From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Youngman Subject: Re: [BUG] non-metadata arrays cannot use more than 27 component devices Date: Wed, 1 Mar 2017 19:50:11 +0000 Message-ID: References: <20170224040816.41f2f372.ian_bruce@mail.ru> <41ea334c-ae1c-dac6-e1a1-480d3700a588@turmel.org> <20170224084024.4dfe83a2.ian_bruce@mail.ru> <1e40da0d-b175-9ff5-d2e5-cf1f25aacc26@turmel.org> <58B2137B.6070608@youngman.org.uk> <5172e2ab-e193-477b-52c4-86fbab0d52fe@turmel.org> <58B21987.6060604@youngman.org.uk> <657e80e9-b1f5-1f58-a4d0-6cbc4cc44927@turmel.org> <58B6E287.9020804@youngman.org.uk> <27373f29-a8ea-012c-102d-9fadf05cb3c2@turmel.org> <2d168cc1-cddc-5e0c-610b-97cbd08de621@turmel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <2d168cc1-cddc-5e0c-610b-97cbd08de621@turmel.org> Sender: linux-raid-owner@vger.kernel.org To: Phil Turmel , ian_bruce@mail.ru, linux-raid@vger.kernel.org List-Id: linux-raid.ids On 01/03/17 18:13, Phil Turmel wrote: > On 03/01/2017 12:23 PM, Phil Turmel wrote: >> I strongly disagree. This procedure, as shown, is an admin cock-up: >> >>> mdadm --build /dev/mdbackup --device-count 2 /dev/md/home missing >>> ... hotplug sd-big ... >>> madam /dev/mdbackup --add /dev/sd-big >>> ... wait for sync to finish ... >>> mdadm --stop mdbackup >>> ... unplug sd-big ... > > One more point. The above is functionally identical in every respect > to just: > > # dd if=/dev/md/home of=/dev/sd-big bs=1M > > Why are you bothering to --build an array? > Because - and this is a point the kernel guys seem to forget - the whole point of having a computer system is TO RUN APPLICATIONS, not to run an OS. As it is, you picked up on the fatal flaw I'd spotted, namely that if "home" is mounted, "backup" is going to be corrupt :-( Defeating the entire purpose of my idea, which was to back up a running system without the need to take down the system to ensure integrity. I work with a database that, not unreasonably, seeks to cache loads of stuff in RAM. I've come across far too many horror stories of corrupt backups because the database hadn't flushed its buffers to the OS, so all the database files on disk were inconsistent, giving a corrupt backup. So the idea was set up the mirror, flush/quiesce the database, break the mirror, wake up the database. System disabled for a matter of seconds. It's all very well saying lvm was created with this in mind, but if the system wasn't installed with this originally in mind, you're up a gum tree. My home system is raid but not lvm, for example - how do I back up the system while it's live? (In reality, I don't care :-) IF it didn't have that fatal flaw, my idea would have been able to back up any system. Oh well, it's flawed, time to drop it :-( Cheers, Wol