2:44 a.m.
On 6/12/25 17:01, Annie Li wrote:
Hello Michal,
On 6/12/2025 9:18 AM, Michal Prívozník wrote:
> On 6/10/25 19:15, Simon Coter wrote:
>> Adding users DL to possibly reach out a wider audience.
>>
>> Simon
>>
> Dropping devel list as this is users list material.
>
>>> On Jun 9, 2025, at 7:28 PM, Annie Li <annie.li(a)oracle.com> wrote:
>>>
>>> Hello,
>>>
>>> I've been looking at source code related to persistent reservation and
>>> got confused a little bit about managed persistent reservation disks.
>>> For disk configured with 'managed=yes' as the following,
>>>
>>> <reservations managed='yes'>
>>> <source type='unix'
path='/var/lib/libvirt/qemu/domain-7-
>>> brml10g19-iscsi-rese/pr-helper0.sock' mode='client'/>
>>> </reservations>
>>>
>>> libvirt is responsible for starting a pr-helper program with a
>>> specific associated socket file. The following source code shows that
>>> there is only one pr-helper and socket file associated with the
>>> managed disks for one VM.
>>>
>>> const char *
>>> qemuDomainGetManagedPRAlias(void)
>>> {
>>> return "pr-helper0";
>>> }
>>> char *
>>> qemuDomainGetManagedPRSocketPath(qemuDomainObjPrivate *priv)
>>> {
>>> return g_strdup_printf("%s/%s.sock", priv->libDir,
>>> qemuDomainGetManagedPRAlias());
>>> }
>>>
>>> So if the VM is booted with multiple disks configured with
>>> 'managed=yes' for reservation, I suppose these multiple disks share
>>> the this managed pr-helper and socket file. However, per the qemu
>>> document, https://urldefense.com/v3/__https://www.qemu.org/docs/
>>> master/interop/pr-helper.html__;!!ACWV5N9M2RV99hQ!
>>> KTvTQA7YxW75PM9pKbPZ3lG5cV-
>>> QT0MDZfDkL1XZmT6gQ3chMSfk63La0TUAg4ZvMk2FCh_zn2p4HnY$
>>> <https://urldefense.com/v3/__https://www.qemu.org/docs/master/
>>> interop/pr-helper.html__;!!ACWV5N9M2RV99hQ!
>>> KTvTQA7YxW75PM9pKbPZ3lG5cV-
>>> QT0MDZfDkL1XZmT6gQ3chMSfk63La0TUAg4ZvMk2FCh_zn2p4HnY$ >
>>> "It is invalid to send multiple commands concurrently on the same
>>> socket. It is however possible to connect multiple sockets to the
>>> helper and send multiple commands to the helper for one or more file
>>> descriptors."
>>>
> This certainly did not use to be the case. IIRC this was discussed in
> this very old thread:
>
> https://urldefense.com/v3/__https://lists.libvirt.org/archives/list/
> devel(a)lists.libvirt.org/thread/UUL3B7ZLAW4WPVUBX2R76GZTOS24Z2SD/__;!!
> ACWV5N9M2RV99hQ!KTvTQA7YxW75PM9pKbPZ3lG5cV-
> QT0MDZfDkL1XZmT6gQ3chMSfk63La0TUAg4ZvMk2FCh_zeWt4DWY$
Thanks for the info.
This thread talks about the socket connection/access, but doesn't touch
the topic of multiple socket.
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?
Are you actually seeing any problems? Or are you just researching the
topic. What is happening is - libvirt starts one pr-helper per guest,
and all disks within the guest share the same pr-helper process. QEMU
needs just one connection for that and per Paolo's reply later in this
thread it has internal mutex that serializes multiple accesses onto the
socket.
I'll dig the qemu-pr-helper source code. Any thoughts are welcome
:)
Again, are you experiencing any bug? If so, please do file an issue so
it can be properly investigated!
https://libvirt.org/bugs.html
Michal