Re: [Xen-users] Shared Storage

[Bart Coninckx <bart.coninckx@xxxxxxxxxx>]

Correct.

If you create the LUNs on top of LVM LV's on the storage server you have 
no issue regarding shrinking or expanding DomU's.
Mind you, this obviously cannnot happen "live" - you need to bring down 
your DomU, bring down the LUN and then resize the LV. Next you need to 
do changes to the partition table on the exported LUN.
B.


On 04/24/11 21:43, Jonathan Tripathy wrote:
> Thanks Bart. Very helpful info
>
> I agree with you about the LVM PV issue. It is indeed very
> uncomfortable. I am looking into CLVM (Cluster LVM) though, however this
> isn't very well documented.
>
> So the current idea is one target per Xen node (hense one target per
> RAID array on the storage server), and one LUN per DomU. Is it easy
> enough to expand and shrink LUNs? This was the advantage of LVM that I
> loved. I guess I would run LVM on the storage server and export the LVs?
>
> Thanks
>
> -----Original Message-----
> From: Bart Coninckx [mailto:bart.coninckx@xxxxxxxxxx]
> Sent: Sun 24/04/2011 20:40
> To: Jonathan Tripathy
> Cc: Jonathan Dye; Xen List
> Subject: Re: [Xen-users] Shared Storage
>
> I think you better take one target and then several LUNs on it (one per
> DomU), that would make more sense. If you don't do that and use just one
> LUN for several DomU's, you need to create PVM LV's on the newly created
> disk for each DomU on the hypervisor side, does not really sound
> comfortable. You would also close any path to HA, unless you maybe
> introduce some locking system, since every hypervisor would be wanting
> to try to write to the LUN.
>
> B.
>
> On 04/24/11 21:35, Jonathan Tripathy wrote:
>  > Hi Guys,
>  >
>  > Please forget the "thousands" number. We would have thousands of DomUs,
>  > but this would be spread over multiple storage servers, so never mind
>  > about that scale.
>  >
>  > If I was exporting "One big LUN" per Xen node, it would contain at most
>  > 80 DomU LVs (In real world usage, closer to 50). Furthermore, each LUN
>  > would be exported from a seperate RAID array. Each storage server would
>  > contain x number of RAID arrays, where x equals the number of Xen nodes
>  > and the number of exported LUNs.
>  >
>  > Of course, if I went with one LUN per DomU, then each storage server
>  > would contain 80x LUNs (closer to 50x though).
>  >
>  > With these numbers, any idea which is better?
>  >
>  > Thanks
>  >
>  >
>  > -----Original Message-----
>  > From: Bart Coninckx [mailto:bart.coninckx@xxxxxxxxxx]
>  > Sent: Sun 24/04/2011 19:36
>  > To: Jonathan Tripathy
>  > Cc: Jonathan Dye; Xen List
>  > Subject: Re: [Xen-users] Shared Storage
>  >
>  > That is completely dependent on your hardware specs and DomU's
> properties.
>  > It sounds like a lot though. I seem to remember some time ago you also
>  > stated to want to run at least 100 DomUs on one hypervisor, maybe this
>  > is again pushing it.
>  > With a decent RAID and 10gbit or infiniband you can go a long way
>  > though. You should also consider using SCST instrad of IET as it is
> faster.
>  >
>  > B.
>  >
>  >
>  >
>  > On 04/24/11 20:31, Jonathan Tripathy wrote:
>  > > We're talking houndreds, if not thousands of DomUs here. Will iSCSI on
>  > > Linux scale to these large numbers?
>  > >
>  > > Thanks
>  > >
>  > >
>  > > On 24/04/2011 19:13, Jonathan Dye wrote:
>  > >> Why not create one iscsi lun per vm disk instead of carving them up on
>  > >> the hypervisor? That's more typical, and a more typical state of
>  > >> affairs in linux is your friend. Also, you would have just one lun
>  > >> queue if you exported one big PV, instead of one lun queue per vbd.
>  > >> That becomes a problem at scale.
>  > >>
>  > >> - Jonathan
>  > >>
>  > >> ----- Original Message -----
>  > >> From: "Jonathan Tripathy"<jonnyt@xxxxxxxxxxx>
>  > >> To: "Xen List"<xen-users@xxxxxxxxxxxxxxxxxxx>
>  > >> Sent: Sunday, April 24, 2011 11:25:38 AM
>  > >> Subject: [Xen-users] Shared Storage
>  > >>
>  > >> Hi Everyone,
>  > >>
>  > >> I am consider such a setup where I export an iSCSI target to a Xen
> node.
>  > >> This Xen node will then use the iSCSI block device as an LVM PV, and
>  > >> create lots of LVs for DomU use.
>  > >>
>  > >> I was wondering if anyone could make me aware of any special
>  > >> consideration I would need to take. I've posted a similar question to
>  > >> the LVM list to ask for further tips more specific to LVM.
>  > >>
>  > >> Am I barking down the wrong path here? I know it would be very easy to
>  > >> just an NFS server and use image files, but this will be for a large
>  > >> scale DomU hosting so this isn't really an option. Additionally, if I
>  > >> wanted to make the LVM VG visible to multiple Xen nodes, is it just a
>  > >> matter of running CLVM on each Xen node? Please keep in mind that only
>  > >> one Xen node will be using an LV at any one time (so no need for
> GFS, I
>  > >> believe)
>  > >>
>  > >> Any help or tips would be appreciated
>  > >>
>  > >> Thanks
>  > >>
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>  > >
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>  >
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