[linux-lvm] Snapshot question... [scaling problem]

[Larry Dickson]

>Then you can't delete an older snapsnot until you delete all newer ones.

Not true of what I was proposing - are we talking past each other? If snap 0
is the current (live) COW, and snap -k refers to time(-k) = time(snap 0) -
k*(interval), then reading the virtual
data for time(-k) involves looking at snap -k, then snap -k+1, ... snap 0,
current data; but stopping the first time your block gets a hit. The only
point with a race is {snap 0, current data}. So you can't delete a NEWER
snapshot until you delete all OLDER ones (because the virtual older snaps
need the newer COWs). That seems a small price to pay, since normally you
throw them away oldest first.

Larry

On 4/24/08, Stuart D. Gathman <stuart at bmsi.com> wrote:
>
> On Thu, 24 Apr 2008, Larry Dickson wrote:
>
> > There's an almost trivial variant on this, where you keep the
> > (read-only) snapshots in a time-ordered sequence, and freeze the last
> > snapshot COW at the same moment as you start the next snapshot. Then
> writing
> > only ever hits the new snapshot COW, and reading from any older snapshot
> > (virtual) volume involves figuring out which is the first after that to
> hold
> > the block, but still involves reading only one block. I wonder why LVM
> does
> > not do this. Perhaps Zumastor does? Or somebody else?
>
> Then you can't delete an older snapsnot until you delete all newer ones.
>
> Zumastor works by using one COW table shared between all snapshots
> for a volume.  Blocks are added to the COW in time order.  The origin
> ignores COW blocks before the last time point (block offset), writing a
> new COW
> block for any modified since that time point.  The snapshots also use
> timepoints in a way that is straightforward, but I don't want to think
> about it at the moment :-)
>
> --
>              Stuart D. Gathman <stuart at bmsi.com>
>    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
> "Confutatis maledictis, flammis acribus addictis" - background song for
> a Microsoft sponsored "Where do you want to go from here?" commercial.
>
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