Re: Interesting qemu/virt-manager bug about the "rotational" attribute on virtio-blk disks

[Max Reitz]

On 16.07.20 10:06, Richard W.M. Jones wrote:
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1857515
> 
> A virtio-blk disk which is backed by a raw file on an SSD,
> inside the guest shows rotational = 1.
> 
> I assumed that qemu must have a "rotational" property for disks and
> this would be communicated by virtio to the guest, but qemu and virtio
> don't seem to have this.  Pretty surprising!  Is it called something
> other than "rotational"?

I sent an RFC last year to inquire the rotational info and expose it in
the block layer:

https://lists.nongnu.org/archive/html/qemu-block/2019-05/msg00911.html

It didn’t expose that info in guest devices, but I wanted to do that in
v1 (if it ever came to v1, which so far it didn’t).  However, Stefan
made me aware that that at least wouldn’t be easy, because this would be
a guest-visible change that could occur on migration, which is forbidden.

So if we really wanted something like this, I suppose we’d need to
ensure that the value can’t just silently change on migration, but
instead the change is somehow explicitly announced to the guest, as if
the disk was swapped out and replaced by an SSD (and I don’t know if
that’s worth the effort).

Max

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