[Lustre-discuss] DRBD + active/active OST, again

[Johann Lombardi]

On Tue, Mar 02, 2010 at 01:09:51PM +0100, Andrew Godziuk wrote:
> Has anything changed in Lustre 1.8 that would make it possible to set
> up two OSS with an OST shared using DRBD, in an active-active
> configuration?

No, a lustre target (OST & MDT) must *never* be active on more than 1 server
at a time and we have no plan to change this. Mounting the same OST on 2
different OSSs results in massive corruptions most of the time.

> I have mounted a shared OST on two OSS nodes, none of them marked with
> "--failover", and it looked as if it was working,

In 1.6, we introduced MMP (stands for multiple mount protection) to prevent
mounting the same target twice. However, we enable MMP in mkfs.lustre only
when a failover node is specified, that's why you are able to mount the
OST twice.

> but I didn't do any stress tests for reliablility. Does such setup ever
> have a chance to work for real or did it only look as if everything was OK?

No, this has no chance to work. For now, all clients are probably still
connected to the OST through the same OSS, but as soon as one client reconnects
to the OST through the 2nd OSS (e.g. after some request timeouts), the two OSSs
will concurrently write to the same ldiskfs filesystem and corrupt it.

Johann