On Fri, May 25, 2012 at 10:42 PM, Alexander Block <ablock84@xxxxxxxxxxxxxx> wrote: > On Fri, May 25, 2012 at 10:27 PM, Peter Maloney > <peter.maloney@xxxxxxxxxxxxxxxxxxxx> 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. >>> -- >>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in >>> the body of a message to majordomo@xxxxxxxxxxxxxxx >>> More majordomo info at http://vger.kernel.org/majordomo-info.html >> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
