[libvirt-users] Locking without virtlockd (nor sanlock)?

[Gionatan Danti]

Hi list,
I would like to ask a clarification about how locking works. My test 
system is CentOS 7.7 with libvirt-4.5.0-23.el7_7.1.x86_64

Is was understanding that, by default, libvirt does not use any locks. 
 From here [1]: "The out of the box configuration, however, currently 
uses the nop lock manager plugin". As "lock_manager" is commented in my 
qemu.conf file, I was expecting that no locks were used to protect my 
virtual disk from guest double-start or misassignement to other vms.

However, "cat /proc/locks" shows the following (17532905 being the vdisk 
inode):
[root at localhost tmp]# cat /proc/locks | grep 17532905
42: OFDLCK ADVISORY  READ  -1 fd:00:17532905 201 201
43: OFDLCK ADVISORY  READ  -1 fd:00:17532905 100 101
Indeed, try to associate and booting the disk to another machines give 
me an error (stating that the disk is alredy in use).

Enabling the "lockd" plugin and starting the same machine, "cat 
/proc/locks" looks different:
[root at localhost tmp]# cat /proc/locks | grep 17532905
31: POSIX  ADVISORY  WRITE 19266 fd:00:17532905 0 0
32: OFDLCK ADVISORY  READ  -1 fd:00:17532905 201 201
33: OFDLCK ADVISORY  READ  -1 fd:00:17532905 100 101
As you can see, an *additional* write lock was granted. Again, assigning 
the disk to another vms and booting it up ends with the same error.

So, may I ask:
- why does libvirtd requests READ locks even commenting the 
"lock_manager" option?
- does it means that I can avoid modifying anything, relying on libvirtd 
to correctly locks image files?
- if so, I should use virtlockd for what use cases?

Thanks.

[1] https://libvirt.org/locking-lockd.html

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
GPG public key ID: FF5F32A8