On Fri, Oct 19, 2007 at 05:42:09PM -0400, Doug Ledford wrote: > The simple fact of the matter is there are only two type of raid devices > for the purpose of this issue: those that fragment data (raid0/4/5/6/10) > and those that don't (raid1, linear). > > For the purposes of this issue, there are only two states we care about: > the raid array works or doesn't work. Yes, but "doesn't work" doesn't mean only that the array fails to start. > If the raid array works, then you *only* want the system to access the > data via the raid array. If the raid array doesn't work, then for the > fragmented case you *never* want the system to see any of the data from > the raid array (such as an ext3 superblock) or a subsequent fsck could > see a valid superblock and actually start a filesystem scan on the raw > device, and end up hosing the filesystem beyond all repair after it hits > the first chunk size break (although in practice this is usually a > situation where fsck declares the filesystem so corrupt that it refuses > to touch it, that's leaving an awful lot to chance, you really don't > want fsck to *ever* see that superblock). Honestly, I don't see how a properly configured system would start looking at the physical device by mistake. I suppose it's possible, but I didn't have this issue. > If the raid array is raid1, then the raid array should *never* fail to > start unless all disks are missing (in which case there is no raw device > to access anyway). The very few failure types that will cause the raid > array to not start automatically *and* still have an intact copy of the > data usually happen when the raid array is perfectly healthy, in which > case automatically finding a constituent device when the raid array > failed to start is exactly the *wrong* thing to do (for instance, you > enable SELinux on a machine and it hasn't been relabeled and the raid > array fails to start because /dev/md<blah> can't be created because of > an SELinux denial...all the raid1 members are still there, but if you > touch a single one of them, then you run the risk of creating silent > data corruption). It's not only about the activation of the array. I'm mostly talking about RAID1, but the fact that migrating between RAID1 and plain disk is just a few hundred K at the end increases the flexibility very much. With superblock at the start, you can't decide to convert a plain disk to RAID1 without shifting all data, with the superblock at the end it's perfectly possible. Also, sometime you want to recover as much as possible from a not intact copy of the data... Of course, different people have different priorities, but as I said, I like that this conversion is possible, and I never had the case of a tool saying "hmm, /dev/md<something> is not there, let's look at /dev/sdc instead". thanks, iustin - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
