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Date:      Fri, 27 Aug 2021 16:45:31 +0200
From:      Christian Groessler <chris@groessler.org>
To:        freebsd-questions@freebsd.org
Subject:   Re: Paritioning scheme on MBR disk doubts
Message-ID:  <afc3d944-08e7-618a-0688-68862d559361@groessler.org>
In-Reply-To: <20210827064821.0678b4da@archlinux>
References:  <MW4PR01MB640175FEDE09CAD451A9AA0BC4C79@MW4PR01MB6401.prod.exchangelabs.com> <d21dc1de-87dc-8e45-260b-cf7baf23998a@holgerdanske.com> <MW4PR01MB6401DA1B3479DE69944C7D5BC4C79@MW4PR01MB6401.prod.exchangelabs.com> <8ab9716c-fdc3-ad09-1883-8210b37ef520@groessler.org> <20210827064821.0678b4da@archlinux>

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On 8/27/21 6:48 AM, Ralf Mardorf wrote:
> On Thu, 26 Aug 2021 20:53:25 +0200, Christian Groessler wrote:
>> If I remember correctly, an extended partition can also only hold 4
>> partitions. Of which one in turn can again be an extended partition
>> which can hold 4 partitions. Of which one in turn can again be an
>> extended partition... etc
> That's incorrect.
>
> "Master Boot Record (partition table)
>
> In the MBR partition table (also known as DOS or MS-DOS partition
> table) there are 3 types of partitions:
>
>      Primary
>      Extended
>          Logical
>
> Primary partitions can be bootable and are limited to four partitions
> per disk or RAID volume. If the MBR partition table requires more than
> four partitions, then one of the primary partitions needs to be
> replaced by an extended partition containing logical partitions within
> it.
>
> Extended partitions can be thought of as containers for logical
> partitions. A hard disk can contain no more than one extended
> partition. The extended partition is also counted as a primary
> partition so if the disk has an extended partition, only three
> additional primary partitions are possible (i.e. three primary
> partitions and one extended partition). The number of logical
> partitions residing in an extended partition is unlimited. A system
> that dual boots with Windows will require for Windows to reside in a
> primary partition.
>
> The customary numbering scheme is to create primary partitions sda1
> through sda3 followed by an extended partition sda4. The logical
> partitions on sda4 are numbered sda5, sda6, etc. Tip: When partitioning
> a MBR disk consider leaving at least 33 512-byte sectors (16.5 KiB) of
> free unpartitioned space at the end of the disk in case you ever decide
> to convert it to GPT. The space will be required for the backup GPT
> header." -
> https://wiki.archlinux.org/title/partitioning#Master_Boot_Record_(partition_table)


The first sector of an extended partition has the same layout as the MBR.
Means that it has 4 slots for partition information. If one of the slots has
the "extended partition" ID it can in turn define more partitions.

That this is only supported 1 level deep is an artificial restriction.

regards,
chris





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