[Kimchi-devel] [RFC] Do not block operations due to ref_cnt
vianac at linux.vnet.ibm.com
Mon Mar 30 23:03:28 UTC 2015
I'd like to propose a change to how Kimchi uses the resource field
Currently, "ref_cnt" - which stands for "reference count" - is one of
the fields returned when looking up a storage volume. Its purpose is to
indicate how many times that resource is being used at the moment. For
example, if the resource /storagepools/pool/storagevolumes/vol has
ref_cnt=1, it means that the disk is attached to 1 VM right now and thus
it cannot be attached to another VM. I believe the original idea of this
feature is to prevent the same resource from being attached more than
once at the same time. However, IMO, that might not always be the
desired behavior and there's no way to enforce it completely as those
resources can be used outside of Kimchi, where "ref_cnt" doesn't exist.
For example, if I have one VM which uses the disk "vol", I'm not able to
attach it to another VM via Kimchi; but if I use another libvirt-based
VM manager (e.g. virsh), I am able to attach that disk to a different
VM. This becomes even trickier when we consider other operations, like
snapshots, which can attach/detach disks while they're being reverted
to. Also, suppose I might want to inspect one VM's disk from another VM,
and then I'd need to attach one disk twice; Kimchi wouldn't allow that
by stating that the disk is already in use.
I propose Kimchi should stop using "ref_cnt" as a blocking method. The
field may still exist for information/warning messages (e.g. "are you
sure you want to attach this disk? it's already being used by another
VM.") but no operation should be blocked because of it, as it is the
case now. As inconsistencies with that value may happen and we have no
way to make sure it will always work, we shouldn't annoy the user by
stopping them from doing something that may be perfectly valid.
Any feedback will be very welcome.
More information about the Kimchi-devel