From mboxrd@z Thu Jan 1 00:00:00 1970 From: Phil Turmel Subject: Re: [BUG] non-metadata arrays cannot use more than 27 component devices Date: Sat, 25 Feb 2017 19:07:06 -0500 Message-ID: <657e80e9-b1f5-1f58-a4d0-6cbc4cc44927@turmel.org> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <58B21987.6060604@youngman.org.uk> Sender: linux-raid-owner@vger.kernel.org To: Wols Lists , ian_bruce@mail.ru, linux-raid@vger.kernel.org List-Id: linux-raid.ids On 02/25/2017 06:55 PM, Wols Lists wrote: > On 25/02/17 23:41, Phil Turmel wrote: >>> Is there a sound technical reason not to go there, or is it simply a >>>> case of "learn another tool for that job"? The less tools I have to know >>>> the better, imho. > >> Um, no, imnsho. Learn new tools when you need them. > > I don't have a problem with that. All too often people use the tool > they're familiar with when it's the wrong tool. But there's a reason > they do that - it's a familiar tool! >> >> Linux raid has no formal mechanism to cleanly separate a mirror from a >> running array, access it as a backup, and not risk corruption when >> re-attaching it to the array. Most filesystems write to the partition >> when mounting, even for read-only mounts. You cannot safely access the >> disconnected member except via pure block reads. > > Because to do so doesn't make sense? Or because nobody's bothered to do > it? I get grumpy when people implement corner cases without bothering to > implement the logically sensible options - bit like those extremely > annoying dialog boxes that give you three choices, "yes", "no", "yes to > all". What about no to all? Because while disconnected, and the array begins accumulating write-intent bits indicating where any disconnected device is out of date, the array has no way to know what writes are happening to that member. And therefore any re-add will introduce unknowable corruptions. There is no way to control what writes happen to that member, and drives don't naturally keep a log of writes that have happened. The data to safely do what you want simply doesn't exist. Your only known safe choice is to disable write-intent bitmaps, forcing complete resync on --re-add. > I feel like mirror-raid is perfect for doing backups. Your feelings are wrong. Sorry. LVM is the perfect tool because it entirely controls the snapshot and doesn't have to re-add it. > I take your point > that linux hasn't implemented that feature (particularly well), but > surely it's a feature that *should* be there. I know I know - "patches > welcome" :-) Good luck creating the necessary data from thin air. It's not a question of writing patches. Phil