Cc: libvir-list for review of the compatibility argument below. Daniel Henrique Barboza <danielhb413@xxxxxxxxx> writes: > Hey David, > > On 9/21/18 9:29 AM, Dr. David Alan Gilbert wrote: >> * Daniel Henrique Barboza (danielhb413@xxxxxxxxx) wrote: >>> changes in v2: >>> - removed the "RFC" marker; >>> - added a new patch (patch 2) that removes >>> bdrv_snapshot_delete_by_id_or_name from the code; >>> - made changes in patch 1 as suggested by Murilo; >>> - previous patch set link: >>> https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg04658.html >>> >>> >>> It is not uncommon to see bugs being opened by testers that attempt to >>> create VM snapshots using HMP. It turns out that "0" and "1" are quite >>> common snapshot names and they trigger a lot of bugs. I gave an example >>> in the commit message of patch 1, but to sum up here: QEMU treats the >>> input of savevm/loadvm/delvm sometimes as 'ID', sometimes as 'name'. It >>> is documented as such, but this can lead to strange situations. >>> >>> Given that it is strange for an API to consider a parameter to be 2 fields >>> at the same time, and inadvently treating them as one or the other, and >>> that removing the ID field is too drastic, my idea here is to keep the >>> ID field for internal control, but do not let the user set it. >>> >>> I guess there's room for discussion about considering this change an API >>> change or not. It doesn't affect users of HMP and it doesn't affect Libvirt, >>> but simplifying the meaning of the parameters of savevm/loadvm/delvm. >> Can you clarify a couple of things: >> a) What is it about libvirt's use that means it's OK ? Does it never >> use numeric tags? > > I am glad you asked because my understanding in how Libvirt was dealing > with snapshots was wrong, and I just looked into it further to answer your > question. Luckily, this series fixes the situation for Libvirt as well. > > I was misled by the fact that Libvirt does not show the same symptoms > we see in > QEMU of this problem, but the bug is there. Here's a quick test with > Libvirt with > "0" and "1" as snapshot names, considering a VM named with no snapshots, > using QEMU 2.12 and Libvirt 4.0.0: > > - create the "0" snapshot: > > $ sudo virsh snapshot-create-as --name 0 dhb > Domain snapshot 0 created > > $ sudo virsh snapshot-list dhb > Name Creation Time State > ------------------------------------------------------------ > 0 2018-09-24 15:47:56 -0400 running > > $ sudo virsh qemu-monitor-command dhb --hmp info snapshots > List of snapshots present on all disks: > ID TAG VM SIZE DATE VM CLOCK > -- 0 405M 2018-09-24 15:47:56 00:04:20.867 > > > - created the "1" snapshot. Here, Libvirt shows both snapshots with > snapshot-list, > but the snapshot was erased inside QEMU: > > $ sudo virsh snapshot-create-as --name 1 dhb > Domain snapshot 1 created > $ > $ sudo virsh snapshot-list dhb > Name Creation Time State > ------------------------------------------------------------ > 0 2018-09-24 15:47:56 -0400 running > 1 2018-09-24 15:50:09 -0400 running > > $ sudo virsh qemu-monitor-command dhb --hmp info snapshots > List of snapshots present on all disks: > ID TAG VM SIZE DATE VM CLOCK > -- 1 404M 2018-09-24 15:50:10 00:05:36.226 > > > This is where I stopped checking out Libvirt at first, concluding > wrongly that it > was immune to the bug. > > Libvirt does not throw an error when trying to apply snapshot 0. In > fact, it acts > like everything went fine: > > $ sudo virsh snapshot-revert dhb 0 > > $ echo $? > 0 Is that a libvirt bug? > Reverting back to snapshot "1" works as intended, restoring the VM > state when it > was created. > > > (perhaps this is something we should let Libvirt be aware of ...) > > > > This series fixes this behavior because the snapshot 0 isn't erased > from QEMU, allowing > Libvirt to successfully restore it. > > >> b) After this series are you always guaranteed to be able to fix >> any existing oddly named snapshots? > > The oddly named snapshots that already exists can be affected by the > semantic > change in loadvm and delvm, in a way that the user can't load/del > using the snap > ID, just the tag. Aside from that, I don't see any side effects with > existing > snapshots and this patch series. Do all snapshots have a tag that is unique within their image? Even snapshots created by old versions of QEMU? -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
