Re: clustered MD - beyond RAID1

From: NeilBrown <neilb@suse.de>
To: Tejas Rao <raot@bnl.gov>
Cc: Scott Sinno <scott.sinno@nasa.gov>,
	linux-raid@vger.kernel.org, "Knister,
	Aaron S. (GSFC-606.2)[COMPUTER SCIENCE CORP]"
	<aaron.s.knister@nasa.gov>
Subject: Re: clustered MD - beyond RAID1
Date: Tue, 22 Dec 2015 15:13:59 +1100	[thread overview]
Message-ID: <87wps72k8o.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <5678A908.6070401@bnl.gov>

[-- Attachment #1: Type: text/plain, Size: 1413 bytes --]

On Tue, Dec 22 2015, Tejas Rao wrote:

> Each GPFS disk (block device) has a list of servers associated with it. 
> When the first storage server fails (expired disk lease), the storage 
> node is expelled and a different server which also sees the shared 
> storage will do I/O.

In that case something probably could be made to work with md/raid5
using much of the cluster support developed for md/raid1.

The raid5 module would take a cluster lock that covered some region of
the array and would not need to release it until a fail-over happened.
So there would be little performance penalty.

The simplest approach would be to lock the whole array.  This would
preclude the possibility of different partitions being accessed from
different nodes.  Maybe that is not a problem.  If it were, a solution
could probably be found but there would be little point searching for a
solution before a clear need was presented.

>
> In the future ,we would prefer to use linux raid (RAID6) in a shared 
> environment shielding us against server failures. Unfortunately we can 
> only do this after Redhat supports such an environment with linux raid. 
> Currently they do not support this even in an active/passive environment 
> (only one server can have a md device assembled and active regardless).

Obviously that is something you would need to discuss with Redhat.

Thanks,
NeilBrown

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 818 bytes --]