On Monday 14 June 2010, Neil Brown wrote:

On Sun, 13 Jun 2010 16:28:34 +0200

Bernd Schubert [off-list ref] wrote:

quoted

On Friday 11 June 2010, Neil Brown wrote:

quoted

On Fri, 11 Jun 2010 00:31:57 -0400

"Graham Mitchell" [off-list ref] wrote:

quoted

Can you do this on a live array, or can it only be done (as the docs
seem to suggest), with the create, build and grow options?

As 'grow' can (and must) be used on a live array you're question
doesn't exactly make sense.
Yes: it can be done on a live array.

While I have done this myself a couple of times, I still do not
understand where it takes the disk space for the bitmap journal from? Is
this space mdadm reserved for this purpose?

Sort-of.
It uses space that the alignment requirements of the metadata assure us is
otherwise unused.
For v0.90, that is limited to 60K.  For 1.x it is 3K.
With recent kernels it is possible for mdadm to tell the kernel where to
 put the bitmap (rather than the kernel *knowing*) so mdadm could use other
 space that was reserved when the array was created, but I haven't
 implemented that in mdadm yet.

Thanks a lot Neil! I added these information to the raid wiki

https://raid.wiki.kernel.org/index.php/Bitmap#Used_disk_space_for_bitmaps


Cheers,
Bernd