[fedora-virt] virt-install disk size problems

[Richard W.M. Jones]

On Mon, Mar 31, 2014 at 09:59:22AM +0100, James Harrison wrote:
[...]

I wasn't aware that cache=none was necessary for live migration.  It
seems really broken that this is necessary, see Stefan's explanation
here:

https://groups.google.com/forum/#!topic/synnefo-devel/9MU5ujOcOt4

> Also Caching=None is set because there is a disk cache on the disk.

The qemu cache mode isn't really relevant to this.  But I guess if
your disk has a huge cache (let's say it's a SAN LUN or a very
expensive RAID controller) then enabling the host cache
(ie. cache=writeback) might be overkill.

> Would double caching would slow down I/O?

Not in my testing.  cache=writeback is a little faster.

There are some caveats:

 - cache=writeback is only safe if your guest sends write barriers,
   ie.  recent Linux and recent Windows guests are OK, Linux and
   Windows prior to ~2008/2009 are NOT safe.

 - Our performance team reckon that cache=none is better than
   cache=writeback, but their test configuration fully provisions host
   memory with guests.  You only benefit if your host actually has
   some free memory to give to the cache ...

 - It may make migration impossible.

 - Libguestfs is not representative of what real guests do.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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