Re: Question about managed/unmanaged persistent reservation disks
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-h...
>>
<https://urldefense.com/v3/__https://www.qemu.org/docs/master/interop/pr-h...
>
>> "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...
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?
I'll dig the qemu-pr-helper source code. Any thoughts are welcome :)
Thanks
Annie
>
> Michal
>
>>> Due to this limitation above, only one persistent reservation disk is
>>> allowed as managed in theory. However, libvirt doesn't throw out any
>>> error or warning when the VM is booted up with multiple managed
>>> persistent reservation disks. I am wondering if I've missed something
>>> here?
>>>
>>> For unmanaged persistent reservation disks, libvirt doesn't start the
>>> pr-helper program for them. It is user's responsibility to start this
>>> program with customized socket file per disk, but the complexity
>>> increases with numbers of persistent reservation disks, especially in
>>> the case of hotplug/hotunplog. Is there any plan to support multiple
>>> managed persistent reservation disks with separate pr-helper/socket file?
>>>
>>> Any suggestions/clarifications are greatly appreciated.
>>>
>>> Thanks
>>>
>>> Annie
>>>