What is the sector size in /sys/block/md*/md items?

What is the sector size in /sys/block/md*/md items?

[Eyal Lebedinsky]

I am reading the kernel Documentation/md.txt.

It explains what md the items in sysfs mean. It refers a few times to "sectors".
- Are we talking 512 bytes or the underlying physical disk sector size?
- Do all disks in an array need to have the same sector size?

Seeing how md can be built over physical disks, partitions, loop mounts and more,
I expect that 'sector' is short for '512 bytes' and real disk sector size is not
relevant here.

Nevertheless, there is md*/queue/hw_sector_size which gives 512. So there are now
a few sector sizes (the array and the underlying devices) and the text does not
make it clear which is used where. I assume the array cluster size is the one
referred to in this document.

TIA

--
Eyal Lebedinsky (eyal@eyal.emu.id.au)

Re: What is the sector size in /sys/block/md*/md items?

[NeilBrown]

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On Sat, 22 Feb 2014 12:11:58 +1100 Eyal Lebedinsky <eyal@eyal.emu.id.au>
wrote:

> I am reading the kernel Documentation/md.txt.
> 
> It explains what md the items in sysfs mean. It refers a few times to "sectors".
> - Are we talking 512 bytes or the underlying physical disk sector size?

Always 512 bytes.

> - Do all disks in an array need to have the same sector size?

No.

> 
> Seeing how md can be built over physical disks, partitions, loop mounts and more,
> I expect that 'sector' is short for '512 bytes' and real disk sector size is not
> relevant here.

Correct.

> 
> Nevertheless, there is md*/queue/hw_sector_size which gives 512. So there are now
> a few sector sizes (the array and the underlying devices) and the text does not
> make it clear which is used where. I assume the array cluster size is the one
> referred to in this document.

I was never happy when 'queue' appeared under md*.  It is almost entirely
irrelevant.

Feel free to post a patch to clarify md.txt.

Thanks,
NeilBrown


> 
> TIA
> 
> --
> Eyal Lebedinsky (eyal@eyal.emu.id.au)
> --
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