Why fdisk wants the first partition to start at 1 MiB?

Why fdisk wants the first partition to start at 1 MiB?

[Francesco Turco]

Hello.

I'm trying to use fdisk (from util-linux 2.19.1) for creating a
partition on a drive. I noticed that the start sector of the first
partition must be at least 2048, that is 1 MiB from the beginning of the
drive. This can be changed by entering the "expert mode" and using the
"move beginning of data in a partition" option. But I'm still wondering
why fdisk reserves so much space by default.

As far as I know the only sector that should not be used for partitions
is the first one, that is, sector 0. It is reserved for the MBR. So the
first partition can start at sector 1. I read that the 1 MiB thing is
Windows related: http://en.wikipedia.org/wiki/Logical_Disk_Manager. Or
it's something that Linux users should also care about? I can't find a
convincing explanation anywhere.

This 1 MiB thing seems to affect parted, too, as it wants partition
boundaries to be multiples of 1 MiB. I don't know if it's related to the
problem I have with fdisk, though.

Any help is much appreciated.

Re: Why fdisk wants the first partition to start at 1 MiB?

[Kay Sievers]

On Tue, Sep 25, 2012 at 10:33 PM, Francesco Turco <fturco@fastmail.fm> wrote:
> I'm trying to use fdisk (from util-linux 2.19.1) for creating a
> partition on a drive. I noticed that the start sector of the first
> partition must be at least 2048, that is 1 MiB from the beginning of the
> drive. This can be changed by entering the "expert mode" and using the
> "move beginning of data in a partition" option. But I'm still wondering
> why fdisk reserves so much space by default.
>
> As far as I know the only sector that should not be used for partitions
> is the first one, that is, sector 0. It is reserved for the MBR. So the
> first partition can start at sector 1. I read that the 1 MiB thing is
> Windows related: http://en.wikipedia.org/wiki/Logical_Disk_Manager. Or
> it's something that Linux users should also care about? I can't find a
> convincing explanation anywhere.
>
> This 1 MiB thing seems to affect parted, too, as it wants partition
> boundaries to be multiples of 1 MiB. I don't know if it's related to the
> problem I have with fdisk, though.

It's a de-facto standard, which Windows does too. The first megabyte
is reserved here for a boot loader or any other management data that
could be needed for a disk or box to boot from.

Boot loader issues are probably not that interesting on EFI boxes and
other non-BIOS hardware, but on usually big sized disks it's still a
safe default.

Kay

Re: Why fdisk wants the first partition to start at 1 MiB?

[Davidlohr Bueso]

On Tue, 2012-09-25 at 22:50 +0200, Kay Sievers wrote:
> On Tue, Sep 25, 2012 at 10:33 PM, Francesco Turco <fturco@fastmail.fm> wrote:
> > I'm trying to use fdisk (from util-linux 2.19.1) for creating a
> > partition on a drive. I noticed that the start sector of the first
> > partition must be at least 2048, that is 1 MiB from the beginning of the
> > drive. This can be changed by entering the "expert mode" and using the
> > "move beginning of data in a partition" option. But I'm still wondering
> > why fdisk reserves so much space by default.
> >
> > As far as I know the only sector that should not be used for partitions
> > is the first one, that is, sector 0. It is reserved for the MBR. So the
> > first partition can start at sector 1. I read that the 1 MiB thing is
> > Windows related: http://en.wikipedia.org/wiki/Logical_Disk_Manager. Or
> > it's something that Linux users should also care about? I can't find a
> > convincing explanation anywhere.
> >
> > This 1 MiB thing seems to affect parted, too, as it wants partition
> > boundaries to be multiples of 1 MiB. I don't know if it's related to the
> > problem I have with fdisk, though.
> 
> It's a de-facto standard, which Windows does too. The first megabyte
> is reserved here for a boot loader or any other management data that
> could be needed for a disk or box to boot from.
> 
> Boot loader issues are probably not that interesting on EFI boxes and
> other non-BIOS hardware, but on usually big sized disks it's still a
> safe default.

Right, in fact GPT only reserves 17Kib and the first partition can start
at sector 34, with 512 sector size.

Davidlohr

Re: Why fdisk wants the first partition to start at 1 MiB?

[Karel Zak]

On Tue, Sep 25, 2012 at 10:50:44PM +0200, Kay Sievers wrote:
> On Tue, Sep 25, 2012 at 10:33 PM, Francesco Turco <fturco@fastmail.fm> wrote:
> > I'm trying to use fdisk (from util-linux 2.19.1) for creating a
> > partition on a drive. I noticed that the start sector of the first
> > partition must be at least 2048, that is 1 MiB from the beginning of the
> > drive. This can be changed by entering the "expert mode" and using the
> > "move beginning of data in a partition" option. But I'm still wondering
> > why fdisk reserves so much space by default.
> >
> > As far as I know the only sector that should not be used for partitions
> > is the first one, that is, sector 0. It is reserved for the MBR. So the
> > first partition can start at sector 1. I read that the 1 MiB thing is
> > Windows related: http://en.wikipedia.org/wiki/Logical_Disk_Manager. Or
> > it's something that Linux users should also care about? I can't find a
> > convincing explanation anywhere.
> >
> > This 1 MiB thing seems to affect parted, too, as it wants partition
> > boundaries to be multiples of 1 MiB. I don't know if it's related to the
> > problem I have with fdisk, though.
> 
> It's a de-facto standard, which Windows does too. The first megabyte
> is reserved here for a boot loader or any other management data that
> could be needed for a disk or box to boot from.

 The real reason is that 1MiB is ideal offset to keep partitions
 aligned on almost all random hardware. This offset is compatible with
 512-byte, 4096-byte sector devices, many raid devices as well as old
 broken WD disks where physical sector size has been incorrectly
 reported. You can use dd(1) to move your PT + data to another device
 without care about physical device topology (I/O limits), etc.

 It's also important to keep your partition sizes aligned to MiB --
 this is for example default in fdisk if you specify the size in
 +<size>{M,G} convention.

> Boot loader issues are probably not that interesting on EFI boxes and
> other non-BIOS hardware, but on usually big sized disks it's still a
> safe default.

 Yes.

 Probably the most complete docs with many references:
 https://ata.wiki.kernel.org/index.php/ATA_4_KiB_sector_issues

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com

Re: Why fdisk wants the first partition to start at 1 MiB?

[Francesco Turco]

> It's a de-facto standard, which Windows does too. The first megabyte
> is reserved here for a boot loader or any other management data that
> could be needed for a disk or box to boot from.

So as far as I understood both starting at 1 MiB and having partition
boundaries at multiples of 1 MiB is for:

- Dealing with all possible situations (Windows, bootloaders, 4K
sectors, ...), both at present and in the foreseeable future
- Avoiding each person to use different partitioning rules (as I were
trying to do)

Am I right?