			Extra debugging information is sent to syslog.

debug			Extra debugging information is sent to syslog.





errors=remount-ro(*)	Remount the filesystem read-only on an error.

errors=remount-ro	Remount the filesystem read-only on an error.

errors=continue		Keep going on a filesystem error.

errors=continue		Keep going on a filesystem error.

errors=panic		Panic and halt the machine if an error occurs.

errors=panic		Panic and halt the machine if an error occurs.

                        (These mount options override the errors behavior

                        specified in the superblock, which can be configured

                        using tune2fs)





data_err=ignore(*)	Just print an error message if an error occurs

data_err=ignore(*)	Just print an error message if an error occurs

			in a file data buffer in ordered mode.

			in a file data buffer in ordered mode.