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Re: [Xen-devel] vscsi and /dev/tape/by-path

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] vscsi and /dev/tape/by-path
From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Date: Tue, 11 Jan 2011 17:00:57 -0500
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On Wed, Jan 12, 2011 at 08:00:47AM +1100, James Harper wrote:
> > 
> > On Tue, Jan 11, 2011 at 01:41:29PM +1100, James Harper wrote:
> > > I use vscsi fairly extensively (under Windows mainly for testing
> > > baremetal restores from tape), and have a problem that's been
> bugging me
> > > for ages. If I use "/dev/st4" it works fine, but obviously that's
> > > hopeless unreliable as it relies on device enumeration order at
> boot. I
> > > want to use something like "
> > > /dev/tape/by-path/pci-0000:01:08.0-scsi-0:0:3:0-st" so that I am
> > > guaranteed of getting the right drive, but vscsi doesn't appear to
> > > support it.
> > >
> > > Digging into it, I see function vscsi_get_hctl_and_devname_by that
> > > attempts to figure out the sg device that corresponds to whatever I
> > > specified in the config file. If I could modify that function to
> simply
> > > follow the symlinks until I get to a real device that would probably
> be
> > > sufficient, but is that the right way to do it?
> > 
> > Yup. Thought I would think that using the /dev/by-uuid/ would be a
> better
> > way of doing it in case you want to passthrough a SCSI disk for
> example?
> > 
> For disk, /dev/disk/by-uuid might be a better option, although it
> depends on your requirements. For tape, my system /dev/tape/by-path is
> the only available option on my system, although I could probably create
> a /dev/tape/by-id path easily enough using the serial number or
> something. by-id would allow the device to be moved across different
> busses and still remain the same, but obviously breaks when a tape drive
> fails and has to be replaced, which happens fairly regularly.

So could the fix you are thinking of, check both of those places?
What would the syntax end up for the vSCSI? I presume not SCSI ID
but now just the UUID (or perhaps the SCSI inq S/N? ?)

> This is the advantage of having so many choices :)
> > Thought what if you want to pass in a SCSI scanner? Would that work?
> Do they still exist? :)

Hehe. Good point. Lets froget that example.

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