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>
diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 174eaff..f75ab10 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -58,13 +58,22 @@
# mount -t ext4 /dev/hda1 /wherever
- - When comparing performance with other filesystems, remember that
- ext3/4 by default offers higher data integrity guarantees than most.
- So when comparing with a metadata-only journalling filesystem, such
- as ext3, use `mount -o data=writeback'. And you might as well use
- `mount -o nobh' too along with it. Making the journal larger than
- the mke2fs default often helps performance with metadata-intensive
- workloads.
+ - When comparing performance with other filesystems, it's always
+ important to try multiple workloads; very often a subtle change in a
+ workload parameter can completely change the ranking of which
+ filesystems do well compared to others. When comparing versus ext3,
+ note that ext4 enables write barriers by default, while ext3 does
+ not enable write barriers by default. So it is useful to use
+ 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
===========
@@ -74,7 +83,7 @@
* ability to use filesystems > 16TB (e2fsprogs support not available yet)
* extent format reduces metadata overhead (RAM, IO for access, transactions)
* 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)
* fix 32000 subdirectory limit
* nsec timestamps for mtime, atime, ctime, create time
@@ -116,6 +125,12 @@
When mounting an ext4 filesystem, the following option are accepted:
(*) == 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
file system will no longer be mountable by ext3.
@@ -144,7 +159,11 @@
identified through its new major/minor numbers encoded
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
written into the main file system.
@@ -219,9 +238,12 @@
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=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
in a file data buffer in ordered mode.