Re: [PATCH 2/2] libxl: switch NetBSD image file handling to Qemu

[Roger Pau Monné <roger.pau%citrix.com@localhost>]

On 28/11/12 16:29, Thor Lancelot Simon wrote:
> On Wed, Nov 28, 2012 at 03:41:56PM +0100, Roger Pau Monn? wrote:
>> On 28/11/12 14:26, Thor Lancelot Simon wrote:
>>>
>>> I am assuming there's some reason we want this, rather than handling
>>> such requests in-kernel.  But what is it?
>>
>> I should have marked this as "experimental", or something like this. I'm
>> not sure if switching to Qdisk for all image file backends is needed.
>> >From the Xen wiki (http://wiki.xen.org/wiki/Blktap) I've found that:
>>
>> "loop device had problems with flushing dirty pages (specifically, doing
>> a large number of writes to an NFS-backed image don't result in the OOM
>> killer going berserk)."
>>
>> I'm not sure if NetBSD is in the same situation, but if I remember
>> correctly (haven't tried that in a long time), trying to use a disk file
>> on a NFS share caused the NetBSD Dom0 kernel to crash. The PR for this
>> issue is: http://gnats.netbsd.org/40726.
> 
> It seems highly unlikely to me that a problem with the loop device driver
> on Linux is tremendously relevant to NetBSD.
> 
> But, even if there is some problem with vnd backed by NFS, that hardly
> seems like it would be a good reason to make a change that reduces I/O
> throughput for the *non* NFS-backed case by at least 20%.  Why would one
> keep disk images for guests, as a general rule, on NFS, rather than simply
> doing the NFS mounts on the guests themselves, or using a more sensible
> protocol like iSCSI?  I have to assume most folks using files as disks are
> storing them on local filesystems on the dom0, and wrecking performance
> for that case to solve a problem with NFS that may or may not be hypothetical
> seems like a very, very bad trade-off.

Quite a lot of people use disk images on NFS, because it's probably the
easiest shared storage that allows migration of guests.

Even if disks on NFS was a remote possibility not used by much people I
don't think it's acceptable to have a Dom0 crash when you try to create
a guest with a disk on NFS, that is by far much more worse than
performance degradation in my opinion.

If this is not accepted, I will have to detect image format somewhere
and use qemu when the format is different than raw.