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

But on the other hand fdisk reports:

# 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.

The 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?

Looking at /proc/mdstat, the raid1 device md8 has 2930265424 blocks, i.e.

3000591794176/2930265424/2 = 512

Does 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)

Under /sys there are actually some more relevant files besides hw_sector_size:

# cat /sys/block/sdb/queue/physical_block_size
# cat  /sys/block/sdb/queue/logical_block_size

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.

1 Answer 1


Chunk size does not apply to raid1 because there is no striping; essentially the entire disk is one chunk. In short, you do not need to worry about the 4k physical sector size. Recent versions of mdadm use the information from the kernel to make sure that the start of data is aligned to a 4kb boundary. Just make sure you are using a 1.x metadata format.

  • If using an mdadm mirror for iscsi for ESXi, and vmfs block size is 1M, is it best to align to 1M for iscsi for vmfs, or does iscsi change alignment with overhead? Commented Dec 23, 2021 at 5:08

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