[Dovecot] Best Cluster Storage

[Eric Shubert]

On 01/14/2011 03:58 PM, Jonathan Tripathy wrote:
>
> On 14/01/11 19:00, Stan Hoeppner wrote:
>> Jonathan Tripathy put forth on 1/13/2011 4:17 PM:
>>
>>> Regarding the servers, I was thinking of having a 2 node drbd cluster
>>> (in
>>> active+standby), which would export a single iSCSI LUN. Then, I would
>>> have a 2
>>> node dovecot+postfix cluster (in active-active), where each node
>>> would mount the
>>> same LUN (With GFS2 on top). This is 4 servers in total (Well, 4 VMs
>>> running on
>>> 4 physically separate servers).
>> Something you need to consider very carefully:
>>
>> drbd is a kernel block storage driver. You run in ON a PHYSICAL
>> cluster node,
>> and never inside a virtual machine guest. drbd is RAID 1 over a
>> network instead
>> of a SCSI cable. Is is meant to protect against storage and node
>> failures.
>> This is how you need to look at drbd. Again, DO NOT run DRBD inside of
>> a VM
>> guest. If you have a decent background in hardware and operating
>> systems, it
>> won't take you 30 seconds to understand what I'm saying here. If it
>> takes you
>> longer, then consider this case:
>>
>> You have a consolidated Xen cluster of two 24 core AMD Magny Cours
>> servers each
>> with 128GB RAM, an LSI MegaRAID SAS controller with dual SFF8087 ports
>> backed by
>> 32 SAS drives in external jbod enclosures setup as a single hardware
>> RAID 10.
>> You spread your entire load of 97 virtual machine guests across this
>> two node
>> farm. Within this set of 97 guests, 12 of them are clustered network
>> applications, and two of these 12 are your Dovecot/Postfix guests.
>>
>> If you use drbd in the way you currently have in your head, you are
>> mirroring
>> virtual disk partitions with drbd _SIX times_ instead of once. Here,
>> where
>> you'd want to run drbd is within the Xen hypervisor kernel. drbd works
>> at the
>> BLOCK DEVICE level, not the application layer.
>>
>> Eric already mentioned this once. Apparently you weren't paying
>> attention.
>>
> I'm sorry I don't follow this. It would be appreciated if you could
> include a simpler example. The way I see it, a VM disk is just a small
> chunck "LVM LV in my case" of a real disk.
>

Perhaps if you were to compare and contrast a virtual disk to a raw 
disk, that would help. If you wanted to use drbd with a raw disk being 
accessed via a VM guest, that would probably be all right. Might not be 
"supported" though.

-- 
-Eric 'shubes'