4KB Sectors

First written 8 January 2011.
Updated 25 July 2015.

I bought a Western Digital WD20EARS-00MVWB0 hard-disk to use in a FreeBSD 8.2 system. These disks have a 4KB physical sector size but lie about it:

# camcontrol identify ada0 | grep size
sector size           logical 512, physical 512, offset 0

Simple disk timings:

# dd if=/dev/ada0 of=/dev/null bs=1M count=1024

Sequential read: 124,100,265 bytes/sec. (median of 11 samples)

# dd if=/dev/zero of=/dev/ada0 bs=1M count=1024

Sequential write: 123,152,588 bytes/sec. (median of 11 samples again)

For UFS tests, I did this:

# fdisk -I -B ada0
# bsdlabel -w -B /dev/ada0s1
# bsdlabel -e /dev/ada0s1
         size   offset    fstype   [fsize bsize bps/cpg]
  a:  8388608   $ofs      unused        0     0
# newfs -f $frag /dev/ada0s1a
# mount /dev/ada0s1a /mnt
# ( cd /mnt; time sh -c 'tar xf ~/tarball.tar; sync' )

I measured that ada0s1 starts at offset 0x7e00 in ada0. (32256 in decimal, or 63 sectors of 512 bytes)

With bsdlabel defaulting to an offset of 16 sectors, ada0s1a starts at 0x9e00 (40448 in decimal, or 79 sectors of 512 bytes), which is exactly one sector away from being 4K-aligned.

The UFS results (time is median of 5 samples):

ofs	frag	time
16	2048	40.38s
17	2048	27.74s
17	4096	20.09s	←
18	4096	35.96s
19	4096	36.29s
20	4096	35.73s
21	4096	38.50s
22	4096	38.13s
23	4096	37.44s
24	4096	37.33s
25	4096	18.41s	←
26	4096	38.97s

Timings with ZFS - median of seven runs, zpool on geli provider with 4K sectorsize, timing untar and sync:

aligned partition:	10.09 secs
unaligned partition:	12.09 secs

I was expecting a bigger difference, but ZFS is excellent at batching writes. Watching zpool iostat 1 shows it sustaining platter speed for 2-3 seconds during its flush cycle.

In conclusion:

Where bsdlabel says 16, change it to 17 and your partitions will be aligned to your drive's 4K sectors.
Make sure your partition sizes are multiples of 8.
For small filesystems, use newfs -f 4096 to specify a 4K fragment size.

Update: (25 June 2011) Newer disks have stopped lying.
For comparison:

device model          WDC WD20EARS-00MVWB0 (2TB)
sector size           logical 512, physical 512, offset 0

device model          WDC WD20EARX-00PASB0 (2TB)
sector size           logical 512, physical 4096, offset 0

Update: (30 October 2011) My first 3TB disk:

device model          WDC WD30EZRX-00MMMB0
sector size           logical 512, physical 4096, offset 0

Update: (17 May 2013) My first 4TB disk:

device model          ST4000DM000-1F2168
sector size           logical 512, physical 4096, offset 0

Update (25 July 2015) An assortment of disks:

Device Model:     WDC WD10EACS-00ZJB0 (1TB)
Sector Size:      512 bytes logical/physical

Device Model:     ST31000528AS (1TB)
Sector Size:      512 bytes logical/physical

Device Model:     ST31500341AS (1.5TB)
Sector Size:      512 bytes logical/physical

Device Model:     ST2000DM001-1ER164 (2TB)
Sector Sizes:     512 bytes logical, 4096 bytes physical