Re: [Xen-API] Alternative to Vastsky?

[Tim Titley <tim@xxxxxxxxxx>]

On 20/04/11 01:51, George Shuklin wrote:
> I see no problem with split brains in case of DRBD between two XCP
> hosts (with DRBD between local drive in fist XCP host and second drive
> via network on second XCP host). XCP assure there is no two copy of
> same VM running in pool (we talking about XCP, not xend?). If some
> pool suddenly go offline or disconnected (same thing), you must
> manually say vm-reset-powerstate. I think this kind of protection is
> fairy normal, except it will delay automatic restart in case of
> unexpected host hangup - but in case of XCP this problem exists for
> every storage solution - problem is not with storage but with XCP way
> to detects HOST_OFFLINE (only after long delay XCP will assume host
> down... or never? I still not test this well).
>
> The main sad thing in DRBD is two host limit, but it still better,
> than plain /dev/sd[abcde] for pack of 'mission critical applications
> with new level of performance and effibla-bla-bla'. And (as far as I
> know XCP internals) it have all capabilities (may be with little
> tweaking) to get DRBD support at logic level. We have shared SR with
> two PBD on two hosts. We calculate vm-vbd-vdi-sr-pbd-host paths before
> sending task to slave (start/migrate/evacuate), we accounting them
> before returning calculated ha-avability (forgot exact names).  To
> avoid 'tripple confilct' we allow only one DRBD per host: if A have
> two different DRBD with B and C, B have same with C and A, and C with
> B and A and we create vm with two vdi on fist and second DRBD volumes,
> we lost any way for successful migration (and, in certain meaning,
> loose some redundancy).
I have not considered using a separate DRBD resource for each vdi for 
this very reason however, if you are sticking to a simple paired host 
setup there are advantages in putting an LVM storage repository on top 
of one large DRBD disk mirrored between both hosts (assuming you don't 
loose network connectivity).
> Than you for reply about two iscsi target for same drbd... I have a
> little doubts about data consistency due iscsi queue...
I never had a problem, probably because it was setup for fail-over not 
performance. I would be inclined to agree with you.
> The last: I DO really wants to see 2.6.38+ in XCP. In 2.6.38 Red Hat
> has add support for blkio-throttle in this version - most wanted
> feature for dom0 - its allow to shape IOPS and bandwidth for every
> process separately (this means 'for every VM'). We have (not very
> good, but working) traffic shaper, so disk shaper is very actual too...
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