Re: RFC: qemu: use uuid instead of name for misc filenames

On 27.02.2020 16:48, Daniel P. Berrangé wrote: > On Thu, Feb 27, 2020 at 03:57:04PM +0300, Nikolay Shirokovskiy wrote: >> Hi, everyone. >> >> I'm working on supporting domain renaming when it has snapshots which is not >> supported now. And it strikes me things will be much simplier to manage on >> renaming if we use uuid in filenames instead of domain names. >> >> 1. Renaming will only involve saving updated config. >> The saving is atomic thanx to tmp file and rename(2) approach. In constast >> current renaming on error paths can leave config with old or new name. Thus >> on libvirt restart extra VM will appear. >> >> And we don't need to rename autostart links, snapshot directories etc. > > Yes, renaming is hard due to the non-atomic nature of the problem. > It ought to be possible to recover/rollback from all but the most > serious failure scenarios though. Problems in the most serious > scenarios, are likely to cause failures in other parts of libvirt > already, such as existing VM shutdown/startup. > > >> 2. Renaming will be possible for running domains with no efforts. >> We only need to pass uuid instead of name in '-name guest=...' command line. > > Passing a uuid for the -name arg would be incorrect. This is a user > visible string, for example displayed in the SDL window title or > the VNC display name. UUIDs are not desirable for exposure to users, > they are for machine usage. Yes, this is not convinient. The broader idea was to stop passing name on command line as we can't change it after process is started. So instead we can omit name parameter altogether and set/update name thru QMP.

Also with domain name in path renaming of an active domain looks really messy to implement.

>> 3. Mgmt can stop using autogenerated names for domains. >> I guess openstack for example uses names like instance-000002ff because we >> have many limitations on domain renaming. And if these limitations are removed >> then openstack can just use user supplied names for domains. > > In openstack the instance names are unique within the scope of a single > project. > > In Libvirt the VM names are unique within the scope of a single libvirt > driver connection (effectively a single host). > > Even if libvirt didn't use the name for on disk files, it will still have > the requirement for unique names per-host in libvirt. > > This is the key reason why openstack uses "instance-$HEX" as the libvirt > guest name. > > It could perhaps use "projectname-guestname" as the name, but i'm not > sure there's much appetite for change in this respect, as it is a long > standing convention now. > >> 4. No issues with long domain names and filename length limit >> >> If the above conversion makes sense I guess the good time to apply it is >> on domain start (and rename to support renaming with snapshots). > > The above has not considered the benefit that using the VM name > has. Essentially the UUID is good for machines, the VM name is > good for humans. Seeing the guest XML files, or VM log files > using a filename based on UUID instead of name is a *really* > unappealing idea to me. I agree. But we can also keep symlinks with domain names for configs/logs etc This can be done as a separate tool as I suggested in the letter or maintain symlinks always. The idea is failing in this symlinking won't affect daemon functionality as symlinks are for humans)