From mboxrd@z Thu Jan 1 00:00:00 1970 From: Adam Goryachev Subject: Re: clustered MD - beyond RAID1 Date: Tue, 22 Dec 2015 09:29:08 +1100 Message-ID: <56787D34.7020108@websitemanagers.com.au> References: <56742652.5040304@nasa.gov> <87si2w66tm.fsf@notabene.neil.brown.name> <567850C4.30108@bnl.gov> <87bn9j4jhr.fsf@notabene.neil.brown.name> <56786EA4.2020209@bnl.gov> <8737uv4fz6.fsf@notabene.neil.brown.name> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <8737uv4fz6.fsf@notabene.neil.brown.name> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown , Tejas Rao , Scott Sinno , linux-raid@vger.kernel.org Cc: "Knister, Aaron S. (GSFC-606.2)[COMPUTER SCIENCE CORP]" List-Id: linux-raid.ids On 22/12/15 09:03, NeilBrown wrote: > On Tue, Dec 22 2015, Tejas Rao wrote: > >> On 12/21/2015 15:47, NeilBrown wrote: >>> On Tue, Dec 22 2015, Tejas Rao wrote: >>> >>>> What if the application is doing the locking and making sure that only 1 >>>> node writes to a md device at a time? Will this work? How are rebuilds >>>> handled? This would be helpful with distributed filesystems like >>>> GPFS/lustre etc. >>>> >>> You would also need to make sure that the filesystem only wrote from a >>> single node at a time (or access the block device directly). I doubt >>> GPFS/lustre make any promise like that, but I'm happy to be educated. >>> >>> rebuilds are handled by using a cluster-wide lock to block all writes to >>> a range of addresses while those stripes are repaired. >>> >>> NeilBrown My understanding of MD level cross host RAID was that it would not magically create cluster aware filesystems out of non-cluster aware filesystems. ie, you wouldn't be able to use the same multi-host RAID device on multiple hosts concurrently with ext3. IMHO, if it was able to behave similar to DRBD, then that would be perfect (ie, enforce only a single node can write at a time (unless you specifically set it for multi-node write)). The benefit should be that you can lose a node without losing your data. After you lose that node, you can then "do something" to use the remaining node to access the data (eg, mount it, export with iscsi/nfs, etc). Currently, this is what I use DRBD for, previously, I've used NBD + MD RAID1 to do the same thing. One question though is what advantage multi-host MD RAID might have over the existing in-kernel DRBD ? Are there plans which show why this is going to be better, have better performance, features, etc? Regards, Adam -- Adam Goryachev Website Managers www.websitemanagers.com.au