ZFS Scrub running at 26% of 6Gb/s SATA 'max speed' - Why?

Hardware
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Hi here,

I notice the ‘speed’ indicated in the scrub operation,

########## ZPool status report for big ##########

  pool: big
state: ONLINE
  scan: scrub in progress since Sun Oct 15 00:00:02 2017
        5.53T scanned out of 22.4T at 201M/s, 24h22m to go
        0 repaired, 24.71% done

My math makes this say (200M/s / 1024) * 8 ~= 1.56 Gb/s; taking that as a percentage of 6Gb/s gives us ~26% theoretical max speed, of at least the SATA 3 interface.

Of course the WD Red 4TBs are only 5,400 rpm drives, so I guess that would affect read/write latencies. So my assumption here is this ‘slowness’ is down to the fact that I have only a single vdev.

Thoughts?

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The intetface speed is totally irrevant, no mechanical HDD is close to saturating Sata under ideal workloads.
Also keep in mind the WD “Internal Transfer Rate” specified for these drives is only 150 MegaBytesPerSecond, or 1,200 MegabitsPerSecond… literally even Sata1 wouldn’t be a bottleneck outside of the 64 MegaBytes of cache.

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3

Woah… I definitely missed that in the specs. FreeNAS reported 200MB/s during the scrub. Took ~33 hrs to scrub the entire 22.4T pool.

Thanks mate!!

4

Story of my life. ZFS Scrubs aren’t super important these days, as long as you have parity, ZFS will repair data on the fly if it detects it’s bad.

That said, I still run them every few months.

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5

Aye, running RAIDz2. Cool I’ll set mine to run once a month then, should be sufficient.

6

That should be perfectly fine.

I’m running z1, but I’ve only got 20TiB raw. (5x4tb)

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