RE: [Xen-users] Limited number of phy disks?

["Dustin Henning" <Dustin.Henning@xxxxxxxxxxx>]

        Well, I understand your point, but mine was that I believe it is
possible to use virtual SCSI to add more drives as well (the OP wanted to go
beyond 4 drives, not necessarily boot from the higher ones), just not to use
only virtual SCSI or to boot from it.  Moreover, assuming it is possible to
have virtual SCSI drives, one could potentially set up a small virtual IDE
drive to use for booting and then have windows on a virtual SCSI drive.
There are more catches to that scenario, though, one would first have to get
floppy emulation to work, the drivers in question would have to be available
in driver disk form, and then it might still not work, depending on whether
or not the BIOS has anything to do with that phase of the boot process in
Windows (the phase where the boot drive tells Windows to load from wherever
it is installed).  Regardless of all that, though, I'm not sure there would
be any speed benefit to SCSI drivers, because the speed of the IDE allegedly
isn't limited to typical IDE speeds in a virtual environment anyway, much
like speed of the NIC allegedly isn't limited by the speed Windows says it
is.  In each of these cases, I say allegedly because 1) I have no system to
provide HD performance beyond the capabilities alleged, and 2) while I have
seen benchmarks for >1Gb virtual networking (with PV drivers), I don't know
whether those systems show the NIC as 1Gb or 10 (or if it is even possible
to show it at 10 with said drivers).
        Dustin

-----Original Message-----
From: Pasi Kärkkäinen [mailto:pasik@xxxxxx] 
Sent: Monday, January 19, 2009 12:50
To: Dustin Henning
Cc: 'Xen Users Mailing List'
Subject: Re: [Xen-users] Limited number of phy disks?

On Mon, Jan 19, 2009 at 12:34:15PM -0500, Dustin Henning wrote:
> OK, that post puts me in the minority, and I haven???t searched 
> or tested to verify this, but I thought the only problem with 
> SCSI emulation in Xen on HVMs was that the QEMU BIOS wouldn???t boot to a
SCSI drive.  
> My understanding is that the drives will be emulated, but you would 
> need drivers for them in order to use them.  I would also go as far as 
> to swear up and down that I have seen SCSI drivers for Xen HVMs more than 
> once (once they weren???t for HDs, but I think they were the other time).

> Regardless, emulating SCSI shouldn't provide any performance increase once
using PV drivers anyway.

Yep, PV drivers are the best choise. 

I just meant that if you're not able to use PV drivers for some reason then 
emulated SCSI should be faster than emulated IDE.

If Qemu BIOS can't boot from SCSI drives then that's obviously a problem..
and leaves IDE as an only choise HVM guests (without PV drivers). 

-- Pasi

> Dustin
> 
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Nick Couchman
> Sent: Monday, January 19, 2009 10:02
> To: pasik@xxxxxx
> Cc: christoffer@xxxxxxxxx; xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Limited number of phy disks?
> 
> As far as I know, the current Qemu implementation in Xen for HVM support
does not support SCSI.  As James pointed out, though, if you use the GPL PV
drivers, you can assign as many devices as you want to the HVM domU.  The
catch to that is that if you ever have to boot with /nogplpv (to disable the
GPL PV drivers), you'll lose everything except disk devices hda-hdd.
> 
> -Nick
> 
> 
> -----Original Message-----
> From: Pasi Kärkkäinen <pasik@xxxxxx>
> To: Nick Couchman <Nick.Couchman@xxxxxxxxx>
> Cc: christoffer@xxxxxxxxx, xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Limited number of phy disks?
> Date: Mon, 19 Jan 2009 16:03:43 +0200
> 
> On Sun, Jan 18, 2009 at 02:45:06PM -0700, Nick Couchman wrote:
> > I assume you're running as an HVM?  What version of Xen are you using?  
> > As far as I know, Xen only support IDE drives in HVM domUs, 
> > and it only supports a total of four drives - hda, hdb, hdc, and hdd.
> > 
> 
> Hi.
> 
> Afaik Qemu supports SCSI disk/adapter emulation nowadays too.. 
> SCSI disk support is not yet implemented in Xen (version of Qemu)? 
> 
> -- Pasi



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