Hi! Some time ago I had to use snapshots of a filesystem which was not on LVM, so I decided to give dmsetup snapshot/snapshot-origin a try. And discovered that besides the very specific LVM-like scenario, this thing just does not work, - it crashes the kernel after a very simple action. Also there's almost no documentation about this. Hence my question. For a test, I made 2 loop devices of two files, loop0 of 1G and loop1 of 100M. They're not used in any way, just losetup'ed. First to be used as a base image, second is for snapshot/cow data. Next I create snapshot-origin device on top of loop0. Documentation _only_ states I should create a linear mapping first and replace it with a snapshot- origin target while I/O is suspended, and LVM does exactly this, but I thought - since this device isn't used, I'd create snapshot-origin right out of it. Created snapshot-origin /dev/mapper/snaporig device, so far so good. But now, any (write) access to this device instantly crashes the kernel with either reboot or hard lockup, depending on the kernel version. So here come first 2 questions. Is this usage wrong somehow? As far as I can see, this is normal usage (besides replacing the table vs creating new table, and besides not locking I/O), and this is how the whole stack looks like whithout an actual snapshot. And if this usage is wrong, shouldn't kernel forbid it somehow, instead of crashing? As far as I understand, one snapshot-origin can be used for several snapshots (again, this isn't exactly clear from the documentation), so my idea was to create snapshot-origin and mount it mount it first, so I'll be able to create snapshots later as needed. Is it okay to have more than one snapshot out of snapshot-origin, or is it always a 1:1 pair, one snapshot-origin and one snapshot? If it's the latter, again, maybe kernel should refuse to have snapshot-origin without snapshot with non-suspended I/O -- eg, when creating snapshot-origin we can have I/O suspended initially, and refuse to resume it without exactly one snapshot. On the other hand, it seems I can remove snapshot table (after creating and playing with it), and _mounted_ snapshot-origin device continues to operate normally without kernel crashing. This is exactly the same stack as I initially created, the only difference is the way it was created - first time (when it crashes) I created only snapshot-origin and mounted it, the second time I created both snapshot-origin and snapshot on top of it, and removed snapshot later, leaving snapshot-origin only. This works, or at least appears to work - again, from the documentation it is unclear whenever this is supposed to work or not. Another question is about the usage of the original device being snapshotted. In almost all examples it is another DM device, and it is used for _both_ the snapshot and snapshot-origin. For me as a end-user, a more logical would be to have the original device as a base for snapshot-origin (which includes the CoW functionality), and this snapshot-origin device being a base for snapshot(s), - snapshots which receive the CoW data. But it seems both snapshot-origin _and_ snapshot have the original device as a base, - it is not exactly clear how the two communicate with each other this way. Can snapshot be made out of snapshot-origin, or will kernel crash there too? I haven't tested all other possible variants and scenarios, thought I'd ask first how this whole thing is _supposed_ to work. A very useful information about this stuff is found on this 2004 email: https://www.redhat.com/archives/dm-devel/2004-July/msg00071.html This and a few other, very scarce, references exists, but it is difficult to get whole picture.. and it definitely is not clear at all why kernel crashes. To me, dmsetup is a basic command like ls or cp, - and running either of them with wrong argument (even if it is not clear _why_ it's wrong) should not result in the system crashing. Whole thing looks very fragile and untested, after these experiments (in a virtual machine so I wont need to reboot larger box 100 times a hour) I now have a conditioned reflex - I expect the system crash on me after hitting <Enter> on the next dmsetup command, so it is always a heart-breaking thing to actually hit the damn Enter :) And BTW, I actually completed the origina task I had with snapshots, but it took me quite some time and amount of crashes/reboots to find the proper sequence of the commands, and to replicate this sequence on a production system. It worked flawlessly, but I knew that any step aside of this is a sure way to lock up the system. Thanks, /mjt -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
