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

[John Madden <jmadden@xxxxxxxxxxx>]

> 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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