Why does the first partition start at sector 34 when I choose "Guided - Use entire disk" during install?

Question

After choosing "Guided - Use entire disk" during installation of Ubunsut Server 12.04 I find that the first partition starts on sector 34. Why that specific sector and not the first one?

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

Number  Start    End          Size         File system  Name  Flags
 1      34s      390659s      390626s      fat32              boot
 2      390660s  890660s      500001s      ext2
 3      890661s  5860533118s  5859642458s

(parted)

In case you prefer bytes as the unit:

(parted) unit B
(parted) print
Model: ATA WDC WD30EZRX-00M (scsi)
Disk /dev/sda: 3000592982016B
Sector size (logical/physical): 512B/4096B
Partition Table: gpt

Number  Start       End             Size            File system  Name  Flags
 1      17408B      200017919B      200000512B      fat32              boot
 2      200017920B  456018431B      256000512B      ext2
 3      456018432B  3000592956927B  3000136938496B

Solution 1

The size of the EFI label is usually 34 sectors, so partitions start at sector 34. This feature means that no partition can start at sector zero (0)

Source.

Solution 2

Colin Ian King's answer is correct; however, it should be noted that hard disks that use the Advanced Format feature (4096-byte physical sectors and 512-byte logical sectors) require partition start sectors to be multiples of 8 in order to get optimal performance. See this article I wrote for all the gory details. Most partitioning tools today (late 2012) align partitions on 1MiB (2048-sector) boundaries by default. Since 2048 is a multiple of 8, such partitions work fine with these disks. Most disks sold today, and certainly most of the larger models, use Advanced Format. Thus, unless you're certain that your disk does not use Advanced Format, it's best to align on 8-sector boundaries. Note that 34 is not divisible by 8; 40 is the smallest start sector for a GPT disk with a standard partition table size that works well with Advanced Format disks.

What version of Ubuntu are you installing, Kent? I haven't checked recently, but I thought that the last version or two used 2048-sector partition alignment. If you're installing something older, you might want to consider installing a newer Ubuntu, since that will give you much more up-to-date software.

Comments

[Boston Kenne]

Great answer! Just to add some information for the curious: The size of the EFI label is usually 34 sectors, so partitions start at sector 34. This feature means that no partition can start at sector zero (0). is from: 82.157.70.109/mirrorbooks/solaris10examprep/0789734613/…

[Boston Kenne]

I got that partitioning structure when installing the 64-bit version of Ubuntu Server 12.04 (so, latest LTS).

[Boston Kenne]

Thanks! I sometimes wish one could up-vote more than once. :-) And of course I want the gory details, thanks for the link, your answer and your article!

[Boston Kenne]

I tried 40s as the start sector, but can't get the partition to align properly according to Parted. Maybe I can get some wisdom from you if you have the time? askubuntu.com/questions/201164/…

[Rod Smith]

Parted is just being conservative. Using the 2048-sector alignment results in such a puny waste of disk space that it's not worth worrying about it; just use the 2048-sector alignment.