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

[Thor Lancelot Simon <tls%panix.com@localhost>]

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.

Thor