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Re: [Xen-devel] blkif requires /dev/hdX for xm create /w devfs

To: Erik@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] blkif requires /dev/hdX for xm create /w devfs
From: Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>
Date: Sun, 28 Nov 2004 11:11:21 +0000
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> We have a host that has no /dev/hd{a,b,c,d}, only serial ATA (/dev/hde 
> through hdi). It runs devfs, so it has no entries for devices that do 
> not seem to be present. /dev/hda1 does not exist.
> When trying to create a new xm domain, it will not be able to stat 
> /dev/hda1 when it tries to look up the device number(s) of it. When 
> /dev/hda1 exists it does work. Is this behavior appropriate? Are 
> domain0's major/minor number better to use than the known standard 
> entries? If not, I suggest modifying it. If it is, the patch could still 
> be aplied for systems that lack the /dev/ entries as fallback information.

Yep, we've been aware of this for a while. The current situation
is particularly daft when you think of Net/FreeBSD domains where
it doesn't make sense to look the target device up on the host at
all. The obvious thing to do would be to pass the device name,
and then look it up within the guest domain. However, this isn't
easy on all OSes, at least for a boot device.

One workaround we added was the ability to specify a device as a
hex number, hence you can always uses '0301' for /dev/hda1.

> It can probably rely on the following information, because it's common 
> to most/all POSIX systems:
> hda: 3,0
> hda1-20: 3,1-20
> hdb: 3,64
> hdb1-20: 3,65-84
> hdc: 22,0
> hdc1-20: 22,1-20
> hdd: 22,64
> hdd1-20: 22,65-84

Having the common hd[abcd],sd[abcd] devices hardwired into xend
as a fall back seems sensible. If someone wants to put together a
patch to do this I'll apply it.

> If you don't want to assume such a numbering scheme it would be nicest 
> to have it recognise devfs, making it devfs compliant, though I'd 
> understand if you have more important things to do!

Devfs is now deprecated, so it doesn't make sense for us to go
there. We do support its replacement 'udev', but that's a
separate issue to the one we're talking about here.


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