From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sebastian Riemer Subject: Re: Simultaneous assembly on multiple hosts Date: Mon, 06 May 2013 11:05:34 +0200 Message-ID: <5187725E.5050803@profitbricks.com> References: <20130504020146.GD31411@chc-5.metaname.net> <20130506163203.7a65786b@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130506163203.7a65786b@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: Neil Matai , linux-raid@vger.kernel.org List-Id: linux-raid.ids On 06.05.2013 08:32, NeilBrown wrote: > On Sat, 4 May 2013 14:01:46 +1200 Neil Matai > wrote: > >> Hi all, >> >> Is it dangerous have an array assembled on two hosts at the same >> time? Say I have servers A and B exporting devices a and b via >> ATA over Ethernet or similar to hosts C and D. If a and b are >> components of an array then both C and D automatically assemble >> the array as soon as the components a and b are exported but is >> this safe (on the proviso that only one of C or D is ever writing >> to the array at any time and that there is no write-behind >> caching above the array device)? >> > > Yes, it is dangerous. Both hosts will try to update the metadata > and will trip over each other. This is why we've implemented automatic super block + bitmap reload upon first IO. Before, the MD device acts as a read-only device with read-only super block. This is rather complex to make it safe. This is also difficult to merge with mainline behavior. Get yourself a kernel developer. ;-) Cheers, Sebastian