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Re: [dm-devel] Shared snapshots



On Mon, Jan 11, 2010 at 04:12:43AM -0500, Mikulas Patocka wrote:
> 
> 
> On Thu, 7 Jan 2010, Pasi Kärkkäinen 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.
>

Ok. Do you know if there has been work making snapshots work with CLVM? 

I remember there was some patches earlier, but no idea about the status
of those..

> 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.
>

This sounds excellent! Thanks for explaining.

-- Pasi


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