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[linux-lvm] Re: [RFC] Multiple Snapshots - Manageability problem

Vijai Babu Madhavan, Thu, 11 Jan 2007 11:18:13 -0700, wrote:

The problem of DM snapshots with multiple snapshots have been discussed in the lists quiet a bit (Most recently @ https://www.redhat.com/archives/dm-devel/2006-October/msg00034.html).

We are currently in the process of building a DM snapshot target that scales well with many snapshots (so that the changed blocks don't get copied to each snapshot). In this process, I would also like to validate an assumption.

Today, when a single snapshot gets created, a new cow device of a given size is also created. IMO, there are two problems with this approach:

a) It is difficult to predict the size of the cow device, which requires a prediction of the number of writes would go into the origin volume during the snapshot life cycle. It is difficult to get this prediction right, as very high value reduces utilization and low value increases the chances of snapshot becoming full.

b) A new cow device needs to be created every time.


Any news on that?

Still, with multiple snapshots write performance degrades linearly - is any work done to change that anytime soon?

Tomasz Chmielewski

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