How to 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:
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) 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/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
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):
As I alluded to before, there are cases where this won’t work: if
optimal_io_size is zero, for example, there are other rules to follow.
Of course it would be nice if parted could do this—the values are all
available as ioctls, after all—but then what would I write about?
Apparently, using % causes parted to automatically align the sectors for best performance: