Re: Injecting delays into block layer

[Paolo Valente]

> Il giorno 21 nov 2019, alle ore 09:00, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto:
> 
> 
> 
>> Il giorno 21 nov 2019, alle ore 08:13, Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx> ha scritto:
>> 
>> Hi Paolo et al.
>> 
> 
> Hi
> 
>> 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?
>> 
> 
> So far I have used scsi_debug [1] for this kind of tests.  In my S
> suite [2], it boils down to setting SCSI_DEBUG=yes in the S config
> file, and then launching any of the benchmarks.  Unfortunately, AFAIK
> scsi_debug gives you only constant delays; but you can emulate delay
> spikes very easily, by changing the delay parameter manually during
> the test.
> 
> If this option sounds reasonable to you, then I'm willing to help you
> for every step.
> 

Hi Oleksandr,
Simone (in CC) and I have worked a little bit on reproducing the I/O
freeze you report.  Simone made a small change in SCSI_debug, which
makes the latter serve I/O with a highly varying random delay (100ms -
1s), about twice a second.

Then, to generate some fluctuating and heavy I/O, he ran the
comm_startup_lat.sh script of my S suite with SCSI_debug a few times.
Unfortunately, he didn't succeed in reproducing the problem.  If you
want, we can send you a patch with his change for SCSI_debug.

Any news on your side?

Thanks,
Simone

> Thanks,
> Paolo
> 
> [1] http://sg.danny.cz/sg/sdebug26.html
> [2] https://github.com/Algodev-github/S
> 
>> Thanks in advance.
>> 
>> -- 
>> Oleksandr Natalenko (post-factum)