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Re: [Xen-users] iSCSI and LVM

On Monday 14 June 2010 17:59:26 Jonathan Tripathy wrote:
> ________________________________
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx on behalf of Serge Fonville
> Sent: Mon 14/06/2010 16:53
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] iSCSI and LVM
> Hi
> >> I am going to get a storage server which will be connected to my Xen
> >> hosts via iSCSI/Ethernet. I wish to use LVM for the DomU disks. The
> >> storage server will have a RAID10 array, and 2 Xen hosts will connect
> >> to this (Each will have a 50% share of the RAID10 array, space wise).
> >>
> >> What is the best way to go about this? Should I:
> >>
> >> a) Split the RAID10 array into 2 partition on the storage server, and
> >> export 1 partition to each xen host, then let the Xen host manage LVM?
> >>
> >> or
> >>
> >> b) Do all LVM stuff on the storage server and export each LVM logical
> >> volume to the correct Xen hosts via iSCSI? Since each host could have
> >> around 100 VMs on it, that's a lot of iSCSI exporting!
> >
> > If you export two "big luns" and do the LVM stuff on each Xens server
> > you will not be able to "switch" a VM from one server to the other (with
> > xm save/restore for example). With that setup a VM can only be started
> > on a specific server. Also you will have two LVM sets to manage.
> >
> > If you do all the storage management on your SAN migrate a VM from one
> > server to the other will be easy and even done "live". Also the storage
> > management will be "unified". The only problem could be the maximum
> > number of iSCSI Luns on yours SAN, but it often is greater than 127
> > distinct units.
> >
> > Ok thanks for the advice. So I think I shall go down the root of doing
> > all LVM stuff on the SAN.
> >
> > However, I think that I will need more than 127 virtual machines, so
> > having an iSCSI target per VM is probably not an option. Is there any
> > other solution for Ubuntu/Debian?
> Would it be an option to have one iSCSI target, used by multiple
> clients where only one clients accesses a directory at a time??
> that way it would be way easier to mgrate a vm from one host to another.
> Regards,
> Serge Fonville
> --
> http://www.sergefonville.nl
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> So you're saying that I could just have one bug LUN, which both xen hosts
>  could connect to, and as long as each host didn't try to run the same DomU
>  as the same time, I'd be ok? Is this safe and done in industry?

A lot depends on whether you plan to use image files or block devices. Also if 
snapshotting comes into the equation is very releveant. In conclusion, whether 
you use cluster software with STONITH functionality is very relevant.


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