1 files changed, 67 insertions, 18 deletions

diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 174eaff7ded9..cec829bc7291 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -58,13 +58,22 @@ Note: More extensive information for getting started with ext4 can be
 
         # 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 @@ Note: More extensive information for getting started with ext4 can be
 * 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
@@ -116,10 +125,11 @@ grouping of bitmaps and inode tables.  Some test results available here:
 When mounting an ext4 filesystem, the following option are accepted:
 (*) == default
 
-extents         (*)     ext4 will use extents to address file data.  The
-                        file system will no longer be mountable by ext3.
-
-noextents               ext4 will not use extents for newly created files
+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.
 
 journal_checksum        Enable checksumming of the journal transactions.
                         This will allow the recovery code in e2fsck and the
@@ -134,17 +144,17 @@ journal_async_commit	Commit block can be written to disk without waiting
 journal=update          Update the ext4 file system's journal to the current
                         format.
 
-journal=inum            When a journal already exists, this option is ignored.
-                        Otherwise, it specifies the number of the inode which
-                        will represent the ext4 file system's journal file.
-
 journal_dev=devnum      When the external journal device's major/minor numbers
                         have changed, this option allows the user to specify
                         the new journal location.  The journal device is
                         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 +229,12 @@ minixdf			Make 'df' act like Minix.
 
 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.
@@ -261,6 +274,42 @@ delalloc	(*)	Deferring block allocation until write-out time.
 nodelalloc              Disable delayed allocation. Blocks are allocation
                         when data is copied from user to page cache.
 
+max_batch_time=usec     Maximum amount of time ext4 should wait for
+                        additional filesystem operations to be batch
+                        together with a synchronous write operation.
+                        Since a synchronous write operation is going to
+                        force a commit and then a wait for the I/O
+                        complete, it doesn't cost much, and can be a
+                        huge throughput win, we wait for a small amount
+                        of time to see if any other transactions can
+                        piggyback on the synchronous write.   The
+                        algorithm used is designed to automatically tune
+                        for the speed of the disk, by measuring the
+                        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
+                        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
+                        the max_batch_time, which defaults to 15000us
+                        (15ms).   This optimization can be turned off
+                        entirely by setting max_batch_time to 0.
+
+min_batch_time=usec     This parameter sets the commit time (as
+                        described above) to be at least min_batch_time.
+                        It defaults to zero microseconds.  Increasing
+                        this parameter may improve the throughput of
+                        multi-threaded, synchronous workloads on very
+                        fast disks, at the cost of increasing latency.
+
+journal_ioprio=prio     The I/O priority (from 0 to 7, where 0 is the
+                        highest priorty) which should be used for I/O
+                        operations submitted by kjournald2 during a
+                        commit operation.  This defaults to 3, which is
+                        a slightly higher priority than the default I/O
+                        priority.
+
 Data Mode
 =========
 There are 3 different data modes: