Re: User feedback

[John Clemens <jclemens-X2/pQFfLUrBl57MIdRCFDg@public.gmane.org>]

On Fri, 2007-10-05 at 07:52 -0500, Anthony Liguori wrote:

> http://hg.codemonkey.ws/qemu-nbd/
> 
> This will let you expose a qcow file (or an individual partition within 
> a qcow file) as an NBD server which you can then mount on your host.  A 
> bit round-about but it gets the job done.

You just made my day. I've been threatening to do the same thing for
weeks :)  A couple of questions if you don't mind:

- I was a bit confused by the statement on the nbd homepage:

"Read-write nbd with client and server on same machine has a rather
fundamental problem: when the system is short of memory, it tries to
write back dirty pages. So the nbd client asks the nbd server to write
back data, but as nbd-server is a userland process, it may require
creating dirty pages to fullfill the request."

The above makes sense, and the README in the source says this is not a
problem on SMP systems since there's more than one kblockd flush
thread... which makes less sense (why can't both kblockd's be blocked in
this way?)

Does qemu-nbd address this, perhaps by opening the qcow file with O_SYNC
or something?  or is this still a problem?

- There's no license file with the code, I'm assuming it's GPL since you
use qemu code directly?

Thanks for writing such a useful bit of software!
john.c


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