[CentOS] drbd xen question
Ross Walker
rswwalker at gmail.com
Fri Aug 21 13:35:34 UTC 2009
On Aug 21, 2009, at 6:27 AM, Karanbir Singh <mail-lists at karan.org>
wrote:
> On 08/20/2009 05:46 PM, Coert Waagmeester wrote:
>> Xen DomU
>> ============
>> DRBD
>> ============
>> LVM Volume
>> ============
>> RAID 1
>>
>
> this makes no sense, you are loosing about 12% of i/o capability
> here -
> even before hitting drbd, and then taking another hit on whatever drbd
> brings in ( depending on how its setup, even on a gigabit link that
> drbd
> load factor can be upto another 15 - 20%, or as low as 2 - 5% )
I don't think LVM adds as much latency as you think it does, tests on
raw partitions show negligible difference between dev-mapper and raw
partitions. It gives a lot of advantages though where you want
multiple phy: disk volumes on the same array.
Now drbd is a different story, it will add latency, a lot for
synchronous replication, a little for asynchronous, but noticeable.
>> What I first wanted to do was:
>>
>> DomU | DRBD
>> ============
>> LVM Volume
>> ============
>> RAID 1
>
> Again, not very clear. Do onething, post some real stuff, eg the proc
> status files for drbd and pvdisplay,gvdisplay and the raid status
> files
> ( whatever raid setup you are using ).
Having a replica per domU will get really complicated really fast
imagine handling dozens of fail-over/split brain scenarios per server,
ouch, forget about it!
If this is for live migration, then I recommend setting up two RAID
arrays one for local only domUs and one for live migration domUs with
active-active replication and synchronous io. If you can't setup two
physical arrays, create two partitions of the existing array. Then
drbd one array/partition, make the drbd device and local partition PVs
and then create your VGs and LVs on them.
If this is for disaster recovery of all domUs then setup a drbd async
replica of the whole array and create LVM on top of that.
-Ross
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