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First I create a properly aligned partition in a new GPT table using parted by specifying percentages for start and end of the partition:

# parted -a optimal /dev/sdb
GNU Parted 2.3
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) mktable gpt
Warning: The existing disk label on /dev/sdb will be destroyed and all data on this disk will be lost. Do you want to continue?
Yes/No? Y
(parted) mkpart primary 0% 1%
(parted) p
Model: ATA WDC WD30EZRX-00M (scsi)
Disk /dev/sdb: 3001GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt

Number  Start   End     Size    File system  Name     Flags
 1      1049kB  2097kB  1049kB               primary

(parted) quit

Note that this disk is using Advanced Format, but correctly reports the physical sector size of 4096B to Parted. Let's look at it again, using sectors as the unit:

# parted -a optimal /dev/sdb
GNU Parted 2.3
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) unit s
(parted) p
Model: ATA WDC WD30EZRX-00M (scsi)
Disk /dev/sdb: 5860533168s
Sector size (logical/physical): 512B/4096B
Partition Table: gpt

Number  Start  End    Size   File system  Name     Flags
 1      2048s  4095s  2048s               primary

(parted) quit
  • Why did it start the partition at 2048s and not 34s which is the first possible sector?
  • 34s is not a properly aligned start sector if the physical sector size is 4096B and the logical (which is the one you specify in Parted) sector size is 512B. A properly aligned start sector is one divisable by 8 (since physical sector size / logical sector size = 8). But that means 40s is the first properly aligned start sector, yet it's not used. Why?

If we try to create a properly aligned partition of 100MiB capacity starting at 40s in a fresh GPT partition table:

# parted -a optimal /dev/sdb
GNU Parted 2.3
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) mklabel gpt
Warning: The existing disk label on /dev/sdb will be destroyed and all data on this disk will be lost. Do you want to continue?
Yes/No? Y
(parted) mkpart primary 40s 204839s
Warning: The resulting partition is not properly aligned for best performance.
Ignore/Cancel? I
(parted) unit MiB
(parted) p
Model: ATA WDC WD30EZRX-00M (scsi)
Disk /dev/sdb: 2861588MiB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt

Number  Start    End     Size    File system  Name     Flags
 1      0.02MiB  100MiB  100MiB  fat32        primary

(parted)
(parted) unit s
(parted) p
Model: ATA WDC WD30EZRX-00M (scsi)
Disk /dev/sdb: 5860533168s
Sector size (logical/physical): 512B/4096B
Partition Table: gpt

Number  Start  End      Size     File system  Name     Flags
 1      40s    204839s  204800s  fat32        primary

(parted)
  • We still get the Warning: The resulting partition is not properly aligned for best performance. warning, even though 40s and 204840s (204839s + 1) are both divisable by 8. Why?
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1 Answer 1

up vote 14 down vote accepted

Parted is just being overly conservative. The usual practice these days is to align partitions on 1MiB (2048-sector) boundaries because this works on Advanced Format disks, on certain type of RAID setups that require alignment, and on most SSDs. For an Advanced Format disk, so long as the alignment is on a multiple of 8, you're fine, and 2048 is a multiple of 8. The lost disk space is puny -- 0.0000336% of your total disk space, if I did the math right and didn't mistype anything. So don't worry about it; just use the 1MiB alignment.

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Yes the lost disk space doesn't matter to me. I just wanted to know that I understood things correctly. I'm checking Parted's source code at the moment, but it takes a bit longer than I have time. So I'll just go ahead and use 1MiB-alignment. Thanks for helping out yet again! –  Deleted Oct 15 '12 at 18:20

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