[pve-devel] [RFC guest-common 09/13] vzdump: schema: add fleecing property string

Fiona Ebner f.ebner at proxmox.com
Thu Feb 1 14:11:20 CET 2024

Am 01.02.24 um 13:39 schrieb DERUMIER, Alexandre:
>>> LVM and non-sparse ZFS need enough space for a copy for the full disk
>>> up-front, so are not suitable as fleecing storages in many cases.
> can't we force sparse for theses fleecing volumes, even if the storage
> don't have sparse enabled ? (I can understand that it could make sense
> for user to have non sparse for production for performance or
> allocation reservation, but for fleecing image, it should be
> exceptionnal to rewrite a full image)

For ZFS, we could always allocate fleecing images sparsely, but would
require a change to the storage API as you can't tell vdisk_alloc() to
do that right now. There could also be a new helper altogether,
allocate_fleecing_image() then the storage plugin itself could decide
what the best settings are.

>>> Should the setting rather be VM-specific than backup job-specific?
>>> These issues
>>> mostly defeat the purpose of the default here.
> can't we forbidden it in storage plugin features ? { fleecing => 1} ?

There is no feature list for storage plugins right now, just
volume_has_feature() and that doesn't help if don't already have a volume.

There is storage_can_replicate() and we could either switch to a common
helper for storage features and deprecate the old or simply add a
storage_supports_fleecing() helper.

But the question remains if the setting should be VM-specific or
job-wide. Most flexible would be both, but I'd rather not overcomplicate
things. Maybe my idea for the default with "use same storage for
fleecing" is not actually a good one and having a dedicated storage for
fleecing is better. Then it needs to be a conscious decision.

>>> IIRC older version of NFS lack the ability to discard. While not
>>> quite
>>> as bad as the above, it's still far from ideal. Might also be worth
>>> trying to detect? Will add something to the docs in any case.
> I never have seen working discard with nfs, I think (never tested) it's
> possible with 4.2, but 4.2 is really new on nas appliance (netapp,...).
> So I think than 90% of user don't have working discard with nfs.

With NFS 4.2 discard works nicely even with raw format. But you might be
right about most users not having new enough versions. We discussed this
off-list too and an improvement would be to use qcow2, so the discards
could happen at least internally. The qcow2 could not free the allocated
blocks, but re-use already allocated ones.

> Is it a problem if the vm main storage support discard , but not
> fleecing storage ? (I don't have looked yet how exactly fleecing is
> working)

It doesn't matter if the main storage does or not. It only depends on
the fleecing storage.

> If it's a problem, I think we should forbind to use a fleecing storage
> not supporting discard, if the vm have discard on 1 disk.

The problem is that the space usage can be very high. It's not a
fundamental problem, you can still use fleecing on such storages if you
have enough space.

There are already ideas to have a limit setting, monitor the space usage
and abort when the limit is hit, but nothing concrete yet.

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