Re: [PATCH v2] docs: Update for stopping small mbr gap support

[Javier Martinez Canillas]

Hello Michael,

On 3/6/20 7:53 AM, Michael Chang wrote:
> Further to the discussion about disabling btrfs zstd support for
> i386-pc[1], this paragraph in manual about mbr gap size doesn't seem to
> hold true any longer.
> 
> "You must ensure that the first partition starts at least 31 KiB (63
> sectors) from the start of the disk"
> 
> As in many occasions we inevitablely have to provide core image with the
> size that goes beyond 31 KiB. For instance, diskfilter and crypto
> modules which are needed by root disk formatted with btrfs, lvm, mdadm
> and so on would add quite a lot space to the image.
> 
> So this misinformation would have people misguided and thought that it
> is still fine to use small MBR gap utill some point of time the update
> has grown the size too much that the grub-install can no longer embed
> the image to the mbr gap. In this case changing the partition layout is
> required but it is never easy to do so.
> 
> The patch tries to correct the paragraph with a more practical size that
> works for grub and also for modern computer systems in general.
> 
> [1] https://lists.gnu.org/archive/html/grub-devel/2019-11/msg00025.html
> 
> Signed-off-by: Michael Chang <address@hidden>
> ---
>

Agreed with the patch.

Reviewed-by: Javier Martinez Canillas <address@hidden>
 
> Changes since v2:
>  * Rework a paragraph in commit message and also some places in manual
>    to be more clear to read
>  * Correct some typos
> 
>  docs/grub.texi | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/docs/grub.texi b/docs/grub.texi
> index 83979af38..4614a2ee1 100644
> --- a/docs/grub.texi
> +++ b/docs/grub.texi
> @@ -845,12 +845,20 @@ only be used if the @file{/boot} filesystem is on the 
> same disk that the
>  BIOS boots from, so that GRUB does not have to rely on guessing BIOS drive
>  numbers.
>  
> -The GRUB development team generally recommends embedding GRUB before the
> -first partition, unless you have special requirements.  You must ensure that
> -the first partition starts at least 31 KiB (63 sectors) from the start of
> -the disk; on modern disks, it is often a performance advantage to align
> -partitions on larger boundaries anyway, so the first partition might start 1
> -MiB from the start of the disk.
> +The GRUB development team generally recommends embedding GRUB before the 
> first
> +partition, unless you have special requirements. You must ensure that the 
> first
> +partition starts at least 1 MiB from the start of the disk; Additionally, on
> +modern disks it is often a performance advantage to align partitions on 
> larger
> +boundaries and 1 MiB is the least common multiple of many used alignment 
> sizes.
> +E.g. SSD, it became crucial to have the partition correctly aligned to avoid
> +excessive read-modify-write cycles and thus modern tools set to use 1 MiB as 
> a
> +standard practice.
> +
> +In case of legacy systems that cannot boot if first partition is not on the
> +cylinder boundary, the fallback blocklist install method should remain 
> working
> +for them if the core image grows too much someday. Here we just can't 
> advertise
> +that 31 KiB (63 sectors) is a sensible size any longer as that would pose 
> great
> +constraint to include new features as time goes by.
>

I think is also worth mentioning that most partition tools these days default to
the optimal alignment for the disk. But could be done as a follow-up patch if 
you
agree that adding this information would be useful.

Best regards,
-- 
Javier Martinez Canillas
Software Engineer - Desktop Hardware Enablement
Red Hat