linux-ext4 - [PATCH] ext4: Clarify impact of 'commit' mount option

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Date:   Wed, 18 Dec 2019 12:12:10 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ted Tso <tytso@....edu>
Cc:     <linux-ext4@...r.kernel.org>,
        Paul Richards <paul.richards@...il.com>,
        Jan Kara <jack@...e.cz>
Subject: [PATCH] ext4: Clarify impact of 'commit' mount option

The description of 'commit' mount option dates back to ext3 times.
Update the description to match current meaning for ext4.

Reported-by: Paul Richards <paul.richards@...il.com>
Signed-off-by: Jan Kara <jack@...e.cz>
---
 Documentation/admin-guide/ext4.rst | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/Documentation/admin-guide/ext4.rst b/Documentation/admin-guide/ext4.rst
index 059ddcbe769d..8d292f5aaea3 100644
--- a/Documentation/admin-guide/ext4.rst
+++ b/Documentation/admin-guide/ext4.rst
@@ -181,14 +181,17 @@ When mounting an ext4 filesystem, the following option are accepted:
         system after its metadata has been committed to the journal.
 
   commit=nrsec	(*)
-        Ext4 can be told to sync all its data and metadata every 'nrsec'
-        seconds. The default value is 5 seconds.  This means that if you lose
-        your power, you will lose as much as the latest 5 seconds of work (your
-        filesystem will not be damaged though, thanks to the journaling).  This
-        default value (or any low value) will hurt performance, but it's good
-        for data-safety.  Setting it to 0 will have the same effect as leaving
-        it at the default (5 seconds).  Setting it to very large values will
-        improve performance.
+	This setting limits the maximum age of the running transaction to
+	'nrsec' seconds.  The default value is 5 seconds.  This means that if
+	you lose your power, you will lose as much as the latest 5 seconds of
+	metadata changes (your filesystem will not be damaged though, thanks
+	to the journaling). This default value (or any low value) will hurt
+	performance, but it's good for data-safety.  Setting it to 0 will have
+	the same effect as leaving it at the default (5 seconds).  Setting it
+	to very large values will improve performance.  Note that due to
+	delayed allocation even older data can be lost on power failure since
+        writeback of those data begins only after time set in
+        /proc/sys/vm/dirty_expire_centisecs.  
 
   barrier=<0|1(*)>, barrier(*), nobarrier
         This enables/disables the use of write barriers in the jbd code.
-- 
2.16.4