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