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...@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/
>> de...@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
