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

[Jeff Rizzo <riz%boogers.sf.ca.us@localhost>]

On 11/28/12 7:48 AM, Roger Pau Monné wrote:
> On 28/11/12 16:29, Thor Lancelot Simon wrote:
>
> > 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.

I agree that this is a case we should be supporting well.  We don't 
support migration yet, but I'd like to see this change in the nearish 
future.

> 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.

And, of course, a crash is suboptimal.  :)

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

I would definitely like to see some middle-ground compromise that allows 
us to keep decent performance for local files while still allowing them 
for NFS.

+j