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Re: [linux-lvm] What is holding back clustered snapshotting?



On Tue, Jan 05, 2010 at 11:30:01AM -0800, Stuart D. Gathman wrote:
> On Tue, 5 Jan 2010, Madison Kelly wrote:
> 
> >   I know that, currently, this isn't supported. Would someone be able to
> > explain or point me at a place to read up on what is holding this feature
> > back? What are the difficulties? Is it just a question of time, or are there
> > certain technical hurdles in the way?
> 
> Setting up the shapshot is just a matter of locking and coordination.
> However, writes to the origin or snapshot (may) require allocating a
> cluster, copying the origin data, then writing the origin.  All of
> this coordinated with all the machines using the VG.  Apart from some
> cleven invention, this requires global locking on many writes.  This
> is just too inefficient.

Writeable snapshots I guess would be a challenge.  But even read only
snapshots would be great as it would theoretically make backing up
large, clustered filesystems simpler.

> 
> However, you can obtain the same effect using a SAN.  Have one
> machine run LVM (and raid, etc), and export LVs via AoE or iSCSI.  Of
> course, that LVM machine now becomes a single point of failure...
> 
> Here's an idea (someone probably already thought of this, but..),
> have one machine in a cluster elected "master" for a VG, and have all
> reads/writes from other machines go through the master via AoE or
> iSCSI.  When failure of the "master" is detected, elect another
> machine to take over as master.  Sort of a rotating SAN server.
> 

Interesting idea. :)

Ray


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