Fixing the ZFS ashift problem on FreeBSD

As you’re probably aware (since you’re reading this), some of the new hard drives with “Advanced Format” lie about their sector size and thus the partitioning and/or file systems end up unaligned with the disk and performance suffers. Boo, Western Digital. You guys are worse than Hitler. But, since the WD20EARS was cheap and i didn’t think anyone would design something so stupid, i bought it.

Ofcourse, ZFS is smart enough to use the larger sector size through the “ashift” setting, but only if it actually knows the sector size of the drive. This option is set at pool (or rather VDEV) creation and can’t be changed.

The easy way around this is to use gnop to prevent the drive from lying.

zpool create tank raidz ada0 ada1 ada2

# zdb | grep ashift ashift: 9 Oe noes! 2^9 is 512, the wrong sector size. Let’s try that again.

zpool destroy tank

# gnop create -S 4096 ada0 # zpool create tank raidz ada0.nop ada1 ada2 # zdb | grep ashift ashift: 12 Woohoo! 2^12 is 4096, as it should be. Note that you only need to tag one of the drives. zpool is clever enough to use the largest size of them all. Also note that you need to do this every time you add a new VDEV, otherwise you’ll have varying ashift values within your pool. Only problem now is that the drive list is a bit ugly, but that’s easily fixed.

zpool status

pool: tank state: ONLINE scan: none requested config: NAME STATE READ WRITE CKSUM tank ONLINE 0 0 0 raidz1-0 ONLINE 0 0 0 ada0.nop ONLINE 0 0 0 ada1 ONLINE 0 0 0 ada2 ONLINE 0 0 0 # zpool export tank # gnop destroy ada0.nop # zpool import tank # zpool status pool: tank state: ONLINE scan: none requested config: NAME STATE READ WRITE CKSUM tank ONLINE 0 0 0 raidz1-0 ONLINE 0 0 0 ada0 ONLINE 0 0 0 ada1 ONLINE 0 0 0 ada2 ONLINE 0 0 0 There we go. Destroying the gnop node just removes the sector size override, the data is safe and sound. You’ve now got ashift set right without fiddling with patches and such.