[Dovecot] Best Cluster Storage

[Jonathan Tripathy]

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.