Re: [PATCH] xl: Special case tap/aio for disk validation

[Jim Fehlig <jfehlig@novell.com>]

Stefano Stabellini wrote:
> On Fri, 28 Jan 2011, Jim Fehlig wrote:
>   
>> Kamala Narasimhan wrote:
>>     
>>>> - phy:/path/to/device
>>>> - file:/path/to/file
>>>> - tap:/path/to/file
>>>>     
>>>>         
>>> Along with phy, file and tap for block device types there appear to be a 'drdb' option too.  Do we actively support it?
>>>       
>> The xend toolstack supports arbitrary external block device types.  We
>> have block-iscsi and block-npiv,  and the drbd project provides
>> block-drbd.  I'm quite sure I've seen other custom block types as well. 
>> Will these arbitrary block device types still be supported in the new
>> toolstack?
>>     
>
> Is it just a matter of forking and executing an external script to
> setup a block device and threat it as phy: afterwards?
>   

Currently, IIUC, the external script is invoked by udev

SUBSYSTEM=="xen-backend", KERNEL=="tap*", RUN+="/etc/xen/scripts/blktap
$env{ACTION}"
SUBSYSTEM=="xen-backend", KERNEL=="vbd*", RUN+="/etc/xen/scripts/block
$env{ACTION}"

The block script will delegate to a helper if needed, which configures
the block device and writes some info (like resulting dev node) to
xenstore.  xend and/or qemu then read the info and continue configuring
the device from backend perspective.

> It shouldn't be too difficult but still non-trivial at this point of the
> release cycle.
> Maybe users could work around the lack of support in 4.1 calling the
> script themselves and using the name of the block device in the VM
> config file directly?
>   

Yes, this is certainly a workaround.  One could argue this is the proper
solution and remove support for external block scripts.  But this
functionality has been around for years.

Jim