TL;DR - SMR drives can take thirteen to sixteen times as long to resilver in your ZFS raid than CMR drives. If they even succeed. This wouldn’t be a big deal, except that Western Digital started using SMR technology in their WD-Red drives that are marketed toward SOHO and small business raid, without any warnings about the RAID performance implications.
I got lucky when I bought my last batch of Reds, they were all CMR, but it was pure luck - I bought them based on Western Digital’s reputation and because I’ve seen multiple NAS vendors recommend WD-REDs in the past.
Here’s some articles with more details.
- Surreptitiously Swapping SMR into Hard Drive Lines Must Stop
- IXSystems (the TrueNAS/FreeNAS vendor)’s statement that they will no longer ship SMR drives in their products
WD has since forked off a RED PLUS product line with CMR.
Anyway, check your spares bin for SMR drives, and check the drives in your NAS - if you have SMR drives in the NAS, pull a backup and start swapping in CMR replacements.