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SubjectInjecting delays into block layer
Hi Paolo et al.

I have a strong suspect that something is going wrong when the
underlying block device responds with a large delay. What makes me
thinking so is that I use a VM on some cloud provider, and they have
substantial block device latency resulting in permanently high (~20%)
iowait. It spikes occasionally when their cluster is overloaded, and
when that happens, the I/O in my VM may stop and never recover. This is
a rare occasion, but it really happens.

What's worse, so far I've seen such a behaviour with BFQ only. I'm still
testing other schedulers though.

Important note: I have no strict evidences that this is *the* case, thus
I'm asking for some suggestions. My idea is to fire up a local VM and
inject delays to a block device while performing some I/O from within
the VM.

So the question is: how can those delays be injected? Using dm-delay?
Can those delays be random?

Thanks in advance.

--
Oleksandr Natalenko (post-factum)

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