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I'm trying to figure out the completion latency of fio benchmark with NVMe SSD.

I made following fio script to test benchmark in fio.

I used following options.

rw=read, ioengine=sync, direct=1

So, I thought there's not much things to make completion times different.

However, the result wasn't what I thought.

enter image description here

The result is 1th : 11us to 99.99th : 111 us.

Synchronous read make no outstanding IOs, so all I/Os are processed sequentially,

And direct option can bypass the buffer in OS.

I thought most of latencies are same.

Any ideas for this result?

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You are talking about microseconds here and 11 microseconds vs 111 microseconds. It isn't going to take much to perturb such a sensitive reading:

  • fio process goes off CPU because your server decided it was time to do a periodic cron job and doesn't get to process some I/O as quickly as others
  • Some of the I/O was in some sort of SSD cache but later I/O actually had to fetched from outside the cache
  • Some of the I/O read happened to have been written in a different order to which is read back (yes reading things back in the order they are written is better for an SSD)
  • Something else decided to I/O to the same device

etc.

You didn't include the full job (and I can see you set iodepth=16 which leaves me wondering what other things you set on your job) so you're asking a bit much from any answer. However I'll point out that 95% of your readings are within 3 microseconds of each other so you're not seeing many outliers across just under a million I/Os. How much determinism are you expecting out of a non-realtime system?

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