Re: Xen and LVM backed domu's

[Adam Hamsik <haaaad%gmail.com@localhost>]

Hi Brian,
On Feb,Friday 26 2010, at 3:25 AM, Brian wrote:

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> Sam Fourman Jr. wrote:
>> 
>> I am confused a bit, doesn't  this fix these issues?
>> 
>> src/sys/arch/xen/xen/xbdback_xenbus.c v 1.31
>> Finaly fix problems with using WEDGE like devices as xen backend devices.
>> 
>> or is this issue specific to HVM's
>> 
>> Sam Fourman Jr.
>> 
> 
> No it does not, as this appears to be HVM specific.  I noticed the same
> problem Hugo reported recently and I have spent a small amount of time
> trouble shooting this.
> 
> I usually use the character devices for my partition-backed domU's, like
> /dev/rraid0j; however, with the dm driver (and phy:/dev/mapper/rv0-lv0)
> I am getting an EBUSY error on VOP_OPEN in xbdback_xenbus.c.  The qemu
> BIOS then reports 0MB as Hugo described.

I think that we are fighting 2 problems here 

1) EBUSY error
2) size 0 in qemu error

The 1) is probably not from dm driver because it do not return EBUSY from its 
sources. Maybe this error comes from specfs_open but I'm not sure why.

The 2) is I think issue in qemu libraries I looked at source code and I have 
found one place which can be wrong. I will try to make some patch which can be 
tested.

> 
> Any combination of /dev/mapper/v0-lv0 or /dev/mapper/rv0-lv0 produces
> different errors from xbdback_xenbus.c while reporting 0MB in the qemu BIOS.
> 
> If I specify /dev/mapper/v0-saturn, I get this in my dmesg:
> xbd backend: attach device v0-saturn (size 62914560) for domain 10
> 
> But qemu BIOS still reports 0MB for the drive.
> 
> If I specify /dev/mapper/rv0-saturn, I get this instead:
> xbdback backend/vbd/11/832: unknown device 0xc202
> 
> With a disk entry like: 'phy:/dev/mapper/rv0-saturn,hdb,w'
> Regardless of what I replace hdb with, the 0xc202 device number just
> changes but still fails.
> 
> I have noticed that on one of my other HVM domU's which uses physical
> drives (/dev/rwd2d) that an unknown device error from xbdback_xenbus.c
> does not affect qemu's detection of the disk size.
> 
> When specifying the character device rv0-saturn, I do not get any
> messages in my /var/log/xen/qemu-dm-saturn.log; however, when I specify
> the block device v0-saturn, I get this:
> 
> qemu: could not open vbd '/local/domain/10/device/vbd/832/phantom_vbd'
> or hard disk image '/dev/mapper/v0-saturn' (drv 'phy' format 'raw')
> 
> I will continue to work on figuring out why this is happening as I know
> Adam is busy.  I do not have any other HVM-capable machines here for the
> time being so my progress is slow as the machine I was debugging with is
> used for production.
> 
> Sincerely,
> Brian Brombacher
> PlanetUnix Networks
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Regards

Adam.