[blfs-support] GUID Partition Tables (GPT)
lfs65 at cruziero.com
Thu Mar 21 12:04:46 PDT 2013
> Date: Thu, 21 Mar 2013 10:48:22 -0500
> From: Bruce Dubbs <bruce.dubbs at gmail.com>
> To: BLFS Support List <blfs-support at linuxfromscratch.org>
> Subject: Re: [blfs-support] GUID Partition Tables (GPT)
> xinglp wrote:
> > 2013/3/21 Bruce Dubbs <bruce.dubbs at gmail.com>:
> >> The example table is outdated. We really don't want the first partition
> >> to start at sector 34. gpt does the right thing by default and aligns
> >> everything at 1 MiB boundaries.
> >> 1 2082 4129 1024.0 KiB EF02 BIOS boot part
> >> 2 4130 208929 100.0 MiB 8300 Linux fs
> >> 3 208930 8392737 3.9 GiB 8300 Linux fs
> >> 4 8392738 11968424 1.7 GiB 8200 Linux swap
> >> 5 11968425 20971486 4.3 GiB 8300 Linux fs
> > if the "BIOS boot part" at 34 - 2047 sector, and then it is just 1007.0 KiB.
> > Number Start (sector) End (sector) Size Code Name
> > 1 34 2047 1007.0 KiB EF02
> > 2 2048 1361886 664.0 MiB 8300 vm1_part1
> > Althrough the first partion is not aligned, but it is just 1007.0 KiB.
> That's a good point. I hadn't thought about that, but I think you're right.
The aim of such alignments is, of course, mainly to avoid write amplification on
4k-sector disks; and to a slightly lesser extent to optimise reads. If a partition
is written to &/or read from only very rarely - like, a boot partition or grub's
boot-part area - then from the point of view of write-amplification & read-opt it
doesn't matter overly much if the partition is aligned or not at start or end
sectors. (You do of course want your normal read-write partitions to be aligned).
A possible gotcha is if some (dumb) piece of software assumed unnecessarily that
the partition had to be aligned (I could imagine grub doing something 'meta' like
that at some point).
And, of course, such userland alignments may be moot for some drives that are
suspected (or known) to do their own internal remappings to/from aligned sectors
>From the point of view of write amplification on 4k-sector disks,
It doesn't matter
For 4k-sector disks,
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