On Sat, 01 Mar 2014 00:23:59 +1100 Eyal Lebedinsky <eyal@eyal.emu.id.au>
wrote:

> Thanks Piergiorgio,
> 
> I did search before, unsuccessfully. I repeated now with different keywords and found
> this entry in the kernel wiki
> 	https://raid.wiki.kernel.org/index.php/RAID_superblock_formats
> 
> It documents the initial fixed fields of the superblock. I still do not know how
> the intent bitmap is laid out. I can see that it starts 4KB into the superblock:
> 	Internal Bitmap : 8 sectors from superblock
> but did not yet find its size (which I expect depends on the array size).
> 
> I guess I can calculate it from the sys/block/md127/md items
> 	component_size * 1024 / 'bitmap/chunksize' / 8
> which comes up to 7452 bytes which is still a small fraction of the 128MB header size.
> 
> What else is there? bad blocks list? Reading an old blog (from Neil) suggests that it
> is not larger than 32KB (but is only 4KB now), so still "small" in this context.
> Don't know where it resides though.
> 
> I need to understand the full layout of the header and so far I do not see anything that
> says what the area past the initial 16KB is used (is always zero when I inspect it).
> I started a heavy file copy on the raid and watched the header and never saw any change.
> I expected to see at least some activity in the bitmap but none encountered.
> 
> My simple question is: is it the case that the reserved header space after 16KB is
> actually still unused in header version 1.2? My bad sector is practically at the end
> of this 128MB area. A trivial question to answer for someone with expert knowledge
> of md.
> 
> Anyone?

The location of the superblock is reported by "mdadm --exmamine".
The size is 4K (Though most of that is unsed).
The location of the bitmap (if present) is reported by "mdadm --examine"
as an offset from the superblock.  It size can be deduced from the output
of "mdadm --examine-bitmap".  Take the number of bits, divide by 8, add 256
and round up to a multiple of 512.  This number is bytes.
The location of the bad-block-log (if present) is reported by "mdadm
--examine" as an offset from the superblocks.  It's size if 4K.
Any other space outside of the data region is currently unused by md.

NeilBrown