Microsoft diskspd Part 3. Oddities and FAQ
By: Date: April 22, 2020 Categories: diskspd Tags: ,

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.
  • Note that in the command below there is no “-t” switch.
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 –o32 (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 –o32 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

Threads and OIO are per disk device.

Using the above (-o32 -t8) with 4 disks will yield 1024 OIO (32*8*4) more than most physical HBA’s can deliver by default

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

In the case where we want to keep the total OIO constant at 128 but experiment with different numbers of disks use for example -O32 -F8 rather than -O32 -t8.

  • 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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