I'm not sure if this is a bug in parted (parted 3.2 on Ubuntu 16.04.1LTS x86-64), or a bug in my understanding of the concepts. Either way I'd like to know.
I've got a new 8TB drive for which the "optimal" alignment calculated by parted doesn't satisfy the "minimal" alignment check. My understanding is the "minimal" check verifies the partition is aligned to physical blocks on the disk, so it seems impossible for an alignment that doesn't pass that check to truly be "optimal". Indeed, the documentation specifies "optimal" is a aligned to a multiple of the physical block size.
For this drive, "--align opt" creates a partition starting at 65535s:
-- 8< --
$ sudo parted --align opt /dev/sdi mkpart 8tb ext4 0% 100% u s p free a min 1 a opt 1
Model: Seagate Expansion Desk (scsi)
Disk /dev/sdi: 15628053167s
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
34s 65534s 65501s Free Space
1 65535s 15628000379s 15627934845s ext4 8tb
15628000380s 15628053133s 52754s Free Space
1 not aligned
1 aligned
-- 8< --
So, I fell back to how we used to do this, but the numbers are... weird:
-- 8< --
$ cat /sys/block/sdi/queue/optimal_io_size
33553920
$ cat /sys/block/sdi/queue/minimum_io_size
4096
$ cat /sys/block/sdi/queue/physical_block_size
4096
$ cat /sys/block/sdi/queue/logical_block_size
512
$ cat /sys/block/sdi/alignment_offset
0
-- 8< --
( 33553920 + 0 ) / 4096 = 8191.875, but that's not going to work. Using the logical block size instead gives the 65535s that parted suggested, but if my understanding is correct that's not actually a good alignment.
I can get a partition that satisfies both checks by scaling the 65535s out to align with a 4k block, and starting the partition at 524280s, at the cost of "wasting" ~300MiB of space:
-- 8< --
$ sudo parted --align opt /dev/sdi mkpart 8tb ext4 524280s 100% u s p free a min 1 a opt 1
Model: Seagate Expansion Desk (scsi)
Disk /dev/sdi: 15628053167s
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
34s 524279s 524246s Free Space
1 524280s 15628000379s 15627476100s ext4 8tb
15628000380s 15628053133s 52754s Free Space
1 aligned
1 aligned
-- 8< --
~300MiB out of 7.3TiB isn't the end of the world, so that's what I've gone with for now, but I wonder if I'm not overthinking it. Was parted's original optimal alignment actually fine and my understanding incorrect? If not, is there a better way than what I've done to calculate an alignment that is?
For the near-term there's no data on this device so I'm perfectly happy to burn it down for testing.
Thanks,
-V
-- 8< --
$ sudo parted /dev/sdi print unit s print unit chs print
Model: Seagate Expansion Desk (scsi)
Disk /dev/sdi: 8002GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
1 268MB 8002GB 8001GB 8tb
Model: Seagate Expansion Desk (scsi)
Disk /dev/sdi: 15628053167s
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
1 524280s 15628000379s 15627476100s 8tb
Model: Seagate Expansion Desk (scsi)
Disk /dev/sdi: 972801,80,61
Sector size (logical/physical): 512B/4096B
BIOS cylinder,head,sector geometry: 972801,255,63. Each cylinder is 8225kB.
Partition Table: gpt
Disk Flags:
Number Start End File system Name Flags
1 32,161,57 972798,8,5 8tb
-- 8< --