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-rw-r--r--Documentation/filesystems/ext4.txt30
1 files changed, 27 insertions, 3 deletions
diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index cec829bc729..97882df0486 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -85,7 +85,7 @@ Note: More extensive information for getting started with ext4 can be
* extent format more robust in face of on-disk corruption due to magics,
* internal redundancy in tree
* improved file allocation (multi-block alloc)
-* fix 32000 subdirectory limit
+* lift 32000 subdirectory limit imposed by i_links_count[1]
* nsec timestamps for mtime, atime, ctime, create time
* inode version field on disk (NFSv4, Lustre)
* reduced e2fsck time via uninit_bg feature
@@ -100,6 +100,9 @@ Note: More extensive information for getting started with ext4 can be
* efficent new ordered mode in JBD2 and ext4(avoid using buffer head to force
the ordering)
+[1] Filesystems with a block size of 1k may see a limit imposed by the
+directory hash tree having a maximum depth of two.
+
2.2 Candidate features for future inclusion
* Online defrag (patches available but not well tested)
@@ -180,8 +183,8 @@ commit=nrsec (*) Ext4 can be told to sync all its data and metadata
performance.
barrier=<0|1(*)> This enables/disables the use of write barriers in
- the jbd code. barrier=0 disables, barrier=1 enables.
- This also requires an IO stack which can support
+barrier(*) the jbd code. barrier=0 disables, barrier=1 enables.
+nobarrier This also requires an IO stack which can support
barriers, and if jbd gets an error on a barrier
write, it will disable again with a warning.
Write barriers enforce proper on-disk ordering
@@ -189,6 +192,9 @@ barrier=<0|1(*)> This enables/disables the use of write barriers in
safe to use, at some performance penalty. If
your disks are battery-backed in one way or another,
disabling barriers may safely improve performance.
+ The mount options "barrier" and "nobarrier" can
+ also be used to enable or disable barriers, for
+ consistency with other ext4 mount options.
inode_readahead=n This tuning parameter controls the maximum
number of inode table blocks that ext4's inode
@@ -310,6 +316,24 @@ journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the
a slightly higher priority than the default I/O
priority.
+auto_da_alloc(*) Many broken applications don't use fsync() when
+noauto_da_alloc replacing existing files via patterns such as
+ fd = open("foo.new")/write(fd,..)/close(fd)/
+ rename("foo.new", "foo"), or worse yet,
+ fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
+ If auto_da_alloc is enabled, ext4 will detect
+ the replace-via-rename and replace-via-truncate
+ patterns and force that any delayed allocation
+ blocks are allocated such that at the next
+ journal commit, in the default data=ordered
+ mode, the data blocks of the new file are forced
+ to disk before the rename() operation is
+ commited. This provides roughly the same level
+ of guarantees as ext3, and avoids the
+ "zero-length" problem that can happen when a
+ system crashes before the delayed allocation
+ blocks are forced to disk.
+
Data Mode
=========
There are 3 different data modes: