Re: Question about managed/unmanaged persistent reservation disks

Adding users DL to possibly reach out a wider audience. Simon

> On Jun 9, 2025, at 7:28 PM, Annie Li <annie.li(a)oracle.com&gt; 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://www.qemu.org/docs/master/interop/pr-helper.html > <https://www.qemu.org/docs/master/interop/pr-helper.html> > "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." >

> 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 >