Re: raid over ethernet

On Mon, Jan 31, 2011 at 06:42:44AM -0200, Denis wrote:

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

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).

What filesystem are you using to enable the primary-primary mode? Have
you evaluated it against any other available option?

The filesystem is whatever the VM is using, usually ext3. But the
filesystem doesn't matter in our use case at all, because:
 - the backing store for drbd  are logical volumes
 - the drbd block devices are directly exported as block devices
   to the VMs
The filesystem is only active inside the VM - and the VM is not aware of
the drbd primary/secondary -> primary/primary -> primary/secondary dance
that happens "outside" to enable live migration.

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