Re: [Qemu-devel] [PATCH] block: add the support to drain throttled requests

[Chris Webb <chris@arachsys.com>]

Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> writes:

> Yesterday I only posted an analysis of the bug but here are some
> thoughts on how to move forward.  Throttling itself is not the problem.
> We've known that synchronous operations in the vcpu thread are a problem
> long before throttling.  This is just another reason to convert device
> emulation to use asynchronous interfaces.
> 
> Here is the list of device models that perform synchronous block I/O:
> hw/fdc.c
> hw/ide/atapi.c
> hw/ide/core.c
> hw/nand.c
> hw/onenand.c
> hw/pflash_cfi01.c
> hw/pflash_cfi02.c
> hw/sd.c
> 
> Zhi Hui Li is working on hw/fdc.c and recently sent a patch.
> 
> I think it's too close to QEMU 1.1 to convert all the remaining devices
> and test them properly before the soft-freeze.  But it's probably
> possible to convert IDE before the soft-freeze.

IDE is the only of these that would affect us as a typical user of
throttling. The others aren't really the kind of devices which you'd be
using in a hosting setting in any case.

However, being able to throttle ide and virtio drives to limit the harm one
VM can cause to another on the same host would make an enormous difference
to us, and for what it's worth, we're desperately keen to see this problem
and the assert() issue fixed so we can deploy this feature, back-porting it
if necessary.

Just to reiterate a previous offer: we'd be very happy to pay for someone's
contracting time to work on this if that would help. The feature is great
and works well apart from the risk of these ide deadlocks and asserts.
Unfortunately, ide represents the bulk of our VMs at the moment.

Best wishes,

Chris.