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

Stuart D. Gathman stuart at bmsi.com
Tue Jan 5 19:30:01 UTC 2010

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.

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.

	      Stuart D. Gathman <stuart at bmsi.com>
    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flammis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.

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