From mboxrd@z Thu Jan 1 00:00:00 1970 From: Shaohua Li Subject: Re: [RFC 1/2]raid1: only write mismatch sectors in sync Date: Wed, 31 Oct 2012 11:25:33 +0800 Message-ID: <20121031032533.GA1487@kernel.org> References: <20120918145710.55394bd4@notabene.brown> <20120919055106.GA1305@kernel.org> <20120919171646.6bc35ba5@notabene.brown> <20120920015655.GB6798@kernel.org> <20121017051113.GA17821@kernel.org> <20121018095601.7aa3238b@notabene.brown> <20121018011735.GA1448@kernel.org> <20121018122959.10bb6c87@notabene.brown> <20121018020134.GB1448@kernel.org> <20121018133657.1bd012f6@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20121018133657.1bd012f6@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Thu, Oct 18, 2012 at 01:36:57PM +1100, NeilBrown wrote: > On Thu, 18 Oct 2012 10:01:34 +0800 Shaohua Li wrote: > > > On Thu, Oct 18, 2012 at 12:29:59PM +1100, NeilBrown wrote: > > > On Thu, 18 Oct 2012 09:17:35 +0800 Shaohua Li wrote: > > > > > > > > > Neil, > > > > > > any further comments on this? This is a usable feature, I hope we can have some > > > > > > agreements. > > > > > > > > > > You still haven't answered my main question, which possibly means I haven't > > > > > asked it very clearly. > > > > > > > > > > You are saying that this new behaviour should not be the default and I think > > > > > I agree. > > > > > So the question is: how it is selected? > > > > > > > > > > You cannot expect the user to explicitly enable it any time a resync or > > > > > recovery starts that should use this new feature. You must have some > > > > > automatic, or semi-automatic, way for the feature to be activated, otherwise > > > > > it will never be used. > > > > > > > > > > I'm not asking "when should the feature be used" - you've answered that > > > > > question a few time and it really isn't an issue. > > > > > The question it "What it the exact process by which the feature is turned on > > > > > for any particular resync or recovery?" > > > > > > > > So you worried about users don't know how to correctly select the feature. An > > > > experienced user knows this, the usage scenario I mentioned describes how to do > > > > the decision. For example, a resync after system crash should enable the > > > > feature. I admit an inexperienced user doesn't know how to select it, but this > > > > isn't a big problem to me. There are a lot of tunables in the kernel (even MD), > > > > which can significantly impact kernel behavior. These tunables are just for > > > > experienced users. > > > > > > > > Thanks, > > > > Shaohua > > > > > > > > > You still aren't answering my question. > > > > > > What exactly, precisely, specifically, will an "experienced user" do? > > > > Set something to a sysfs entry to enable the feature (like my RFC patch does to > > have a new sysfs entry for the feature), and readd disk. resync then does 'only > > write mismatch data'. Is this what you asked? sorry for the delay. > Yes, that is the sort of thing I was asking for. > When you say "readd disk" I assume you mean to use the --readd option to > mdadm. > The only works when there is a bitmap active on the array, so relatively few > blocks will be resynced so does it really matter which approach is taken? > Always copy, or read-and-test? > > Though maybe you really mean to "--add" the device. In that case it would > probably make sense to add some other option to mdadm to say "enable > read-mostly recovery". I wonder what a good name would be. > --minimize-writes ?? Yep, it's '--add' case. For the '--readd' with bitmap case, bitmap can already avoid a lot of write already. The useage case is something like: one disk is broken; trim whole disk of a new disk; add the new disk If source disk has a lot of 0 and we only write mismatch data, we can avoid write a lot. I believe we need such mechanism for '--create' too, if the first disk has some data, but the second disk is empty. > You earlier gave a list of scenarios in which you thought this would be > useful. It was: > > > > > For 'compare and avoid write if equal' case: > > > > 1. update SSD firmware. This doesn't change the data, but we need take one disk > > > > off from the raid one time. > > > > 2. One disk has errors, but these errors don't ruin most of the data (for > > > > example, a pcie error) > > > > 3. driver/os crash. > > > > In all these cases, two raid disks must be resync, and they have almost identical > > > > data. write avoidness will be very helpful for these. > > > For case '3', it would be a "resync" rather than a "recovery". How would you > expect an "advanced user" to choose read-and-test recovery in that case? > There is no "readd" command happening. If there is bitmap, maybe we don't need do read-and-test, so this one isn't very necessary in current stage. If not, what I suggested is: 1. user suspends resync (write something to a sysfs file) 2. user enables read-and-test (again, write a sysfs file) 3. resume resync Thanks, Shaohua