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


From: Michael Chang
Subject: [PATCH] docs: Update for stopping small mbr gap support
Date: Thu, 5 Mar 2020 18:40:01 +0800
User-agent: Mutt/1.10.1 (2018-07-13)

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 size
that goes beyond 31 KiB, this statement becomes a true liability as
people would be misguided and think it is still fine to use small MBR
gap, that has always been a headache in distribution's upgrade path as
growing new feature would render the size requirement bigger but no way
for the user to relocate their partitions.

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>
---
 docs/grub.texi | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/docs/grub.texi b/docs/grub.texi
index 83979af38..651468268 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; 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. For 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 stardard
+practice.
+
+In case legacy systems that cannot boot if first partition not on the cylinder
+boundary, the fallback blocklist install method should remain working for them
+if the core image growing 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.
 
 @heading GPT
 
-- 
2.16.4




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