[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>  

> On 08/20/2009 05:46 PM, Coert Waagmeester wrote:
>> Xen DomU
>> ============
>> ============
>> 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.


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