Re: RAID6 and crashes (reporting back re. --bitmap)

[Bernd Schubert <bernd.schubert@fastmail.fm>]

On Monday 14 June 2010, Neil Brown wrote:
> On Sun, 13 Jun 2010 16:28:34 +0200
> 
> Bernd Schubert <bernd.schubert@fastmail.fm> wrote:
> > On Friday 11 June 2010, Neil Brown wrote:
> > > On Fri, 11 Jun 2010 00:31:57 -0400
> > >
> > > "Graham Mitchell" <gmitch@woodlea.com> wrote:
> > > > 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