Re: [Xen-API] [xs-devel] lvmthin

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Fri, Jan 09, 2015 at 03:59:26PM -0700, Tobias Kreidl wrote:
>    So sorry, I meant of course to write that the utility is lvmthin (not
>    thinlvm).
>

Hello,

I *think* the lvmthin stuff is currently designed for single-host systems,
where the storage is used by a single dedicated host only. That's the usual 
model for LVM aswell.

If multiple hosts access the same LVM volumes one needs to make sure the LVM 
metadata changes are properly synchronized across all hosts,
so all the hosts sharing the same physical LUNs/PVs/VGs/LVs have exactly the 
same idea of the LVM settings, and I believe XAPI does this for normal (thick) 
LVM volumes.

But it gets much more difficult with thin volumes, because the "metadata" can 
change on *every* write IO (when one needs to allocate more blocks from the 
thin pool?).

Note I haven't looked at the internals of LVM thinprovisioning, but I assume 
it'll be more difficult to get working in shared-LUN/multi-host environments 
compared to normal thick-LVM.

I guess there would have to be some "free-blocks-per-host-for-thin-volumes", so 
each host would allocate new blocks from its dedicated pool, without the risk 
of corrupting the LVM PVs or the volumes.. and avoiding the need of syncing 
metadata on every write IO?


Just random thoughts :)

-- Pasi
 
>    On 1/9/2015 3:54 PM, Tobias Kreidl wrote:
> 
>      As of around CentOS 6.5 (and for sure in RHEL 6.4) that there is a
>      thinlvm lvmthin utility that seems to take care of supporting
>      thinly-provisioned LVM volumes. There is an associated snapshot design
>      based on LVM thin provisioning that even supports the ability to do
>      snapshots of snapshots, etc. down the chain.
> 
>      Unfortunately, from a cursory look, it doesn't look like a back port to
>      CentOS 5 would be that easy or even possible, but that it is integrated
>      into both CentOS 6 and CentOS 7 gives some hope for a possibly
>      standardized support of it in a future XenServer release, doesn't it?
>      The other big question would be how readily something like this could be
>      integrated into XenServer.
> 
>      -=Tobias

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