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 <mch...@suse.com>
> ---
>
Agreed with the patch.
Reviewed-by: Javier Martinez Canillas <javi...@redhat.com>
> 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
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