On Thu, Aug 20, 2009 at 5:25 PM, Robert Milkowski <mi...@task.gda.pl> wrote:
> Matthew Stevenson wrote: > > Ha ha, I know! Like I say, I do get COW principles! > > I guess what I'm after is for someone to look at my specific example (in txt > file attached to first post) and tell me specifically how to find out where > the 13.8GB number is coming from. > > I feel like a total numpty for going on about this, I really do, but despite > all the input I still don't see an answer to this basic question. > > I promise this will be my last post querying the subject! > > > > The problem is how do you account for data which is referenced by more > than one snapshot? Which snapshot would you "charge" then? The used property > only tells you how much *unique* data is being held by snapshot (so the data > is not referenced by any other snapshot or filesystem). If some data is > referenced by more than one snapshot it is accounted as used for a file > system for lack of better way of presenting it to user. The usedbysnap > property represents the amount of storage to be regained if all snapshots > were removed. > > Lets say you created 1GB file: /pool/file1. > Now you created snapshot: zfs snapshot p...@snap1 > > so far the snapshot does not consume any data. > > now create another 1GB file: /pool/file2 > and create another snapshot: zfs snapshot p...@snap2 > > Notice that file1 is referenced by both snapshots while file2 is referenced > only by snap2. > > Now remove file1 from the filesystem. Because both snapshots reference it > against which one should you report 1GB of usage? In such a case the 1GB > will be charged against main filesystem only. If you remove file2 used space > will be charged against filesystem and snap2 as it is only snap2 which > references to it so there is no confusion as what it should be charged > against. > > Example: > > > mi...@r600:~# zfs create rpool/test > mi...@r600:~# mkfile 10m /rpool/test/file1 > mi...@r600:~# zfs snapshot rpool/t...@snap1 > mi...@r600:~# mkfile 10m /rpool/test/file2 > mi...@r600:~# zfs snapshot rpool/t...@snap2 > > mi...@r600:~# zfs list -r -t all rpool/test > NAME USED AVAIL REFER MOUNTPOINT > rpool/test 20.0M 93.0G 20.0M /rpool/test > rpool/t...@snap1 17K - 10.0M - > rpool/t...@snap2 0 - 20.0M - > > mi...@r600:~# rm /rpool/test/file1 > > mi...@r600:~# zfs list -r -t all rpool/test > NAME USED AVAIL REFER MOUNTPOINT > rpool/test *20.1M * 93.0G *10.0M * /rpool/test > rpool/t...@snap1 17K - 10.0M - > rpool/t...@snap2 17K - 20.0M - > mi...@r600:~# > > mi...@r600:~# rm /rpool/test/file2 > > mi...@r600:~# zfs list -r -t all rpool/test > NAME USED AVAIL REFER MOUNTPOINT > rpool/test 20.1M 93.0G 19K /rpool/test > rpool/t...@snap1 17K - 10.0M - > rpool/t...@snap2 10.0M - 20.0M - > mi...@r600:~# > > Since file2 is referenced uniquely only by snap2 it is reported as used > space of snap2. > file1 is referenced by both snapshots so its 10MB is charged against > filesystem only as it is not clear which snapshot should heave represent it. > > Usedbysnashot should point to ~20MB: > > mi...@r600:~# zfs get usedbysnapshots rpool/test > NAME PROPERTY VALUE SOURCE > rpool/test usedbysnapshots 20.0M - > mi...@r600:~# > > > > > So going back to your case (I haven't read you attachment but I think I got > it right): all your snapshots are using 13.8GB as that much data is being > referenced by them but no longer by filesystem (so data was modified or > deleted from the filesystem). Now some of that data is referenced by > multiple snapshots so it is not clear against which one it should be > reported. Some of data is being uniquely referenced by only a one snapshot > so such data is easy to be represented as being used by that specific > snapshot. > > > I hope it is clear now... > > -- > Robert Milkowski > http://milek.blogspot.com > - > Kudos: Very nicely written Robert! :) Perhaps we can add this to the ZFS FAQs Regards, Al Hopper Logical Approach Inc,Plano,TX a...@logical-approach.com Voice: 972.379.2133 Timezone: US CDT OpenSolaris Governing Board (OGB) Member - Apr 2005 to Mar 2007 http://www.opensolaris.org/os/community/ogb/ogb_2005-2007/
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