On Fri, 01 Feb 2013 12:50:45 -0500
Phil Turmel [off-list ref] wrote:

On 02/01/2013 12:17 PM, Dominique wrote:

quoted

Hi list,

I was wondering what would be the best way to convert a 6 hdd raid 5 to
raid 6. Ideally without having to reformat everything and lose all the
data in the process. I have backups of the data but I don't look forward
reinstalling everything on that server.

...

3) Convert array to raid6 w/ "mdadm --grow --level=raid6"

--layout=preserve will make it an order of magnitude faster. Read "man mdadm"
for more details. (And "man md" to learn about things in general.)

That's all while still assuming you *can* or want to add a 7th drive.
If not, things become more complex.

-- 
With respect,
Roman