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

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.


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