Microsoft diskspd Part 3. Oddities and FAQ

Default OIO is “2” if no other parameters are specified

This is documented on the diskpsd page but most workload generators that i use will default to a single OIO, so it’s worth pointing out.

Default case:

8KB Random IO, no thread or OIO specified
Resource monitor shows a Disk Q (OIO) of ~2 (1.9)

The -o parameter is per-disk and per thread.

Run diskspd with -o 32 (single thread) generates a total of 32 OIO

8KB Random IO, 32 OIO (no thread count specified)
Resource monitor shors a Disk Q (OIO) of ~32


Run diskspd with -o 32 and -t2 (two threads) generates a total of 64 OIO

Run diskspd with -o32, -t8 (eight threads) generates a total of 256 OIO

diskpd -o32 -t8

Run diskspd with -o32, -t8 (eight threads) and 4 Disks – generates 1024 total OIO

Threads and OIO are per disk device.

Using the above with 4 disks will yield 1024 OIO more than most physical HBA’s can deliver by default

To keep total OIO constant across disk count use -O and -F

4 Devices

2 Devices

1 Device

Creating a file with -Zr does not fill the file.

To create a file with non-null bytes, you will have to do a separate write phase after initial creation.

The default fill pattern is highly compressible

  • Writing out a file without a switch writes a Repeating pattern every 256 bytes. A 1FFF file will compress by KKK% to JJJbytes
  • Writing out a file with “-Z” writes NULL bytes
  • Writing out a file with “-Zr” writes totally random bytes.

“Raw” devices are supported

diskspd supports writing directly to the drive (e.g. without a filesystem) using the format #<disknum>

diskspd writing direct to disk device

Writing to a file/device without a time period will default to 10 seconds

Anyone coming from e.g. fio might expect the write to fill the entire file.

diskspd with no time parameter defaults to 10s
diskspd writes only 3.5GB – not the entire 33GB

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