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

Tomasz Chmielewski mangoo at wpkg.org
Wed Jan 23 16:16:42 UTC 2008

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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