As drives have been getting larger and larger, having a 512B block size becomes inefficient. Drive manufacturers have slowly been moving to a 4KB block size, also known as Advanced format. An interesting article on the transition can be found over at Anadtech: “Western Digital’s Advanced Format: The 4K Sector Transition Begins”

You can determine if your drive using 4KB sector sizes by using smartctl:

1
2

# smartctl --all /dev/sdb | grep "Sector Size"
Sector Sizes:     512 bytes logical, 4096 bytes physical

When creating a zpool using ZFS, it queries the drive for the physical sector size to attempt to optimize disk operation. Unfortunately, some vendors emulate / report 512B sector size. The means for each 512B the disk actually has to read 4KB, modify, and write 4KB back. This can be a significant performance impact. The Illumos Wiki has more details on the problem.

Therefore it is best to look at your drive manufacturers tech sheet to see what the real sector size is. If you find that your drive is mis-reporting, you can for AFS on Linux to use a different sector size with the ashift option. It takes a bitshift value, therefore 9 is 2^9 (512B) and 12 is 2^12 (4096B).

1

zpool create -o ashift=12 data mirror sda sdb

Or if you’re adding a mirror to an existing pool

1

zpool add -o ashift=12 -f data mirror sdc sdd