[Discuss] ZFS scrub performance degrading

[Rich Pieri]

Configuration: 16GB RAM, 64GB NVMe L2ARC, 4x6TB RAIDZ2 zpool. Earlier
this year I bumped the server from 16GB to 64GB RAM, and I removed the
L2ARC. I made no other hardware changes.

Prior to the upgrade the monthly scrub process took 15-20 hours and the
four disks would run at their full performance of around 130MB/s
for the duration. Now, read performance starts dropping at around the
50% completion mark down to under 40MB/s by the time it finishes.

If I pause the scrub, reboot, and resume, then I get the full read
performance again, and it will finish the scrub in a reasonable time.
Adding the L2ARC back does not make any appreciable difference.

Any suggestions about what to look at? I'm genuinely confused by this.

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