linux-kernel - Re: atime and filesystems with snapshots (especially Btrfs)

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Message-ID: <CAB9VWqAk5mKVJy4reiTyKDcq7HgwQmnTVj5qNEqg84-mQgynvQ@mail.gmail.com>
Date:	Fri, 25 May 2012 22:48:41 +0200
From:	Alexander Block <ablock84@...glemail.com>
To:	Peter Maloney <peter.maloney@...ckmann-consult.de>
Cc:	linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: atime and filesystems with snapshots (especially Btrfs)

On Fri, May 25, 2012 at 10:42 PM, Alexander Block
<ablock84@...glemail.com> wrote:
> On Fri, May 25, 2012 at 10:27 PM, Peter Maloney
> <peter.maloney@...ckmann-consult.de> wrote:
>> On 05/25/2012 09:10 PM, Alexander Block wrote:
>>> Just to show some numbers I made a simple test on a fresh btrfs fs. I
>>> copied my hosts /usr (4 gig) folder to that fs and checked metadata
>>> usage with "btrfs fi df /mnt", which was around 300m. Then I created
>>> 10 snapshots and checked metadata usage again, which didn't change
>>> much. Then I run "grep foobar /mnt -R" to update all files atime.
>>> After this was finished, metadata usage was 2.59 gig. So I lost 2.2
>>> gig just because I searched for something. If someone already has
>>> nearly no space left, he probably won't be able to move some data to
>>> another disk, as he may get ENOSPC while copying the data.
>>>
>>> Here is the output of the final "btrfs fi df":
>>>
>>> # btrfs fi df /mnt
>>> Data: total=6.01GB, used=4.19GB
>>> System, DUP: total=8.00MB, used=4.00KB
>>> System: total=4.00MB, used=0.00
>>> Metadata, DUP: total=3.25GB, used=2.59GB
>>> Metadata: total=8.00MB, used=0.00
>>>
>>> I don't know much about other filesystems that support snapshots, but
>>> I have the feeling that most of them would have the same problem. Also
>>> all other filesystems in combination with LVM snapshots may cause
>>> problems (I'm not very familiar with LVM). Filesystem image formats,
>>> like qcow, vmdk, vbox and so on may also have problems with atime.
>>> --
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>> Did you run the recursive grep after each snapshot (which I would expect
>> would result in 11 times as many metadata blocks, max 3.3 GB), or just
>> once after all 10 snapshots (which I think would mean only 2x as many
>> metadata blocks, max 600 MB)?
>>
>
> I've run it only once after creating all snapshots. My expectation is that
> in both cases the result is the same. If all snapshots have the file /foo/bar,
> then each individual snapshotted copy of it would have a different atime
> and thus an own metadata block for it. As this happens with all files, no
> matter how i iterated the files, then nearly all metadata blocks get their
> own copy.

Hmm, you did maybe assume the snapshots were r/o. In my test, the
snapshots were all r/w. In the r/o case, I would have to do the recursive
grep after each snapshot creation to get the same result.
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