On Fri, Sep 21, 2012 at 12:05 AM, Stefan Ring <stefan...@gmail.com> wrote:
> On Fri, Sep 21, 2012 at 6:31 AM, andy thomas <a...@time-domain.co.uk> > wrote: > > I have a ZFS filseystem and create weekly snapshots over a period of 5 > weeks > > called week01, week02, week03, week04 and week05 respectively. Ny > question > > is: how do the snapshots relate to each other - does week03 contain the > > changes made since week02 or does it contain all the changes made since > the > > first snapshot, week01, and therefore includes those in week02? > > Every snapshot is based on the previous one and store only what is > needed to capture the differences. > > > To rollback to week03, it's necesaary to delete snapshots week04 and > week05 > > first but what if week01 and week02 have also been deleted - will the > > rollback still work or is it ncessary to keep earlier snapshots? > > No, it's not necessary. You can rollback to any snapshot. > > I almost never use rollback though, in normal use. If I've > accidentally deleted or overwritten something, I just rsync it over > from the corresponding /.zfs/snapshots directory. Only if what I want > to restore is huge, rollback might be a better option. > > I wasn't going to jump into this quagmire, but I will. To the second question, if you've got snaps 1-5, and you roll back to snap 3, you will lose snaps 4 and 5. As part of the rollback, they will be discarded. As will any other changes made since snap 3. If you delete snap 1 or snap 2, any blocks they have in common with snapshot 3 will be retained, you will simply see snap 3 "grow" because those blocks will now be accounted for under snap 3 instead of snap 1 or snap 2. Any blocks that were not shared with snap 3 will be discarded. Another point since you seem to be new to snapshots that I'll illustrate with an example. Say you've got snap 1, and in snap 1 you've got file 1. File 1 is made up of 20 blocks. If you overwrite blocks 1-10 of file 1 50 times before you take snapshot 2, snapshot 2 will only capture the final state of the file. You will not get 50 revisions of the file. This is not continuous data protection it's a point in time copy. --Tim
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