[parted-devel] The new alignment options

[Phillip Susi]

It looks like the new optimal/minimal alignment options were implemented 
half in libparted and half in parted.  libparted provides the 
ped_device_get_optimium_alignment(), but it is parted's do_mkpart() that 
attempts to enforce it.

Contrast this with the way that cylinder alignment used to work.  It was 
implemented by having parted set the PED_DISK_CYLINDER_ALIGNMENT flag, 
and that caused the disklabel to enforce the alignment requirement.  It 
seems to me that the new alignment modes should have been added in an 
orthogonal way; by adding new PED_DISK_XXX_ALIGNMENT flags and letting 
the disklabel enforce it.  Also the synthesized free space partitions 
should be subjected to the same constraints.

There are a few issues that involve the shown free space being 
misleading that I think would be fixed by this change.  When using the 
old cylinder alignment, print would show the free space starting at 
sector 63, even though you technically could create a partition starting 
at sector 1.  When using optimal alignment, print still shows free space 
starting at sector 63, even though you get an alignment exception if you 
try to start a partition there.  Not only is this confusing to a user, 
but to scripts as well.

For example, I am aware of a bug involving a script uses parted to 
automatically create a single partition in an image file.  Parted 
advertises via print free that space from 16.5kb onward is free, so the 
script creates a partition starting at 16.5kb.  This creates an 
improperly aligned partition, and prevents the installation of grub.  If 
the script requested optimal alignment, then parted should not advertise 
misaligned spaces.

Finally, if you are using optimal alignment, the free space lists small 
bits at the start and end of the disk that aren't usable without 
breaking alignment.  This is especially frustrating for gparted users 
who see the extra space at the end of the disk and keep wanting to drag 
the partition to the right to use it, and it mysteriously won't work. 
When using cylinder alignment, the advertized free space is cylinder 
aligned, so you don't see the sub-cylinder gaps, even though they are 
there, so I think that optimal mode should behave the same way.

On IRC Jim was concerned that this change would negatively impact other 
libparted users, but it would only make a difference if they choose to 
set the PED_DISK_XXX_ALIGNMENT flag, so it would be a backward 
compatible change.

I would like to hear any additional thoughts that anyone has.