Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/linville/wirel...

[karo-tx-linux.git] / Documentation / filesystems / ext4.txt

diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index cec829bc7291623da6189de984fedd753ce51450..608fdba97b72c5c95dec4d28cbb8364132a8a90d 100644 (file)
--- 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
@@ -288,7 +294,7 @@ max_batch_time=usec Maximum amount of time ext4 should wait for
                        amount of time (on average) that it takes to
                        finish committing a transaction.  Call this time
                        the "commit time".  If the time that the
-                       transactoin has been running is less than the
+                       transaction has been running is less than the
                        commit time, ext4 will try sleeping for the
                        commit time to see if other operations will join
                        the transaction.   The commit time is capped by
@@ -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
+                       committed.  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:
@@ -334,7 +358,7 @@ written to the journal first, and then to its final location.
 In the event of a crash, the journal can be replayed, bringing both data and
 metadata into a consistent state.  This mode is the slowest except when data
 needs to be read from and written to disk at the same time where it
-outperforms all others modes.  Curently ext4 does not have delayed
+outperforms all others modes.  Currently ext4 does not have delayed
 allocation support if this data journalling mode is selected.
 
 References