Re: [Gptfdisk-general] gdisk skip sectors when suggesting the 1st sector of a partition

[Rod S. <rod...@ro...>]

On 8/7/20 4:51 PM, u3...@ne... wrote:
> All the existing partitions have their first sector immediately following 
> the last sector of their prior partition.
> Why gdisk suggest to start the 8th partition at sector #59424768? The last sector
> of the 7th partition is at sector #59422999.
> gdisk is 1.0.5.
> A non destrcutive read only badblocks -sv doesn't find any badblock.

By default, GPT fdisk begins partitions on 2048-sector (1 MiB)
boundaries, if the disk uses 512-byte logical sectors (as most do). If
the final sector of partition #7 is 59422999, then the next free sector
would be 59423000, which is not divisible by 2048, so GPT fdisk won't
start a partition there. Instead, it will round up to the next free
divisible-by-2048 value, which is 59424768, just as you report.

This is done because it optimizes performance on many disk devices. Most
modern hard disks use what's called Advanced Format, which uses
4096-byte physical sectors that are broken up into eight 512-byte
logical sectors. These require alignment on 8-sector boundaries for
optimal performance. Many RAID arrays have requirements to align on 256
KiB, 512 KiB, or similar power-of-two boundaries. SSDs vary a lot in
their requirements, but some of them have a power-of-2 optimal
alignment, too. A 1 MiB alignment meets all of these technologies' needs
for optimal alignment. (There are some oddball SSDs and other
configurations that might work better with other types of alignment.)
For this reason, most modern partitioning tools align partitions' starts
to 1 MiB boundaries. (Partition end points are not important, AFAIK.)

If you understand your disk's alignment needs and want to adjust the
setting in GPT fdisk, you can do so from the experts' menu with the "l"
(lowercase L) option. You can see the current alignment with the "d"
option on the experts' menu. For instance, you could set this to 8 if
you're using an Advanced Format disk without RAID; or to 1 if you're
using a non-Advanced-Format hard disk without RAID. Changing this value
will enable you to reclaim a little otherwise lost disk space; but the
amount is puny on modern hard disks. Even modern USB flash drives have
capacities measured in the tens of GB, so saving under 1 MiB at the
start of the disk, or between partitions if they aren't sized to precise
multiples of 1 MiB, represents less that 0.1% of the disk's capacity. On
a hard disk with a capacity measured in the TB range, the savings would
be two or three orders of magnitude less, on a percentage basis.

I don't know how you created your existing partitions. You can check for
proper alignment (that is, alignment matching GPT fdisk's current
setting) via the "v" option in gdisk. It's possible that partition #7 is
not properly aligned; or it could simply have a size that's not an exact
multiple of 2048 sectors. Changing the alignment would require a tool
like GParted, which understands filesystems as well as partitions, and
it might or might not be worth the bother depending on the disk type and
partition contents.

-- 
Rod Smith
rod...@ro...
http://www.rodsbooks.com