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Re: [Xen-devel] [PATCH] libxl: stat the path for all non-qdisk backends (including unknown)

On 10/05/13 15:09, Dave Scott wrote:
On 10/05/13 14:46, George Dunlap wrote:
On Fri, Apr 26, 2013 at 3:29 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
On Fri, 2013-04-26 at 15:17 +0100, Sylvain Munaut wrote:
Since the intention of that commit was to allow for qdisk backends with no
explicit file in dom0 (i.e. network remote backend such as ceph) the lowest
impact fix appears to be to make that explicit. This should probably be
revisited to rationalize the probing.
What about the remote disk case of blktap ?  blktap2.5 supports NBD
already AFAIK
and I'm pretty sure I'll hit that same stat issue soon for another
remote blktap case.
Right, sounds like I should have gone with my first instinct which was:


  From 884beff4a897d785f61705dcfa2f048536982d7c Mon Sep 17 00:00:00 2001
From: Ian Campbell <ian.campbell@xxxxxxxxxx>
Date: Fri, 26 Apr 2013 12:41:43 +0100
Subject: [PATCH] libxl: stat the path for all non-qdisk backends (including 

The commit a8a1f236a296 "libxl: Only call stat() when adding a disk if we
expect a device to exist." changed things to only stat the file when the phy
backend was explicitly requested. This broke the case where we are probing and
would normally be able to decide on the phy option.
So at the moment qdisk backends aren't checked at all with this --
which means that if you give a path to a file that doesn't exist via,
for example, xl cd-insert, things fail in weird ways:

1. In qemu-traditional, the command silently completes; the effect is
that the disk currently in the drive is ejected

2. in qemu-upstream, qmp returns an error which is reported.  The disk
is ejected from the guest, but the xenstore entries are not updated
(still contain the old values)

It seems like we should probably also at least check if disk_format is RAW.

OTOH, I don't seen an option for disk_format to be ceph; is it just
listed as "raw" as well?  That doesn't seem right...
AFAICT a ceph disk is in the "raw" format but it uses a custom network
protocol to actually read and write the blocks. I imagine on the ceph
servers the disk is stored in a cleverer format, but all qemu/tapdisk
see are plain blocks with no fancy encoding, no .vhd or .qcow.

Oh, hang on -- does qdisk / tapdisk then just open a plain file? Then it *is* just a raw file for these purposes; and we should still be able to call stat() on it.

I guess I'm not yet clear why qdisk files are excluded from the stat(). Under what circumstances would disk->pdev not be a valid file if disk->backend_domid == LIBXL_TOOLSTACK_ID?


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