[Xen-users] block-* scripts multiple issues

[Zakk <zakk@xxxxxxxxx>]

Greetings,

Let me preface this by saying I'm currently running Xen 3.2.0 under  
CentOS 5.1, although I believe these issues were present in 3.1.x also.
I'm experimenting with a setup that uses the block-drbd disk script  
provided by the drbd team. I've come across multiple problems, some of  
which I believe would apply to any other block-* script...
1) The xm python libs (specifically the code path for 'create') sanity  
check the VBD to make sure it's accessible. This is especially a  
problem with the drbd type since the 'device' specification is just a  
label, so xm create has no idea how to find the real device. This I  
can work around with a quick patch that calls the drbdadm tool and  
gets the actual device name.
2) If you're using pygrub, it is executed before any of the block  
scripts; so if there's any setup that needs to occur before the VBD is  
accessible, pygrub will fail. Again, this is the case with drbd  
because it won't let you much around in the device if you haven't  
become 'primary'. However, fixable with a workaround where you specify  
an alternate bootloader that's just a wrapper around pygrub that does  
whatever magic you need.
And here's the one I can't work around...


3) If you use qemu-dm as the device model, it is started before the  
block scripts are run. So just like pygrub, it fails to access the VBD  
in some instances and then the VBD isn't there in your domU. Which  
usually breaks your domU. This one I can't just simply put a wrapper  
around since the VBD isn't passed in on the command line.
It seems to me like it may make some sense to have an interface into  
the block scripts so xm create can say 'I need your real device name,  
and do any setup needed assuming you're about to become active'. This  
would solve all three of the issues I listed, with a bit of added  
complication of how to pass the correct device name to qemu-dm.  
However, I don't know the code well enough to know how the case of  
live migration/qemu-dm/block-* works and what (if anything) needs to  
be done there. I plan on testing that next.
Any ideas as to a reasonable solution to this stuff?

Thanks, and sorry about the length.

-=Zakk




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