[linux-lvm] [RFC] Multiple Snapshots - Manageability problem
Vijai Babu Madhavan
mvijai at novell.com
Thu Jan 11 18:18:13 UTC 2007
The problem of DM snapshots with multiple snapshots have been discussed
in the lists quiet a bit (Most recently @
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.
This really gets messy and creates a management problem once many
snapshots of a given origin are created, and gets worse with multiple origins.
I am thinking, having a single device that would hold the cow blocks of any
number of snapshots of a given origin (or more) would help solve this issue
(Apart from this, having a single device helps share the cow blocks among
snapshots very effectively in a variety of scenarios).
But, it does require that LVM and EVMS be changed to suit this model and
also makes the snapshot target quiet complex.
I would like to receive some comments about what users, developers
and others think about this.
P.S:- BTW, apologizes for cross posting.
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