I want to use two 3 TB drives in a mdadm raid1 setup (using Debian Sequeeze).
The drives use 4k hardware sectors instead of the traditional 512 byte ones.
I am a bit confused because on the one hand the kernel reports:
$ cat /sys/block/sdb/queue/hw_sector_size 512
But on the other hand
# fdisk -l /dev/sdb Disk /dev/sdb: 3000.6 GB, 3000592982016 bytes 255 heads, 63 sectors/track, 364801 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 4096 bytes I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Thus, it seems that the kernel has some idea that the drive uses 4k sectors.
mdadm man page is a bit cryptic about the chunk size and raid1:
-c, --chunk= Specify chunk size of kibibytes. The default when creating an array is 512KB. To ensure compatibility with earlier versions, the default when Building and array with no persistent metadata is 64KB. This is only meaningful for RAID0, RAID4, RAID5, RAID6, and RAID10.
Why is it not meaningful for raid1?
/proc/mdstat, the raid1 device md8 has 2930265424 blocks, i.e.
3000591794176/2930265424/2 = 512
mdadm use then a blocksize of 512 bytes? (/2 because it a two-way mirror)
And is chunk-size a different concept than blocksize?
Trying to let
mdadm explain a device:
# mdadm -E /dev/sdb -v -v Avail Dev Size : 5860531120 (2794.52 GiB 3000.59 GB) Array Size : 5860530848 (2794.52 GiB 3000.59 GB)
3000591794176/5860530848 = 512
With a default
mkfs.xfs on the md device, it reports:
I corrected this with a call of
mkfs.xfs -s size=4096 /dev/md8
Edit: Testing a bit around I noticed following things:
It seems that the initial resync is done with a block size of 128k (and not 512 bytes):
md: resync of RAID array md8 md: minimum _guaranteed_ speed: 1000 KB/sec/disk. md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for resync. md: using 128k window, over a total of 2930265424 blocks.
The displayed speed via
/proc/mdstat is consistent for that blocksize (for 512bytes one would expect a performance hit):
[>....................] resync = 3.0% (90510912/2930265424) finish=381.1min speed=124166K/sec
(For example when disabling the write cache the displayed speed immediately drops to 18m/sec)
/sys there are actually some more relevant files besides
# cat /sys/block/sdb/queue/physical_block_size 4096 # cat /sys/block/sdb/queue/logical_block_size 512
That means that the drive does not lie to the kernel about its 4k sector size and the kernel has some 4k sector support (as the output of
fstab -l suggested).
Googling a bit around resulted in a few reports about WD disks, which do not report the 4k size - fortunately this 3 TB WD disk does not do that - perhaps WD fixed their firmware with current disks.