Re: [pve-devel] [PATCH qemu-server 20/22] blockdev: add helpers to generate blockdev commandline

From: Fiona Ebner <f.ebner@proxmox.com>
To: Proxmox VE development discussion <pve-devel@lists.proxmox.com>
Subject: Re: [pve-devel] [PATCH qemu-server 20/22] blockdev: add helpers to generate blockdev commandline
Date: Tue, 17 Jun 2025 12:02:11 +0200	[thread overview]
Message-ID: <a4d7b7ff-741e-4490-bc0b-e16408b6b29e@proxmox.com> (raw)
In-Reply-To: <mailman.393.1750072060.395.pve-devel@lists.proxmox.com>

Am 16.06.25 um 13:07 schrieb DERUMIER, Alexandre via pve-devel:
>>> The 'snapshot' option, for QEMU's snapshot mode, i.e. writes are only
>>> temporary, is not yet supported.
> 
> from qemu manpage:
> 
> "
>        -snapshot
>               Write to temporary files instead of disk image files. In
> this case, the raw disk image you use is not written back. You can
> however force the write back by pressing C-a s (see the Disk Images
> chapter in the System Emulation Users Guide).
> 
>               WARNING:
>                  snapshot is incompatible with -blockdev (instead use
> qemu-img to manually create snapshot images to attach to your
> blockdev).  If you have mixed -blockdev and -drive declarations you can
> use the 'snapshot' property on your drive declarations instead of this
> global
>                  option.
> "
> 
> So, if we want to keep this option. (I never used it to be honest),

I've briefly talked with Fabian about this a while ago. Yes, we do want
to keep it and it will be useful for
https://bugzilla.proxmox.com/show_bug.cgi?id=5187#c3

> I think we should create a temporary qcow2 file at vm start and delete
> it at vm stop.

Yes, we need an overlay. Note that there already is a FIXME comment too ;)

>> +    # FIXME use overlay and new config option to define storage for temp write device
>> +    die "'snapshot' option is not yet supported for '-blockdev'\n" if $drive->{snapshot};

> I'm not sure of current behaviour when you use a block device ? 

Note that what you linked above is for the global '-snapshot' option,
not the drive one. It just doesn't apply to blockdevs AFAIU.

> how qemu known where to write a temp snapshot file ?

It just uses /var/tmp which of course is not ideal.

> maybe we could reuse state storage ? only with a temp qcow2 file ? or
> do we want to reuse snapshot feature of storage ?

We want to add a dedicated option to select a snapshot storage. There
could be a sensible default selection (depending on restrictions for the
overlay image).

> Also, I think that some features should be disable when it's used.
> (vm snapshots for example, maybe move disk,...)

Haven't looked into that, but would be pre-existing issues, so nothing
urgent. We just shouldn't break the feature if not necessary. If we
really don't get around to support it in time for PVE 9 we could also
initially note it as a breaking change or fallback to -drive.

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