mdadm format for manual destriping

mdadm format for manual destriping

[Marek]

Hi,

is there a documentation available on how mdadm stores data? E.g.
1.  super-block
2.  offset of first block on each drive
3. whether mdadm stores blocks as raw data or stores some kind of
metadata associated with each block e.g. disk 1 -> data_block1
immidiatelly followed by data_block 5 or
metadata_for_block_1 | data_block 1 |  metadata_for_block_5 | data_block 5 |
4. how and where mdadm stores its backup superblock
5. what klind of XOR algorithm is used by mdadm

thanks,

Marek

Fwd: mdadm format for manual destriping

[Marek]

I have found the following link which answers question 1.
But there's more stuff that's seems undocumented
https://raid.wiki.kernel.org/index.php/RAID_superblock_formats
Question 2
Does the superblock of size 256+2 start at 0?
Does the first block start immediately after 258 bytes?
Also question 3,4,5

Marek

On Saturday, October 24, 2015, Marek <mlf.conv@gmail.com> wrote:
>
> Hi,
>
> is there a documentation available on how mdadm stores data? E.g.
> 1.  super-block
> 2.  offset of first block on each drive
> 3. whether mdadm stores blocks as raw data or stores some kind of
> metadata associated with each block e.g. disk 1 -> data_block1
> immidiatelly followed by data_block 5 or
> metadata_for_block_1 | data_block 1 |  metadata_for_block_5 | data_block 5 |
> 4. how and where mdadm stores its backup superblock
> 5. what klind of XOR algorithm is used by mdadm
>
> thanks,
>
> Marek

Re: Fwd: mdadm format for manual destriping

[Phil Turmel]

Hi Marek,

{ Top-posting fixed.  Please don't do that. }

On 10/26/2015 02:09 PM, Marek wrote:
> On Saturday, October 24, 2015, Marek <mlf.conv@gmail.com> wrote:

>> is there a documentation available on how mdadm stores data? E.g.

Yes, the man pages.  Try "man md" and "man mdadm" in a console.  This
documentation is provide by the kernel team and, other than the source
code itself, is expected to be definitive.

>> 1.  super-block

The location of the superblock is described in "man md".  The precise
content of the superblock is only detailed in the source code.

>> 2.  offset of first block on each drive

Zero for metadata versions 0.90 and 1.0.  Configurable for metadata
versions 1.1 and 1.2.  See the --data-offset option in "man mdadm".

>> 3. whether mdadm stores blocks as raw data or stores some kind of
>> metadata associated with each block e.g. disk 1 -> data_block1
>> immidiatelly followed by data_block 5 or
>> metadata_for_block_1 | data_block 1 |  metadata_for_block_5 | data_block 5 |

No metadata per block.

>> 4. how and where mdadm stores its backup superblock

None, although each device has one.

>> 5. what klind of XOR algorithm is used by mdadm

Straight parity for raid4, raid5, and the 'P' blocks of raid6.  Galois
field arithmetic for the 'Q' blocks of raid6.

https://www.kernel.org/pub/linux/kernel/people/hpa/raid6.pdf

[trim /]

> Does the first block start immediately after 258 bytes?

No.

> Also question 3,4,5

See above and in the man pages.

HTH,

Phil