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Re: Feature - Add delay to NVMe device model

From: Klaus Jensen
Subject: Re: Feature - Add delay to NVMe device model
Date: Tue, 2 Aug 2022 09:03:41 +0200

On Aug  1 19:22, Stephen  Bates wrote:
> Hi
> I am interested in exploring the impact of increased storage IO delay
> on applications and was thinking of doing this by adding a delay
> argument to the NVMe device model in QEMU. Is this something that
> others would be interested in? If so I would be happy to work up a
> patch for upstream consideration.
> If this is already do-able in QEMU I would appreciate a pointer to that.

Hi Stephen,

If this is exclusively for reads/writes, then it might better fit in the
QEMU block layer. There is already the throttle filter[1] which can
limit iops and bandwidth, I'm not sure if that would be a solution for
you. I have not tried using it myself. You could potentially also do a
simpler delay filter instead.

However, the block layer filters are oblivious to the device models, so
it would not work for Copy where we would end up introducing a delay on
each range. In that case we would need to introduce a timer delay in
QEMU. If this can be done without being too intrusive, then I think I'd
be okay with adding something like that.

  [1]: https://github.com/qemu/qemu/blob/master/docs/throttle.txt

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