Re: Evenly distribute jobs and iodepth over a 1 TiB device so that every byte is written to in parallel

[Thomas Glanzmann <thomas@glanzmann.de>]

Hello Sitsofe,

* Sitsofe Wheeler <sitsofe@gmail.com> [2025-07-15 22:44]:
> (https://fio.readthedocs.io/en/latest/fio_doc.html#cmdoption-arg-scramble_buffers)
> which may be enough but you would have to check. The trick would be to
> see what happens with a single stream as that's easier to reason
> about.

I see, I stuck with refill_buffers because it was good enough.

> https://fio.readthedocs.io/en/latest/fio_doc.html#cmdoption-arg-offset_increment
> and size https://fio.readthedocs.io/en/latest/fio_doc.html#cmdoption-arg-size

Thank you that resolved my problem. I used the following command line:

fio --ioengine=libaio --refill_buffers --offset=0 --offset_increment=26G \
        --size=25G --ramp_time=2s --numjobs=80 --direct=1 --verify=0 \
        --randrepeat=0 --group_reporting  --filename /dev/nvme0n1 --name=1mhqd \
        --blocksize=1m --iodepth=3 --readwrite=write

> Do you get similar results in terms of space used with a single fio
> stream? Start small and then work your way up!

Yes, that works, but I also wanted to benchmark parallel performacne.
Also the single fio stream takes an hour while the parallel one only
takes 8,5 minutes.

> But perhaps you're thinking of device queue depths?

Yeah, that I was looking for Keith Bush answered me. The commands I was
looking for, are:

# How many IO queues are there:

ls -1 /sys/block/nvme0n1/mq/ | wc -l

# How large is each IO queue:

cat /sys/block/nvme0n1/queue/nr_requests

> Given you're running 40 jobs I'd be surprised if you can hit a depth
> of over 1000 per job (that would be over 65000 I/Os in total) without
> some serious tuning. You may want to look at
> /sys/block/[disk]/queue/nr_requests (see
> https://www.kernel.org/doc/Documentation/block/queue-sysfs.rst ) and
> /sys/block/[disk]/device/queue_depth but you may also find you run
> into libaio limits...

I could not. The netapp only has 8 * 128 queue depth, so now that I knew
the exact values I adopted the same.

Cheers,
        Thomas