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Re: [Xen-users] High availability and live migration with just LVMand ha

To: Bart Coninckx <bart.coninckx@xxxxxxxxxx>
Subject: Re: [Xen-users] High availability and live migration with just LVMand hardware fencing
From: John Madden <jmadden@xxxxxxxxxxx>
Date: Thu, 20 May 2010 09:21:10 -0400
Cc: Jeff Sturm <jeff.sturm@xxxxxxxxxx>, "xen-users@xxxxxxxxxxxxxxxxxxx" <xen-users@xxxxxxxxxxxxxxxxxxx>, Javier Guerra Giraldez <javier@xxxxxxxxxxx>
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I did some more thinking in regards to the snapshotting and came up with the following: the backend storage is an IET storage target. I could build the LUNs on top of LV's. This would give me snapshotting for the Xen setup. Granted, not very elegant, since for a restore I need to put the snapshot back and as a added action I need to connect to it from an iSCSI initiator in order to access the data.

You'll have the same problems here as you will snapshotting from dom0 -- filesystem cache and application state. If the OS hasn't flushed its cache and frozen the filesystem, any snapshot from the block level would contain an inconsistent filesystem. This is not an acceptable backup mechanism and I think doing it from IET (i.e., the SAN) is even worse because you have yet another layer of abstraction and potential for caches.

Note Jeff's note on SAN snapshots and gfs_tool. His snapshots work because the OS is made aware of the need to flush caches and temporarily queue writes to the disk.

John





--
John Madden
Sr UNIX Systems Engineer
Ivy Tech Community College of Indiana
jmadden@xxxxxxxxxxx

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