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

[Bart Coninckx <bart.coninckx@xxxxxxxxxx>]

On Wednesday 19 May 2010 15:43:19 Jeff Sturm wrote:
> > -----Original Message-----
> > From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-
> > bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Bart Coninckx
> > Sent: Wednesday, May 19, 2010 9:00 AM
> > To: xen-users@xxxxxxxxxxxxxxxxxxx
> > Subject: Re: [Xen-users] High availability and live migration with
> 
> just LVMand hardware
> 
> > fencing
> >
> > Did some further research in the meantime and so far the best reason I
> 
> can
> 
> > find for using cLVM, is the making changes to the volume groups: with
> 
> cLVM
> 
> > they seem to be "published" to all Xen hosts at once, while with LVM
> 
> one needs
> 
> > to reconnect to the volume group (probably causing weird things to the
> > guests).
> 
> We've done it with and without CLVM.  It's not a requirement, but it
> works, and can compensate for storage devices that make it cumbersome to
> manage the number of LUNs needed for your deployment.
> 
> Advantanges of CLVM:
> 
> - Fully clustered, uses robust fencing, no single point of failure
> - Integration with device mapper allows for intuitive logical volume
> names
> - Works identically atop any network block storage (iSCSI etc.)
> - Supports mirroring of block devices in recent releases
> - Synchronizes volume group changes across cluster
> 
> Disadvantages:
> 
> - No support for volume snapshots (maybe someday--last I tried it didn't
> work)
> - Not available or convenient to use with all OS distributions
> - Requires cluster infrastructure--difficult to use with system
> partitions (e.g. /, /var)
> - No thin provisioning
> 
> For my money I prefer the LUN management that ships with certain SAN
> products (such as Dell's Equallogic arrays) over CLVM.  As we consider
> moving away from a RHEL-supported dom0 kernel towards something like XCP
> or XenServer, it'll become a necessity.
> 
> -Jeff
> 

Jeff,

a well put conclusion. 
Your second disadvantage particularly is relevant to me, since I need to use 
the commercially supported SLES10SP3, which has no cLVM available in it. 

Also the first one is a, to say the least, disappointing. Having the 
possibility of snapshotting is most valuable in the context of 
updating/rollback. 

By the way: would it be correct to state that cLVM is only an advantage when 
dealing with more than one volume group (being the result of having more than 
one LUN)?


thx!

B.

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