Re: very large data-offset?

[NeilBrown <neilb@suse.com>]

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On Thu, Nov 30 2017, Duane wrote:

> Why is the data offset set so big? I created a 3x4TB RAID5 array and the 
> data offset was 128MB. Chunk size was the default 512kB.

It is less than 0.1% of the device...

>
> I cannot see why such a large offset is used. I would think the data 
> offset need only be at most the chunk size plus the space (1 sector) for 
> the superblock and bitmap.

It is insurance.  If you want to change the chunksize later, having a
lot of head-room will allow the reshape to go much faster.

>
> When reshaping the array, I am prompted to use an external file, so I 
> don't see that mdadm ever uses the space.

Citation needed.... what version of mdadm, what kernel?  What reshape
command?

>
>
> I tried making some test arrays and got much smaller sizes. A 3x1GB 
> RAID5 array with 64k chunks had a 1MB data offset.
>
>
> If I make a 7x4TB RAID5 array with 64kB chunks, is there a problem with 
> setting the data offset to around 2MB?

Only that it might reduce your options in the future, though probably
not by much.

NeilBrown

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