Advanced Format Drive ?

[Warren Block]

On Thu, 15 Nov 2012, Ronald F. Guilmette wrote:
> In your tutorial document, you say:
>
>     "Create a boot partition to hold the loader, size of 512K."
>
> How big is that thing (gpart boot loader), actually?  Half a megabyte
> seems rather a bit large-ish, certainly relative to ye olde MBR loader,
> which I gather was limited to... what?  32KB (minus a little for the
> partition table) ?

/boot/gptboot is 15K, /boot/gptzfsboot is 39K.  A code limitation makes 
512K the largest this partition can be made.  So I make it that big so 
it won't have to be increased for bigger boot loaders later.  And the 
space is not wasted because of the next partition...

> Also, when creating the partition to hold the GPT boot loader, shouldn't
> that "gpart add" operation include a "-b 4k" option, you know, on a
> modern "Advanced Format" disk?  If not, why not?

-a 4k, yes.  It doesn't really matter.  The loader is read only at boot, 
once, and it's tiny.  So it doesn't really matter if it reads at 
30M/second or 500M/second.  But yes, for consistency, I'll modify that 
so the start of the freebsd-boot partition is at 40.

> You also go on to say:
>
>     "Create partition for /. It should start at the 1M boundary for proper
>     sector alignment on 4K sector drives."
>
> Come again?  Sorry, but you just lost me entirely.  In order to get "proper
> sector alignment" on one of these newer Advanced Format (4k) drives, why
> on earth should it be necessary to begin a partition at some alignment
> which is greater than the obvious minimum, i.e. 4KB ?

Starting the first filesystem partition at 1M is a semi-standard, used 
by various vendors including Microsoft.  Besides being aligned to 4K, 
it's also aligned to bigger values that can be important for performance 
on devices like SSDs.

And that explains the oversized boot partition.  It's space that would 
be unused otherwise.