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Re: Free space after partitioning



On Wed, 2012-03-21 at 14:32 +0100, Jan Safranek wrote:
> On 03/21/2012 02:18 PM, David Lehman wrote:
> > On Wed, 2012-03-21 at 13:55 +0100, Jan Safranek wrote:
> >>
> >>   start: 110592  end: 112454  length: 1863
> >> This is a free space between two logical partitions.
> >> -> Again, why is the free region here? And there are 186 'hidden'
> >> sectors between this free space end and start of the next logical partition.
> > 
> > We are somewhat more liberal with rounding start sector up for logical
> > partitions, basically because parted is more aggressive in aligning
> > start of logical partitions than it is with normal partitions. Also, we
> > need to account for at least one sector of metadata before each logical
> > partition. In some unfortunate cases this bump to the next MB boundary
> > can amount to 2047 520-byte sectors. Again, parted is so aggressive with
> > this that we had to do the same in order to ensure that parted
> > magic/fixups do not cause unwanted surprises when the partitions are
> > added to disk.
> 
> Please correct me if I am wrong - I understand it as the logical
> partition metadata is at sector 112455 (=end of the free space
> before+1), then there are 185 unused sectors because of alignment and
> the usable data on a partition starts at sector 112640, as reported by
> fdisk and storage.partitions. Nothing can ever touch these 186 sectors,
> because it might overwrite the metadata.

Correct.

> 
> >> If the alignment to cylinder boundary is cause of these weird numbers,
> >> can I turn the alignment off in pyanaconda.storage? The numbers
> >> reported by disks are completely artificial anyway these days.
> > 
> > We turn off cylinder alignment already.
> 
> What alignment is used then? Why the partitions don't follow each other?

The kernel has specified that partitions on that disk should always
start on a 2048-sector boundary. Aligning up from sector 1 or 63 gets
you to sector 2048. Likewise, the rest of the start sectors will all be
multiples of 2048.

> 
> Jan
> 
> >> /dev/sda6           90112      110591       10240   83  Linux
> >> /dev/sda7          112640      133119       10240   83  Linux
> 
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