How do I set 'write-behind' on existing array?

How do I set 'write-behind' on existing array?

[Hans Kraus]

Hi,

I want to expand an existing root array with an SSD. For that purpose I
try to set firstly 'write-behind' on that array. Afterwards I will add
the SSD, remove the HDs and re-add them with 'write-mostly'. On that
array is the root filesystem, therefore stopping is no option. In
 >/home/kraush/work/mdadm/mdadm< is the latest mdadm version from git.
I tried:
-----------------------------------------------------------------------
root@elefant:/home/kraush/work/mdadm/mdadm# ./mdadm --manage 
--write-behind=4096 /dev/md4
mdadm: :option --write-behind not valid in manage mode
root@elefant:/home/kraush/work/mdadm/mdadm# ./mdadm --grow 
--write-behind=4096 /dev/md4
mdadm: no changes to --grow
-----------------------------------------------------------------------
What is the correct syntax?

Regards, Hans

Re: How do I set 'write-behind' on existing array?

[NeilBrown]

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On Mon, 20 Oct 2014 14:33:31 +0200 Hans Kraus <hans@hanswkraus.com> wrote:

> Hi,
> 
> I want to expand an existing root array with an SSD. For that purpose I
> try to set firstly 'write-behind' on that array. Afterwards I will add
> the SSD, remove the HDs and re-add them with 'write-mostly'. On that
> array is the root filesystem, therefore stopping is no option. In
>  >/home/kraush/work/mdadm/mdadm< is the latest mdadm version from git.
> I tried:
> -----------------------------------------------------------------------
> root@elefant:/home/kraush/work/mdadm/mdadm# ./mdadm --manage 
> --write-behind=4096 /dev/md4
> mdadm: :option --write-behind not valid in manage mode
> root@elefant:/home/kraush/work/mdadm/mdadm# ./mdadm --grow 
> --write-behind=4096 /dev/md4
> mdadm: no changes to --grow
> -----------------------------------------------------------------------
> What is the correct syntax?
> 

Not quite.

"write-behind" is a property of the bitmap.
To enable write-behind on an array you first need to remove the bitmap if
there is one
  mdadm /dev/md4 --grow --bitmap=none
then add a new bitmap with the required property
  mdadm /dev/md4 --grow --bitmap=internal --write-behind=4096

NeilBrown

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