KVM vs ZFS

[Tom Buskey]

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.
>
>
I've been running ZFSonLinux for awhile.  Now on CentOS 7, but previously
on Ubuntu.  And OpenSolaris before that.

I typically do a minimal OS with 2 smaller disks with RAID1 mdadm.  I like
to make my OS disks independant of any driver or OS addons.  I don't know
how good Linux booting on ZFS is either.  Actually, I don't even know if
it's possible.  I think it is with BSD.

I do ZFS on my data disks (no dedup!).  ZFS could do a RAIDZ of the unused
space in a partition of the OS drive + the same partitions of the other
drives, but it really prefers whole disks and works better.  Plus, all
drives should be the same size.


> 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:
>
> So you're going to use one drive for the OS w/o ZFS?  Then 2 drives for
ZFS & data?
Then using zfs commands to allocate space to the guests?  I do this all the
time.


>
>    1. Giving each guest its own disk. (At least one of the guests will be
>    running ZFS).
>
> I wouldn't ever run ZFS on a single disk if I cared about the data.  It's
like running RAID0; get an error, you lose your all your data.  Actually,
you might recover data from a RAID0 non-ZFS.

You can use iSCSI on ZFS to give your KVMs a a raw block device instead of
a zfs partition w/ a QCOW2 file.  I've only done the zfs partition & qcow2,
not the iSCSI block.

I'd do the 1st setup and get the benefits of ECC and on the fly
partitioning.  I'd imagine the snapshots would be big for either qcow or an
iSCSI block.  I think you'd have to benchmark qcow vs iSCSI block to see
which is faster w/ various compressions (in qcow, in ZFS, etc)

ZFS will eat up unused RAM, but Linux does that for filesystems already so
we're used to that.  I don't see any huge performance hits with modern
multicore systems.


>
>
> 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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