KVM vs ZFS

[Alan Johnson]

The host is some linux or other?  Running with that assumption, I don't
expect you will get anything extra out of zfs if you are only using it for
pooling vs. LVM.  As long as you are handing a block device to the guest,
the IO overhead of the virtualization is pretty much nil.  Now, if you were
using regular files to back your virtual drives instead of block devices,
ZFS (not just zpool) on the host might get you something, but I depends on
a lot of things, and I would never advocate putting a file system on a file
system if you care at all about IO.  That's where your IO overhead comes
in. Passing it to a KVM/libvirt guest does not introduce anything of
concern on modern hardware (maybe an extra processor instruction or 2 here
and there).

If you don't already know and love ZFS with all its beauty and risks, and
you don't have a specific critical need for it, I recommend against it.
 ext4 on LVM does a great job for 99.99% of use cases and the support and
user community for it is second to none.  I found this not to be the case
with FreeNAS/BSD/ZFS when I had a year long nightmare with it some time ago
on what is admittedly now likely and aged version of ZFS.


On Fri, Aug 21, 2015 at 3:33 PM, Bruce Dawson <jbd at codemeta.com> wrote:

> For this rainy weekend, please consider the following:
>
> I'm constructing a new server and want 2 KVM guest systems on it. There
> are 3 4TB drives on it. At the moment, assume one 4TB drive will be
> reserved for the KVM host. The server has 16GB of RAM.
>
> What are the advantages/disadvantages of:
>
>    1. Putting all disks in a ZFS pool on the host and dividing the pool
>    between each guest. Or:
>    2. Giving each guest its own disk. (At least one of the guests will be
>    running ZFS).
>
> The guests will be:
>
>    * Both guests will be running DNS servers
>    * One guest will be running a Postfix/Dovecot mail server (including
> mailman)
>    * The other guest will be running a LAMP stack.
>
> Hints:
>    * I don't particularly like option 2 as I'll lose the benefits of ZFS
> (snapshot backups, striping, ...)
>    * I don't know if the performance benefits of ZFS will outweigh the
> overhead of KVM/libvirt.
>
> --Bruce
>
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>


-- 
Alan Johnson
alan at datdec.com
Date Format PSA <http://xkcd.com/1179/>
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