On 16/11/2016 15:47, Patrick M. Hausen wrote:
Hi, all,
we are just starting a project that will run a couple
of Ubuntu guests on top of bhyve instead of ESXi
that we used in the past.
As far as I could find out, more or less all bhyve
manager/wrapper tools use zvols as the backing
store for "raw" guest disk images.
I looked at
* chyves
* iohyve
* vm-bhyve
So far so good. Yet, this blog article has some very
valid (IMHO) points against using them:
http://jrs-s.net/2016/06/16/psa-snapshots-are-better-than-zvols
Afaik this is only a problem if you create a reservation for your ZVOLs.
By default ZFS does create ZVOLs with reservations matching their size.
The problem is that there is no way to signal to a VM that the
overcommited SAN just ran out of space and your stupid operator is
looking for ways to increase free space *NOW* and the VM should just
retry. Instead the VM gets a general write error. To keep this promise
ZFS has to reserve enough space to change each bit in the ZVOL if you
take a snapshot of a ZVOL. Other datasets are created without
reservations by default because the POSIX file system API can report
ENOSPC if the file system ran out of space. Of course you get the same
problem if you storage VM images in a file system there is just no sane
default setting protecting you, because you just outsmarted your poor
operating system.
Another thing I'm pondering is: wouldn't it be better to
run on UFS so you can dedicate as much memory
as possible to VMs?
ZFS isn't a memory eating boogie man. Yes ZFS uses a lot of memory and
requires more memory than UFS for stable performance, but you get a lot
in return:
* End to end checksumming
* Snapshots
* Remote replication of snaphosts
* Painless volume management
* ...
So I'm a bit puzzled now on what to do. Any opinions,
experiences, war stories to share?
ZFS saved my bacon more than once. Twice it detected and corrected
datacorruption on dying hardware in time. With a normal FS both cases
would have turned into bitrot spreading into the backups until it's too
late.
Without ZFS you would require a reliable hardware RAID controller (if
such a magical creature exists) instead (or build a software RAID1+0
from gmirror and gstripe). IMO money is better invested into more RAM
keeping ZFS and the admin happy.
-- Jan Bramkamp
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