[Qemu-devel] Re: NBD server for QEMU images

[Anthony Liguori <anthony@codemonkey.ws>]

Salvador Fandino wrote:
> Anthony Liguori wrote:
>> Salvador Fandiño wrote:
>>> Hi,
>>>
>>> The patch available from http://qemu-forum.ipi.fi/viewtopic.php?t=2718
>>> adds a new utility, qemu-nbds, that implements a NBD server (see
>>> http://nbd.sf.net) for QEMU images.
>>>
>>> Using this utility it is posible to mount images in any format
>>> supported by QEMU.
>>>
>>> Unfortunatelly, only read access works (locally) due to a limitation
>>> on the Linux Kernel :-(
>> http://hg.codemonkey.ws/qemu-nbd/
>>
>> And write access works for me.  What's this limitation you speak of?
> 
> Mounting a partition being served on the same host as read-write can
> cause deadlocks. From nbd-2.9.0 README file:

This text is pretty old.  Is this still valid?  This would imply that 
things like loop can result in dead locks.  I don't see why flushing one 
device would depend on the completion of another device.  Otherwise, if 
you had two disk adapters, they would always be operating in lock step.

As I've said, I've never seen a problem doing-write with nbd on localhost.

Regards,

Anthony Liguori

> "When you write something to a block device, the kernel will not
> immediately write that to the physical block device; instead, your
> changes are written to a cache, which is periodically flushed by a
> kernel thread, 'kblockd'. If you're using a single-processor system,
> then you'll have only one kblockd, meaning, the kernel can't write to
> more than one block device at the same time.
> 
> If, while your kblockd is emptying the NBD buffer cache, the kernel
> decides that the cache of the block device your nbd-server is writing to
> needs to be emptied, then you've got a deadlock."
> 
> Regards,
> 
>  - Salva