KVM vs ZFS

[Bruce Dawson]

Thanks for your reply!

Ah. Sigh.

Its on Ubuntu 14.04 LTS. The "critical need" is for backups and 
failover. I know I can use LVM, but we have had more problems with LVM 
that we have had with bare disks (and tar backups); especially when it 
came to disk failures.

On 08/21/2015 04:23 PM, Alan Johnson wrote:
> 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 
> <mailto: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 <mailto:alan at datdec.com>
> Date Format PSA <http://xkcd.com/1179/>

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