Is it ok to run scylla_io_setup while service is running?

[Kiswono Prayogo]

Is the /usr/lib/scylla/scylla_io_setup 

are this command destroy the database?

Also need clarification about the result:

# my own pc

Generating evaluation file sized 10GB...10GB written in 38 seconds
Refining search for maximum. So far, 99096 IOPS
Maximum throughput: 99096 IOPS
Recommended --max-io-requests: 36
Written the above values to /etc/scylla.d/io.conf

# our dedicated server

Generating evaluation file sized 10GB...10GB written in 23 seconds
Refining search for maximum. So far, 136592 IOPS
Maximum throughput: 136592 IOPS
Recommended --max-io-requests: 54
Written the above values to /etc/scylla.d/io.conf

# shared vps

Generating evaluation file sized 10GB... timed out before we could write the entire file. Will continue but accuracy may suffer.
4.23889GB written in 144 seconds
Refining search for maximum. So far, 3652 IOPS
Maximum throughput: 3652 IOPS
Recommended --max-io-requests: 147
Written the above values to /etc/scylla.d/io.conf

Why the result for lower iops/slower ssd higher than the first two?

[Glauber Costa]

On Fri, Jun 23, 2017 at 6:24 AM, Kiswono Prayogo <kis...@gmail.com> wrote:
> Is the /usr/lib/scylla/scylla_io_setup
> are this command destroy the database?
>

It will not destroy your existing data, but that doesn't mean it is
okay to run it with the running database.

scylla_io_setup is very sensitive to other processes running, and that
can skew the results greatly. It is intended to be run alone.

You can re-run it in an existing environment, as long as Scylla is not
running at the moment (or anything else for that matter)

It is not clear to me if you want to know why the disk is slower (1)
or why is it that a slower disk gets a biggger number (2)

Can you clarify ?

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[Kiswono Prayogo]

The number 2, why slower disk get bigger number.

38 sec with 99096 iops -> 36

23 sec with 136592 iops -> 54

144 sec (339 sec scaled) with 3652 -> 147 

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Best Regards,
Kiswono P

GB

[Glauber Costa]

There are two reasons in theory. I'll explain both for generality, but
in your case it really is the second reason:

1) bigger numbers don't mean faster disks, it means more parallel
disks. That number represents how many requests the disk can handle in
parallel. There are cases in which the disk can have very little
parallelism, but each request is very fast so it can sustain a very
high IOPS with a very low number.

2) it timed out. scylla_io_setup uses a statistical process, and if it
times out before it can complete the evaluation, then the results will
be given with the data points it currently has. The results are
refined in many phases, and the early phases are really innacurate.

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> GB
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[Avi Kivity]

Probably, the last disk was a spinning disk, not an SSD. iotune has a bug on spinning disks and generates incorrect results.

Note that the output of iotune doesn't correlate directly to performance. There can be fast disks with low concurrency; for example a ramdisk will generate low concurrency numbers on a small system but will be very fast.

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