On Mon, Sep 14 2009 at 1:21pm -0400, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > Hi > > When lvconvert -M --namesnapshot is invoked, there is a bug, it ends up > with a message like > device-mapper: snapshots: exception for block 0 is on disk but not in > memory > > The bug is not in the kernel but in the lvm. It loads -real and -cow > devices badly, so that the kernel treats the origin volume as the > snapshot. > > The --namesnapshot option causes that the snapshot becomes the origin and > mergin starts, if we have > lv-origin owi-a- origin > lv-snapshot swi--- snapshot > , after lvconvert -M --namesnapshot vg/lv-snapshot, we'll have > lv-snapshot Owi-a- mergin snapshot, containing the previous content of > lv-snapshot > > The trick that needs to be done is to preserve the logical volume > structure and the device, but move it from one storage to another. Sadly, > LVM2 is not designed to do it. That --pivotorigin option that Alasdair > implemented needed the same trick and it was buggy too. > > I'd suggest to remove --namesnapshot option at all (and remove patches > lvm-merge-swap-uuid.patch and > lvm-merge-swap-lvid-of-underlying-devices.patch that do most of this > stuff) and leave only --nameorigin and --onactivate. > > The implementation of --namesnapshot is very dirty. I was writing that > option --namesnapshot for a month, implemented it three times before > getting it right. And now some other changes to lvm made it nonworking > again. It is unmaintainable over the long term. It is not worth fixing, > because it would be beroken again. > > Other that that, I think that snapshot merging could be applied to the > kernel and lvm. I agree with the above. The two times I rebased those swap uuid patches they were fairly fragile/subtle/challenging. Eliminating --namesnapshot seems reasonable. I updated the LVM2 snapshot-merge patches accordingly: http://people.redhat.com/msnitzer/patches/snapshot-merge/lvm2/LVM2-2.02.52/ -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
