Re: raid over ethernet

On Sat, Jan 29, 2011 at 09:54:55PM +0000, John Robinson wrote:

On 29/01/2011 21:08, Alexander Schreiber wrote:

quoted

On Sat, Jan 29, 2011 at 12:23:14PM -0200, Denis wrote:

quoted

2011/1/29 Alexander Schreiber[off-list ref]

quoted

plain disk performance for writes, while reads should be reasonably
close to the plain disk performance - drbd optimizes reads by just reading
from the local disk if it can.

 However, I have not used it with active-active fashion. Have you? if yes,
what is your overall experience?

We are using drbd to provide mirrored disks for virtual machines running
under Xen. 99% of the time, the drbd devices run in primary/secondary
mode (aka active/passive), but they are switched to primary/primary
(aka active/active) for live migrations of domains, as that needs the
disks to be available on both nodes. From our experience, if the drbd
device is healthy, this is very reliable. No experience with running
drbd in primary/primary config for any extended period of time, though
(the live migrations are usually over after a few seconds to a minute at
most, then the drbd devices go back to primary/secondary).

Now that is interesting, to me at least. More as a thought
experiment for now, I was wondering how one would go about setting
up a small cluster of commodity servers (maybe 8 machines) running
Xen (or perhaps now KVM) VMs, such that if one (or potentially two)
of the machines died, the VMs could be picked up by the other
machines in the cluster, and only using locally-attached SATA/SAS
discs in each machine.

I guess I'm talking about RAIN or RAIS rather than RAID so maybe I'd
better start reading the Wikipedia pages on those and not talk about
it on this list...

For the "survive single node total machine failure" case your problem has
already been solved: http://code.google.com/p/ganeti/

We run a large number of clusters with that and the VMs routinely survive
disk failures and recover (come back from what looks like a power failure
to the VM) from node failure.

Kind regards,
           Alex.
-- 
"Opportunity is missed by most people because it is dressed in overalls and
 looks like work."                                      -- Thomas A. Edison