Re: fdisk and alignment

[David Brownlee <abs%absd.org@localhost>]

On Mon, 10 Jul 2023 at 10:06, Martin Husemann <martin%duskware.de@localhost> wrote:
>
> On Sun, Jul 09, 2023 at 08:54:56PM +0100, David Brownlee wrote:
> > In practice I think it would be better not to penalise current use of
> > "small" (120GB SSD or 64GB or or CF/USB devices) for an edge case of
> > older hardware
>
> I am not sure what you see as penalty here.

An offset of 63 sectors is guaranteed to be non-optimal for any device
with larger than 512 byte internal block size. As such it's likely a
poor choice for any solid state storage, and potentially some
traditional disks (though any of the latter with > 512 byte sectors
are almost certainly > 128GB and so avoid the behaviour).

> I think two factors made the original size decisions sensible back then:
>
>  - for large disks the space lost to alignment does not matter
>  - native block sizes > 512 byte only happened for larger disks initially
>
> Overall it would be better to enforce alignment as least as large as the
> native block size on *any* disk - but IIRC it is quite hard to find that
> value in all cases.

We currently effectively have two values - 63 sectors and 1MB. We have
the cutoff between them at 128GB. I propose moving that to 2GB, to
gain the more optimal alignment on devices from 2GB to 128GB.

Thanks

David