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

Il 28-12-2019 01:39 Gionatan Danti ha scritto: > 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@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@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 Ok, maybe I found some answers: from what I read here [1] and here [2], Qemu started to automatically lock disk image files to prevent corruption from processes outside libvirt scope (ie: manually issues "qemu-img" commands). Do you suggest relying on Qemu own locks or using virtlockd (in addition to Qemu locks)? Whatever the answer is, can you explain why? Thanks. [1] https://qemu.weilnetz.de/doc/2.12/qemu-doc.html#disk_005fimage_005flocking [2] https://bugzilla.redhat.com/show_bug.cgi?id=1378241