On 08/21/2015 05:30 PM, Tom Buskey wrote:
On Fri, Aug 21, 2015 at 3:33 PM, Bruce Dawson <j...@codemeta.com
<mailto:j...@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.
Ubuntu 14.04 will supposedly boot from a ZFS root.
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.
Oh - but I thought ZFS will mirror "filesystems" within the pool
(probably with much poorer performance)? At any rate, I'm thinking the
first approach is the best.
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 didn't know ZFS would provide that. Guess I've got more reading - I
wonder if it'll be faster.
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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