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I see this in my dmesg log

EXT4-fs (md1): re-mounted. Opts: commit=0
EXT4-fs (md2): re-mounted. Opts: commit=0
EXT4-fs (md3): re-mounted. Opts: commit=0

I think that means that dealloc is disabled? does mdadm not support dealloc?

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It appears that for some reason when a system is using ext4 on a RAID set, it wants to remount the set with the "commit=0" option set. I am currently seeing the same thing happen with 3ware hardware RAID. Like the original poster, I have no idea why this is happening. I am running two different OS's on the same hardware. Mandriva 2010.2 is NOT remounting with the commit option set, Mageia 1 is. –  user7836 May 27 '11 at 14:03
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2 Answers

up vote 4 down vote accepted

mdadm supports dealloc.

commit=sec is the time, the filesystem syncs its data and metadata. Setting this to 0 has the same effect as using the default value 5.

So I don't get the link between mdadm and commit=0 in your question?

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I guess I'm not sure why the system is remounting my FS's like this... I'm certainly not setting it to anything, and before I moved to mdadm raid I never saw this. 5 is something like what ext3 uses where I think the default for ext4 is something like 600 –  xenoterracide Sep 1 '10 at 22:49
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I asked this question the LKML and the answer I got was that something though KDE/Hal was remounting the drives for some reason. –  xenoterracide Oct 30 '10 at 12:42
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To xenoterracide: Default 600 sec.? Here Kernel.org/doc/Documentation/filesystems/ext4.txt is something different: "... The default value is 5 seconds. ... Setting it to 0 will have the same effect as leaving it at the default (5 seconds). ..." –  user7578 May 18 '11 at 15:19
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RTFM mount(8)

   commit=nrsec
          Sync  all  data  and  metadata  every nrsec seconds. The default
          value is 5 seconds. Zero means default.

Very short.

New drives unload heads. Would then need base on commit size and run the risk of loosing 10 min of work to save wear and tear on disc.

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