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bug#17994: Linux RAID MBR type code

From: Chris Murphy
Subject: bug#17994: Linux RAID MBR type code
Date: Sun, 13 Jul 2014 19:07:24 -0600

On Jul 13, 2014, at 4:41 PM, Phillip Susi <address@hidden> wrote:

> Hash: SHA512
> On 07/10/2014 07:58 PM, Chris Murphy wrote:
>> This is in master branch.
>> libparted/labels/dos.c 98    #define PARTITION_LINUX_RAID    0xfd
>> This type code and metadata version 0.9 are long deprecated.
>> Parted lacks support for the "non-fs data" partition type code
>> 0xda, which is what should be used for mdadm metadata 1.x
>> partitions.
>> man 8 mdadm: "When  creating  a partition based array, using mdadm 
>> with version-1.x metadata, the partition type should be set to
>> 0xDA (non fs-data).  This type selection allows for greater
>> precision since using any other [RAID auto-detect (0xFD) or a
>> GNU/Linux partition (0x83)], might create problems in the event of
>> array recovery through a live cdrom."
> Why does it matter?  Linux doesn't pay attention to the partition type
> code anyhow.  I've always just used 0x83.


I find this logic troubling. It's rather similar to the logic that lead to 
parted using the pre-existing Microsoft basic data GUID when making Linux 
partitions on GPT disks; out of a pool of just under infinite alternative 
GUIDs. "Oh it doesn't really matter" on Linux, but meanwhile on dual boot 
systems, Windows recognizes its partitiontype GUID, but not the contents of the 
partition, and actively invites the user to reformat it.

For example, 0x83 partition type, and mdadm metadata 1.0 on md raid1 suggests 
that the partition can be mounted stand alone rather than first assembling the 
raid. If something actually were to do this, the array would become 
inconsistent and unrepairable without rather knowledgable manual intervention. 
A partition with md metadata is in fact not a Linux filesystem, so really we 
shouldn't lie about what it is by using the wrong partition type code.

Chris Murphy

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