Re: atime and filesystems with snapshots (especially Btrfs)

[Peter Maloney]

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)?

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