FIO (Flexible I/O Tester) Part4 – fixed, ranges or finer grained block sizes?

“fio” Flexible IO Tester post series from installing to a deep dive explanation and usage.

full course
  1. FIO (Flexible I/O Tester) Part1 – Installation and compiling if needed
  2. FIO (Flexible I/O Tester) Part2 – First run and defining job files
  3. FIO (Flexible I/O Tester) Part3 – Environment variables and keywords
  4. FIO (Flexible I/O Tester) Part4 – fixed, ranges or finer grained block sizes?
  5. FIO (Flexible I/O Tester) Part5 – Direct I/O or buffered (page cache) or raw performance?
  6. FIO (Flexible I/O Tester) Part6 – Sequential read and readahead
  7. FIO (Flexible I/O Tester) Part7 – Steady State of SSD,NVMe,PCIe Flash with TKperf
  8. FIO (Flexible I/O Tester) Part8 – Interpret and understand the result/output
  9. FIO (Flexible I/O Tester) Part9 – fio2gnuplot to visualize the output

Using fixed block sizes is the most common way when doing storage (device) tests. There is more then one reason for that. The obvious reason is that a lot of tools only support fixed block size. Another reason is that the storage vendors present their performance counters mostly based on fixed block sizes with 4K. But this doesn’t means it’s the best way to test or measure your storage (device).

There are two common tests which you would like to perform:

  • RAW peak performance
  • Simulating production workload

Raw peak performance means a test where you try to test your storage (device) to achieve the same or similar values as the vendor presents on his website/documentation. This is by nature the highest possible value (peak) without the influence of page caches or file systems. Why should you do this? It is not to make sure the numbers at the vendor website are true or false. It is to make sure the device is running as expected and all best practices have been done.

Simulating production workload means a test where you try to test your storage (device) against a workload production. But most of the time you are not able to run the production workload directly on the storage (device). So then you need to simulate the production workload with a tool like “fio”,”sqlio“,”swingbench“… etc. If possible always test with real production workloads instead of simulations.

Fixed block size

You may used the option “blocksize=4K” or “bs=4K” which is the way to specify a fixed block size of 4K. But the output shows “BS=4K-4K/4K-4K/4K-4K”. So why 6 numbers instead of one?

There are three parts ( / is the separator) in the output. The first is related to the block size for reads, the second for writes and the third for discards. 4K-4K means use a block size from the range of 4K to 4K which is exactly 4K. But this gives you an idea that ranges could be used. There is even more then ranges. Block sizes can be fine grained.

You are able to specify fixed block size with different values for read,write,discard.

Example: Lets run a job with 50% reads and 50% writes. The read should use a fixed block size of 4K but the writes should use 64K. So we need to set the option “rw=readwrite” or “rw=rw” and the option “rwmixread=50” or “rwmixwrite=50”.

file: read-4K-write-64K


Block size ranges

Instead of using a fixed block size you can specify a range.

Example: Let’s run a read workload with a block size in the range of 1K to 16K. This can be done by set the “blocksize_range=1K:16K” or “bsrange=1K:16K”. But how will the mix look like?

file: read_bs1K-16K


1400 IO have been issued. At this point I am note sure about the real distribution. I tried a few calculation but have not been able to provide an explanation yet.The documentation states:

fio will mix the issued io block sizes. The issued io unit will always be a multiple of the minimum value given (also see bs_unaligned).

 Finer grained block sizes

Sometimes it maybe useful to control how the weight for a block size within a range should be set. This can be done with the option “bssplit”. What could be the reason why you would like to control the block size? Lets assume you know a production workload and would like to use “fio” to act similar inside a new VM/container to evaluate if the production workload would run fine.

The format for this option is:

for as many block sizes as needed.

Example: Let’s run a read workload with a block size 10% 1K, 80% 4k, 10% 16K. This can be done by set the option “bssplit=1K/10:4K/80:16K/10”.

file: read_bs1K10p_4K80p_16K10p


2029 IO issued. The simple math is:

  • 2029 * 0,8 (80% of IO) * 4K (is 4K) = 6492,8 K
  • 2029 * 0,1 (10% of IO) * 1K (is 1K) = 202,9 K
  • 2029 * 0,1 (10% of IO) * 16K (is 16K) = 3246,4 K
  • 6492,8K + 202,9K + 3246,4K = 9942,1K ~ 10MB

Yes there is a derivation with means my approach is only a simple approximation of the real algorithm. Remember you can not issue one half of an IO 🙂

Go Zookeeper.

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