65

I'm partitioning a non-SSD hard disk with parted because I want a GPT partition table.

parted /dev/sda mklabel gpt

Now, I'm trying to create the partitions correctly aligned so I use the following command to know where the first sector begins:

parted /dev/sda unit s p free

Disk /dev/sda: 488397168s
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start  End         Size        File system  Name      Flags
        34s    488397134s  488397101s  Free Space

We can see that it starts in sector 34 (that's the default when this partition table is used).

So, to create the first partition I tried:

parted /dev/sda mkpart primary 63s 127s

to align it on sector 64 since it's a multiple of 8 but it shows:

Warning: The resulting partition is not properly aligned for best performance.

The logical and physical sector sizes in my hard disk are both 512 bytes:

cat /sys/block/sda/queue/physical_block_size
512

cat /sys/block/sda/queue/logical_block_size 
512

How do I create partitions correctly aligned? What am I doing wrong?

  • Are you sure sector numbers start at zero? – Burton Samograd May 8 '12 at 19:35
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    I tried too parted /dev/sda mkpart primary 64s 128s (in case it starts in 1) but it returns the same warning. – Marc May 8 '12 at 20:23
  • It looks like there's a command to check if a value is aligned: gnu.org/software/parted/manual/html_node/align_002dcheck.html. Maybe try that with various values using a for loop and spit out the values that are ok? – Burton Samograd May 8 '12 at 20:26
  • Burton, I didn't know about that command but it cann't be used until to have a partition. – Marc May 8 '12 at 20:34
  • Sorry about that, I didn't read the description carefully enough. – Burton Samograd May 8 '12 at 20:52
65

In order to align partition with parted you can use --align option. Valid alignment types are:

  • none - Use the minimum alignment allowed by the disk type.
  • cylinder - Align partitions to cylinders.
  • minimal - Use minimum alignment as given by the disk topology information. This and the opt value will use layout information provided by the disk to align the logical partition table addresses to actual physical blocks on the disks. The min value is the minimum alignment needed to align the partition properly to physical blocks, which avoids performance degradation.
  • optimal Use optimum alignment as given by the disk topology information. This aligns to a multiple of the physical block size in a way that guarantees optimal performance.

Other useful tip is that you can set the size with percentages to get it aligned. Start at 0% and end at 100%. For example:

parted -a optimal /dev/sda mkpart primary 0% 4096MB

  • 4
    Should the command not end 0% 100%? Where does 4096MB come from? – njahnke Jun 8 '14 at 18:08
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    Value 4096MB is used here just an example to show ability of mixing % and specific size numbers – lik Jun 9 '14 at 6:24
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    Just a note, I had to enclose the '0%' in quotes for my shell to execute the command properly. – Adam Eberlin Jan 26 '15 at 14:48
  • @lik do you have suggestion for this post unix.stackexchange.com/questions/248939/… – AnkurTank Dec 12 '15 at 12:58
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    Nevermind the above, I just noticed the parted print uses kB as 1000 bytes... – jiggunjer Feb 1 '17 at 11:45
9

My solution has been to use gdisk which can perform partitions alignment automatically on a 2048 sectors (1024KiB) by default, although it can be changed in the expert menu.

6

From the Arch Wiki:

When creating a partition, parted might warn about improper partition alignment but does not hint about proper alignment. For example:

(parted) mkpart primary fat16 0 32M
Warning: The resulting partition is not properly aligned for best performance.
Ignore/Cancel?                                                     

The warning means the partition start is not aligned.

Enter Ignore to go ahead anyway, print the partition table in sectors to see where it starts, and remove/recreate the partition with the start sector rounded up to increasing powers of 2 until the warning stops. As one example, on a flash drive with 512B sectors, Parted wanted partitions to start on sectors that were a multiple of 2048, which is 1MB alignment.

Additionally, just above this section, they state that the more precise IEC binary units of KiB, MiB, GiB, etc., are acceptable for units as well as the less precise KB, MB, GB, etc.

Personally, my exact command that solved this was:

mkpart hd ext4 1024KiB 8470MB

It didn't complain, I guess it wanted 1MB alignments.

  • Nope, I tried 1048576KiB, no dice. – Behrooz Mar 15 at 20:23
5

A minor correction. As I understand it, GPT disks don't have partition types anymore, they are all "primary". Command 'mkpart primary' doesn't create a primary partition like it does on msdos disks, it just creates a partition called primary. You can just as well use command

(parted) mkpart Parted-FUN! 21476MB 4000787MB
(parted) print
Model: ATA ST4000DM000-1F21 (scsi)
Disk /dev/sdf: 4000787MB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt

Number  Start    End        Size       File system  Name         Flags
 1      1.05MB   17181MB    17180MB    ext4         raid-var
 2      17181MB  21476MB    4295MB                  raid-swap
 3      21476MB  4000786MB  3979310MB               Parted-FUN!

This may cause confusion if you try creating logical partitions, I know it did for me.

(parted) mkpart extended 21476MB 4000787MB
(parted) mkpart logical 21476MB 1000787MB
Warning: You requested a partition from 21476MB to 1000787MB.
The closest location we can manage is 4000786MB to 4000786MB.
Is this still acceptable to you?
Yes/No? Yes
(parted) print
Model: ATA ST4000DM000-1F21 (scsi)
Disk /dev/sdf: 4000787MB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt

Number  Start      End        Size       File system  Name       Flags
 1      1.05MB     17181MB    17180MB    ext4         raid-var
 2      17181MB    21476MB    4295MB                  raid-swap
 3      21476MB    4000786MB  3979310MB               extended
 4      4000786MB  4000786MB  0.00MB                  logical

You can't create a partition called "logical" inside a partition called "extended".

  • thank you, I just wasted quite a bit of time thanks to this tools bad cli, and you made me figure out what's going on. – Florian Heigl Mar 16 '16 at 23:59
5

The accepted answer by @lik shows how you can specify the preferred alignment. However, parted always uses that alignment for checking the resulting partition, but not always creates partitions with that alignment.

TL;DR: When 1MiB is the optimal alignment, specifying 0% will work for disks of 200MiB and bigger. For smaller disks or larger alignment, specifying 0G could work where 0% does not. See below about why.

Parted does try to satisfy the alignment constraints when creating a partition, but it also will not deviate from your requested values too much. What counts as "too much" depends on how you specified the postions.

When you specify a start (or end) position to the mkpart command, it internally generates a range of acceptable values. This range is centered on the value you specify, and extends equally on both sides by half the unit size you used (that's what I read from the code, the comments say one full unit size on both ends).

For example, when you specify "10M", it will try to use any position between 9.5M and 10.5M. The same goes for percentages, so if you specify 0% it will use any value between 0% and 0.5% (it does not go negative, obviously).

One exception is that when using power-of-two-units (such as KiB for 1024, as opposed to K for 1000) parted assumes you're trying to specify an exact position and only considers the exact value you specified.

Since optimal alignment typically seems to be 1MiB-aligned, the K and M units will often not have sufficient room to reach optimal alignment. Specifying positions in G should have plenty room, but % is usually also fine.

So, this is why 0% usually works, though even that has limits.

  • Could you summarize a practical TLDR of your answer: what exactly should be better done in a different way as compared to the accepted answer? – yurkennis Jan 30 at 11:36
  • Good suggestion, I added a TL;DR. In practice, I think the 0% will work for all but really small disks, but my answer adds a little background on how it works exactly. – Matthijs Kooijman Jan 30 at 15:14

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