Thursday, August 20, 2020

iostat output format changes again

The format for iostat differs across the last 3 Ubuntu LTS (16.04, 18.04, 20.04). Some of my scripts scrape that output and now it is time to update them to detect the new format. I wrote about this before. There are other ways to get this data so this is a risk I accept for being lazy. The changes are from added columns, removed columns, renamed columns and reordered columns. I wish the last two weren't done. There are a few more projects out there that scrape iostat output and must update how they parse it.

In Ubuntu 20.04 the new columns are for trim, which is useful. The svctm column was removed and fields were reordered. I assume that the bytes written metrics no longer include bytes trimmed -- which inflated by 2X the write rate for anything that does frequent unlike (like RocksDB).

New fields in 20.04 are:

  • d/s - number (after merges) of discard requests completed per second for the device
  • dsec/s (dkB/s, dMB/s) - number of sectors (KB, MB) discarded for the device per second
  • drqm/s - number of discard requests merged per second that were queued to the device
  • %drqm - percentage of discard requests merged together before being sent to the device
  • dareq-sz - average size (in kilobytes) of the discard requests that were issued to the device
  • d_await - average time (in milliseconds) for discard requests issued to the device to be served. This includes the time spent by the requests in queue and the time spent servicing them

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