Re: mdadm - level change from raid 1 to raid 5

[Christoph Hellwig <hch@infradead.org>]

On Mon, Oct 03, 2011 at 09:07:44PM +1100, NeilBrown wrote:
> The default for chunksize should be 512K I thought..

It is.

> I once saw a mathematical formula, but it was a function of the number of
> concurrent accesses and the average IO size - I think.
> 
> Big is good for large streaming requests.  Smaller is good for lots of random
> IO.  Only way to know for sure is to measure your workload on different sizes.
> 
> You can change it once the array is build, but it is a very slow operation as
> it has to move every block on every disk to somewhere else.
> 
>  mdadm -G /dev/md2 --chunk=32

FYI: For XFS I always get much better results using 32k chunk size, even
for simple streaming reads/writes.  I haven't really tracked down why.

Also for any modern system I always have to massively increase the
stripe cache size.