Align partitions for best performance using parted

There are two common problems when creating partitions in Linux on big storage arrays. The first is easy, and the warning message from fdisk is a bit of a giveaway:

WARNING: The size of this disk is 8.0 TB (7970004230144 bytes).
DOS partition table format can not be used on drives for volumes
larger than (2199023255040 bytes) for 512-byte sectors. Use parted(1) and GUID 
partition table format (GPT).

The answer: use parted. Don’t have it? Install it!

The second problem is this warning from parted:

(parted) mklabel gpt
(parted) mkpart primary 0 100%
Warning: The resulting partition is not properly aligned for best performance.

…and no matter what combination of numbers you use, the message just keeps coming back. It’s tempting to ignore it, but don’t.

There are a few posts on the subject, but this one from HP really gets to the guts of the problem.

Here’s a quick step-by-step guide to aligning partitions properly. It’s just an abstraction of the HP post, but hopefully easier to follow. This will work for most arrays (in fact it works for all the arrays that I’ve seen); there are more options in HP’s post, but I’ve included the most common configuration here.

  1. Get the alignment parameters for your array (remember to replace sdb with the name of your device as seen by the kernel).
    # cat /sys/block/sdb/queue/optimal_io_size
    # cat /sys/block/sdb/queue/minimum_io_size
    # cat /sys/block/sdb/alignment_offset
    # cat /sys/block/sdb/queue/physical_block_size
  2. Add optimal_io_size to alignment_offset and divide the result by physical_block_size. In my case this was (1048576 + 0) / 512 = 2048.
  3. This number is the sector at which the partition should start. Your new parted command should look like
    mkpart primary 2048s 100%

    The trailing ‘s’ is important: it tells parted that you’re talking about sectors, not bytes or megabytes.

  4. If all went well, the partition will have been created with no warnings. You can check the alignment thusly (replacing ’1′ with the partition number if necessary):
    (parted) align-check optimal 1                                            
    1 aligned
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