fio is a versatile tool for measuring I/O performance, and it can produce a lot of output, which can sometimes be difficult to interpret. When fio reports IOPS (Input/Output Operations Per Second), it often provides separate results for read and write operations, as well as an aggregate total.

The output from fio might look something like this:

read: IOPS=10k, BW=40MiB/s (42MB/s)(1000MiB/25003msec)
write: IOPS=15k, BW=60MiB/s (63MB/s)(1500MiB/25003msec)

In this example:

  • The read: IOPS=10k means that the system was able to perform 10,000 read operations per second.

  • The BW=40MiB/s (42MB/s) indicates the read bandwidth, i.e., the system was able to read 40 Mebibytes (which is about 42 Megabytes) per second.

  • The (1000MiB/25003msec) is the total amount of data read (1000 MiB) and the total time it took (25003 milliseconds).

The same interpretation applies to the write operations.

If you see a line like this:

IOPS=25k, BW=100MiB/s (105MB/s)(2500MiB/50006msec)

This is an aggregate result that combines both reads and writes:

  • The IOPS=25k means that the system was able to perform a total of 25,000 I/O operations (reads and writes combined) per second.

  • The BW=100MiB/s (105MB/s) indicates the total bandwidth (for both reads and writes), i.e., the system was able to read and write a total of 100 Mebibytes (which is about 105 Megabytes) per second.

  • The (2500MiB/50006msec) is the total amount of data read and written, and the total time it took.

Remember, the results of the fio test can be impacted by numerous factors, such as your disk type (HDD vs SSD), RAID configuration, filesystem, block size, I/O scheduler, and other hardware and software factors. Also, different workloads (random vs sequential, reads vs writes, etc.) can produce dramatically different results.