libvirt users - Jan 2020 - Re: Locking without virtlockd (nor sanlock)?

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@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).
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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Il 03-01-2020 11:26 Daniel P. Berrangé ha scritto:
> 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).
Hi Daniel,
if I understand the docs correctly, this locking scheme is really useful 
for raw block devices, right?

Now that Qemu automatically locks file-based vdisks, what is the main 
advantage of virtlockd locking?
> 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.
While I am not an heavy user of external snapshot and other 
block-related operation, I occasionally use them (and, in these cases, I 
found them very useful).

Does it means that I should avoid relying on virtlockd for locking? 
Should I rely on Qemu locks only?

Thanks.

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

On Fri, Jan 03, 2020 at 02:56:50PM +0100, Gionatan Danti
wrote:
> Il 03-01-2020 11:26 Daniel P. Berrangé ha scritto:
> > 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).
> 
> Hi Daniel,
> if I understand the docs correctly, this locking scheme is really useful
for
> raw block devices, right?
> 
> Now that Qemu automatically locks file-based vdisks, what is the main
> advantage of virtlockd locking?
QEMU's locking should be good enough for file based images. There isn't
a clear benefit to virtlockd in this case.
> 
> > 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.
> 
> While I am not an heavy user of external snapshot and other block-related
> operation, I occasionally use them (and, in these cases, I found them very
> useful).
> 
> Does it means that I should avoid relying on virtlockd for locking? Should
I
> rely on Qemu locks only?
As above, QEMU's locking is good enough to rely on for file based
images.

The flaws I mention with libvirt might actually finally be something we
have fixed in 5.10.0 with QEMU 4.2.0, since we can finally use
"blockdev"
syntax for configuring disks.  Copying Peter to confirm/deny this...

Regards,
Daniel
-- 
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