Re: Question about managed/unmanaged persistent reservation disks
Il mar 17 giu 2025, 18:02 Annie Li <annie.li(a)oracle.com> ha scritto:
Hello Paolo,
On 6/16/2025 5:11 PM, Paolo Bonzini wrote:
Il lun 16 giu 2025, 21:00 Annie Li <annie.li(a)oracle.com> ha scritto:
> > My understanding of the qemu document is that it's OK to run one
> > helper per QEMU or even per host, but multiple disks shouldn't share
> > the same socket since it is possible that multiple commands may be
> > sent concurrently.
> >> Maybe QEMU has some internal lock that does the right thing and
> >> serializes requests?
>
Multiple disks can share the socket, the serialization of requests is
handled with a mutex in scsi/pr-manager-helper.c.
Thanks a lot for the clarification.
I was only focusing on the qemu-pr-helper source code, haven't checked
pr-manager-helper yet, will definitely take a look.
Hi Annie
that's the part of QEMU that talks to the helper process.
Looks the following document is misleading,
https://www.qemu.org/docs/master/interop/pr-helper.html
Since there is a mutex handling the requests from multiple disk over one
socket, I suppose the statement "It is invalid to send multiple commands
concurrently on the same socket." can be removed
No, I don't think it should be removed: the mutex is exactly what makes
QEMU obey that statement. Remember that the documentation is written for
everyone that needs to implement a qemu-pr-helper replacement (say one that
does persistent reservations using a shared database), or a client that may
not be QEMU (ok in practice it will be).
Paolo
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