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Re: [dm-devel] Shared snapshots

On Thu, 7 Jan 2010, Pasi K?rkk?inen wrote:

> On Wed, Jan 06, 2010 at 09:38:00AM -0500, Mikulas Patocka wrote:
> > Hi
> > 
> > I uploaded new code for shared snapshots at
> > http://people.redhat.com/mpatocka/patches/kernel/new-snapshots/devel/
> > 
> > Changes:
> > - two queues, one for reads and one for writes, to improve read latency 
> > when the device is write-congested
> > - limit number of reallocations in flight (VM subsystem sends all writes 
> > at once and without limit it creates big lags).
> > - fixed one place where it didn't handle on-disk errors
> > - simplified bitmap creation, removed some unneeded code
> > - new macro for error reporting, as suggested by Zdenek
> > - some comments added
> > - DM_ prefix added to some macros
> > 
> > Userspace for lvm 2.02.53 is at
> > http://people.redhat.com/mpatocka/patches/userspace/new-snapshots/lvm-2.02.54/
> > 
> > Please, someone, do offensive testing with this --- i.e. apply the 
> > patches, try it, and try all possible lvm commands with it, try to stress 
> > and crash it in various ways, etc. If we wait with testing until Alasdair 
> > reads the code, it won't get much testing...
> > 
> Sorry for a stupid question, but what does "shared" mean? Support for CLVM?
> -- Pasi

No, it is not clustered.

It means that you can create more snapshots (actually 2^32), they have a 
common snapshot store and data in this store are shared.

Normal snapshots get extremely ineffective when using multiple snapshots 
--- for example, if you have 10 snapshots, data have to be written 10 
times and any writes to the origin are 10 times slower. Shared snapshots 
write data only once.

A possible use for them is to take periodic snapshot, for example once per 
5 minutes, to record system activity.

Another possible use is to create one master image with system image and 
take many snapshots, each snapshot for one virtual machine.


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