[Dovecot] Best Cluster Storage

[Eric Rostetter]

Quoting Stan Hoeppner <stan at hardwarefreak.com>:

> DRBD is for mirroring physical devices over a network.  You might be  
> able to do
> DRBD inside a VM guest, but to what end?  What sense does it make to do so?

It doesn't really make sense, and it can cause problems...  What problems
depends on your VM implementation (Xen, KVM, VMWare, VirtualBox, etc).

Some things might be mount table propogation between hosts/guests causing
problem unmounting drbd paritions or even shutting down the VM, running
lots of drbd instances instead of only one (as Stan mentioned) which can
mean more processes and more (buffer) memory being used than is needed and
more configuration files needed, performance issues of all kinds, and so on.

Think about it: you are increasing the number of processes in the VM guests,
you are increasing the amount of memory used in the VM guests,
you are increasing traffic to the virtual switch/bridge, you are potentially
increasing the complexity of your configuration, possibly taking a big
performance hit (depending on VM type and config), limiting your flexibiilty,
increases difficulty of debugging and performance tuning, and so on.  Is it
worth it?

Plus, if you run DRBD in the VM, then the VM must run DRBD.  If you run
DRBD in the physical host, you can then export it to any VM, even a VM
that doesn't support DRBD.  Things like this can impact VM flexibility
(migrations, OS support, backups, etc).

Can you do it? Yes.  Can you get away with it? Probably.  Should you do
it? No.  Would I do it? Never...

-- 
Eric Rostetter
The Department of Physics
The University of Texas at Austin

Go Longhorns!