Jonathan - Bytes issued during a zpool scrub

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What does the number of bytes issued mean during a zpool scrub? Previously,
the scrub just showed how many bytes had been scanned. The number of bytes
issued seems to be new with FreeBSD 12, and I can't find anything explaining
what it means. Example shown below:

  pool: zdata
 state: ONLINE
  scan: scrub in progress since Mon Dec 17 04:00:00 2018
        28.3T scanned at 555M/s, 22.0T issued at 432M/s, 33.9T total
        0 repaired, 65.09% done, 0 days 07:57:43 to go
Pasted 1 month, 1 week ago — Expires in 328 days
URL: http://dpaste.com/0YTPYV4