On Fri, May 25, 2012 at 10:27 PM, Peter Maloney
[off-list ref] wrote:

On 05/25/2012 09:10 PM, Alexander Block wrote:

quoted

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.