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

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

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:
> > >
> > > 8<------------------------------------------------------------
> > >
> > >   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 
> > > unknown)
> > >
> > > 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?
 -George

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