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BIOS hard disk geometry is set suchwise that if you want good performance the partitions have to be aligned correctly. The smallest increment of a disk partition is a cylinder. You cannot have a partition with a fraction of a cylinder in it. That means that the partition cannot be any size. The partitioner will round up or round down to the nearest size to the desired size that lines up properly with the disk geometry
The reason they're being rounded down is as AwesomeMachine posted. They're automatically being aligned to the nearest full cylinder. I wish I knew how to make it round UP instead of down (I've had this issue quite a few times myself), but other than figuring out that, those numbers are the closest to what you want it's able to give you while still aligning to a cylinder (which gets REALLY odd when you start using SSD's, which of course don't have cylinders anymore but it still does it).
The smallest increment of a disk partition is a cylinder. You cannot have a partition with a fraction of a cylinder in it. That means that the partition cannot be any size.
That is nonsense. For hard disks manufactured in the last 30 years, the actual layout of data on the disk bears no relationship whatsoever to the CHS geometry that some partitioning tools still treat as meaningful. Modern disks have a variable number of sectors per track, with the longer, outer cylinders holding more sectors than the shorter tracks on the inner cylinders. The actual track layout is known only to the firmware on the drive. It can be inferred only by sensitive tests examining timing, and even there the drive's internal read-ahead and buffering confuse the result.
Yes, there are alignment considerations, the principal ones being the alignment of 512-byte LBA addresses with 4096-byte physical sectors, and the (unspecified) size of SSD erase blocks. Neither of these has anything to do with CHS addressing. Modern partitioning tools align partitions on 1 MiB boundaries, ignoring CHS completely.
Filesystem Size Used Avail Use% Mounted on
/dev/dm-0 9.1G 780M 7.8G 9% /
looks like its from 'df -h'.
1. There is an overhead of structuring the disk block layout ie it needs to use some space from the raw (see also the GB v GiB above) and also default 4k block for the top dir inode.
2. Default reserved space for recent mkfs eg mkfs.ext4 is 5% http://linux.die.net/man/8/mkfs.ext4
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 465.8G 0 disk
Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes