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



>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 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 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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