Technical background
Block device partitioning has a long history, related to the physical construction of hard disk drives – to their cylinders, heads and sectors. Current hard disk drives present themselves to the operating system as a simple sequence of blocks. For practical partitioning purposes the physical construction no longer matters except that Zone Bit Recording means that a partition at the beginning of the drive (physically the outer edge) transfers data faster than one at the end of the drive. With the introduction of 4k sector sizes, aligning partitions on sector boundaries has become important.
Wikipedia zone bit recording (ZBR a.k.a Zone Constant Angular Velocity (Zone CAV, Z-CAV or ZCAV))
Wikipedia advanced format (> 512 B sectors)
The EBR/EPBR
Each logical partition is preceded by an Extended Boot Record (EBR) a.k.a. Extended Partition Boot Record (EPBR) of one sector followed by a number of unused sectors. There are commonly 62 unused sectors which, with the EBR, fill a historical 63 sector track.
Which partition management utility to use?
From the fdisk man page:
There are several *fdisk programs around. Each has its problems and strengths. Try them in the order cfdisk, fdisk, sfdisk. (Indeed, cfdisk is a beautiful program that has strict requirements on the partition tables it accepts, and produces high quality partition tables. Use it if you can. fdisk is a buggy program that does fuzzy things - usually it happens to produce reasonable results. Its single advantage is that it has some support for BSD disk labels and other non-DOS partition tables. Avoid it if you can. sfdisk is for hackers only -- the user interface is terrible, but it is more correct than fdisk and more powerful than both fdisk and cfdisk. Moreover, it can be used noninteractively.)
These days there also is parted. The cfdisk interface is nicer, but parted does much more: it not only resizes partitions, but also the filesystems that live in them.
Find an exact partition size
The easiest way to find an exact partition size is to read the /sys/block/sd*/sd*/size file, for example cat /sys/block/sda/sda2/size. This gives the size of the sda2 partition in sectors.
TODO: check what happens with a 4 kB sector size HDD. If /sys/block/sd*/sd*/size then lists the size in 4k sectors, how best to find the sector size? fdisk?
The alternatives are:
fdisk
The partition size shown by fdisk (in 1024 bytes Blocks) is rounded; if an exact partition size is required it can be calculated by End - Start + 1 (fdisk shows End and Start in sectors). For example:
root@CW8:~# fdisk -l /dev/sdb[snip]Device Boot Start End Blocks Id System[snip]/dev/sdb7 1224268508 1250261614 12996553+ 83 Linux
End - Start + 1 is 1250261614 - 1224268508 + 1 = 25993107.
Exact size in sectors, converted to blocks: 25993107 / 2 = 12996553.5
cfdisk
The most informative listing is produced by the print option -P with the display sectors option -s (other units are rounded for display).
In the output, illustrated below:
Length includes the EPBR, any following unused sectors and the logical partition itself.
Offset is the EPBR plus any following unused sectors.
First Sector is the first (and only) sector of the EPBR
The partition size is not displayed. It can be calculated from Length less Offset. For example:
root@CW8:~# cfdisk -Ps /dev/sdbPartition Table for /dev/sdb First Last # Type Sector Sector Offset Length Filesystem Type (ID) Flag-- ------- ----------- ----------- ------ ----------- -------------------- ----[snip] 7 Logical 1224268445* 1250261614* 63 25993170 Linux (83) None
Length less Offset: 25993170 - 63 = 25993107.
sfdisk TBC
parted TBC
/proc/partitions The blocks listed are 1024 byte blocks, rounded down to the nearest integer.
Create a partition with an exact size
fdisk
The u option ("change display/entry units") toggles between cylinders and sectors. It changes the Start and End addresses but not the Blocks which are always shown as 1024 byte blocks. (Tested using util-linux 2.20.1).TODO: test the text below
When creating a new partition, the default starting block leaves 62 unused sectors after the EPBR. This results in a historically conventional 63 sector track before the partition itself.
cfdisk The size entered is the EPBR+unused+partition. If an exact partition size is required, the EPBR+unused size must be added to the required size. This can most easily be done by specifying the size in sectors (append an S) to avoid rounding effects.
sfdisk TBC
parted TBC