On Thu, 18 Oct 2012 10:01:34 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote: > On Thu, Oct 18, 2012 at 12:29:59PM +1100, NeilBrown wrote: > > On Thu, 18 Oct 2012 09:17:35 +0800 Shaohua Li <shli@xxxxxxxxxx> 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? 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 ?? 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. NeilBrown
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