Re: [pve-devel] [PATCH storage 1/1] storage/plugins: pass scfg to parse_volname

[Wolfgang Bumiller <w.bumiller@proxmox.com>]

On Tue, Mar 05, 2024 at 12:13:05PM +0100, Thomas Lamprecht wrote:
> Am 23/02/2024 um 10:24 schrieb Roland Kammerer:
> > This passes the well known $scfg to parse_volname and bumps the API
> > versions accordingly. This allows plugins to access their configuration
> > if necessary.
> 
> We discussed this another time here and effectively it can be fine, while
> the need for it seems like a slight smell from our architecture POV, as
> it basically always means that the VMID -> volume mapping is not encoded
> in the name any more (worse UX), it still does not hurt our, or other
> external, existing plug-ins.
> 
> So fine to add, but please also the parameter also to the base
> "parse_volname" method including a comment that mentions that this
> is in general not used and only required if the storage cannot
> provide all required information in the volume name.

My thoughts on this: (TLDR: we should just merge it and probably also
consider adding a separate method to get the *format* of a volid)

- Adding the parameter itself is fine, not thinking about how/why it is
  used. Generally, it makes sense for all storage API methods to also
  know the storage's config anyway.
- Most (if not all) invocations that actually need the owner vmid (which
  is the part which becomes expensive here) AFAICT are already within a
  more expensive context anyway.
- We have a *lot* of callers which actually only want the disk *format*,
  which IMO means we could introduce a separate storage API call for
  this (this can be backward compatible with a fallback to the parse
  method if the plugin does not provide the new method) 

./API2/LXC/Config.pm:199:                           $format = (PVE::Storage::parse_volname($storage_cfg, $volid))[6];
^ update_vm call, attaching volumes - needs storage access anyway
./API2/LXC.pm:2345:             my $format = (PVE::Storage::parse_volname($storecfg, $source_volid))[6];
./API2/LXC.pm:2368:                 my $fmt = (PVE::Storage::parse_volname($storecfg, $source_volid))[6];
^ both in 'move_volume', storage access already expected
./API2/Qemu.pm:155:             my ($vtype) = PVE::Storage::parse_volname($storecfg, $volid);
^ check_storage_access called from create_vm - creation is already needs
to access the storage anyway and can take different amounts of time
based on storages and is not a hot path anyway
./API2/Qemu.pm:441:             my ($vtype) = PVE::Storage::parse_volname($storecfg, $volid);
^ disk creation... also already expensive and not a hot path...
./API2/Qemu.pm:1640:        $format = (PVE::Storage::parse_volname($storecfg, $volid))[6];
^ update_vm call...
./API2/Qemu.pm:4143:            my $format = (PVE::Storage::parse_volname($storecfg, $source_volid))[6];
./API2/Qemu.pm:4166:                my $fmt = (PVE::Storage::parse_volname($storecfg, $source_volid))[6];
^ both in move_disk... duh :)
./API2/Qemu.pm-4866-        my (undef, undef, undef, undef, undef, undef, $format) =
                                PVE::Storage::parse_volname($storecfg, $drive->{file});
^ resize_vm - also fine...
./CLI/pct.pm-247-               my (undef, undef, undef, undef, undef, undef, $format) =
                                    PVE::Storage::parse_volname($storage_cfg, $volid);
^ in 'pct fsck', fine

Vs the rest:

./API2/LXC.pm-1966-         my (undef, undef, $owner, undef, undef, undef, $format) =
                                PVE::Storage::parse_volname($storage_cfg, $volid);
^ a check in a resize operation (not necessarily cheap anyway)

./API2/Qemu.pm:164:             (my $vtype, undef, $src_vmid) = PVE::Storage::parse_volname($storecfg, $src_image);
./API2/Qemu.pm:242:    my $src_vmid = (PVE::Storage::parse_volname($storecfg, $src_volid))[2];
^ both are part of the import-disk code - certainly not cheap to begin
with

./API2/VZDump.pm:295:	    my (undef, undef, $ownervm) = PVE::Storage::parse_volname($storage_cfg, $volume);
./CLI/pvesm.pm:181:	    my (undef, undef, $ownervm) = PVE::Storage::parse_volname($storage_cfg, $volume);
^ in both cases: permission check in extracting a config from a backup,
so only affects path based storages and isn't particularly cheap anyway


./CLI/pvesr.pm:41:    my ($vtype, undef, $ownervm) = PVE::Storage::parse_volname($storecfg, $volid);
^ replication...
./API2/Storage/FileRestore.pm:201:      my (undef, $snap) = PVE::Storage::parse_volname($cfg, $volid);
^ PBS specific...
./CLI/pve7to8.pm:895:	    ($vtype) = eval { PVE::Storage::parse_volname($storage_cfg, $volid); };
^ happens only on major debian releases...

While within the storage implementations a lot of times the need for it
is storage dependent (eg. path based storages need it because the files
are in vmid-named subdirectories), and besides, it's usually within the
context of doing actual storage operations.

Also, the `volume_is_base_and_used()` call Dietmar mentioned only
happens when destroying or migrating VMs and so I'm not all that worried
about storage access (especially when there's a burst-cache anyway) for
that kind of access anyway...