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

[Daniel P. Berrangé]

On Sat, Dec 28, 2019 at 02:36:27PM +0100, Gionatan Danti wrote:
> 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 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
> 
> 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).

Yes, this is correct, the OFDLCK you are seeing are held by QEMU itself
and can't be turned off.

> Do you suggest relying on Qemu own locks or using virtlockd (in addition to
> Qemu locks)? Whatever the answer is, can you explain why?

The QEMU locks use fcntl() as their impl and as such they only apply to the
local machine filesystem, except when using NFS which is cross node.

virtlockd also uses fcntl(), however, it doesn't have to acquire locks on
the file/block device directly. It can use a look-aside file for locking.
For example a path under /var/lib/libvirt/lock. This means that locks on
block devices for /dev/sda1 would be held as /var/lib/libvirt/lock/$HASH(/dev/sda1)

If you mount /var/lib/libvirt/lock on NFS, these locks now apply across
all machines which use the same block devices. This is useful when your
block device storage is network based (iSCSI, RBD, etc).


There are some issues with libvirt's locking though where we haven't
always released/re-acquired locks at the correct time when dealing
with block jobs. As long as your not using snapshots, block rebase,
block mirror APIs, it'll be ok though.

Regards,
Daniel
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