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raid6 reshape wasn't added until 2.6.21.  Before that only raid5 was
supported.
You also need to ensure that CONFIG_MD_RAID5_RESHAPE=y.
        

I don't see that in the config. Should I add it? Then reboot?

Don't know how I missed it first time, but that is in my config.

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You reported that you were running a 2.6.20 kernel, which doesn't
support raid6 reshape.
You need to compile a 2.6.21 kernel (or
   apt-get install linux-image-2.6.21-1-amd64
or whatever) and ensure that CONFIG_MD_RAID5_RESHAPE=y is in the
.config before compiling.
    

There only seems to be version 2.6.20 does this matter a lot? Also how do I
specify what is in the config when using apt-get install?
  

2.6.20 doesn't support the feature you want, only you can tell if that
matters a lot. You don't, either get a raw kernel source and configure,
or run what the vendor provides for config. Sorry, those are the option.

I have finally managed to compile a new kernel (2.6.21) and boot it.

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I used apt-get install mdadm to first install it, which gave me 2.5.x then
I
downloaded the new source and typed make then make install. Now mdadm -V
shows "mdadm - v2.6.2 - 21st May 2007".
Is there anyway to check it is installed correctly?

The "mdadm -V" check is sufficient.

Are you sure because at first I just did the make/make install and mdadm -V
did tell me v2.6.2 but I don't believe it was installed properly because it
didn't recognise my array nor did it make a config file, and cat
/proc/mdstat said no file/directory??

mdadm doesn't control the /proc/mdstat file, it's written by the kernel.
The kernel had no active array to mention in the mdstat file.

I see, thanks. I think it is working OK.

I am currently growing a 4 disk array to an 8 disk array as a test, and if
it that works I'll use those 8 and add them to my original 8 to make a 16
disk array. This will be a while yet as this first grow is going to take
2000 minutes. It looks like it's going to work fine, but I'll report back in
a couple of days.

Thank you so much for your help; Dan, Bill, Neil, Justin and everyone else.

The last thing I would like to know is if it is possible to 'clean' the
super blocks to make sure they are all OK. TIA.


Iain