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

[Neil Brown <neilb@suse.de>]

On Mon, 14 Jun 2010 17:53:27 +0600
Roman Mamedov <roman@rm.pp.ru> wrote:

> On Mon, 14 Jun 2010 19:47:42 +1000
> Neil Brown <neilb@suse.de> wrote:
> 
> > When you create an array an specify that a bitmap be added at the same time,
> > there is more flexibility for size and location of the bitmap.  It can easily
> > be more that 3K in that case.
> > 
> > So presumably this array was created with a bitmap, rather than created
> > without a bitmap and had a bitmap added later with --grow.  Correct?
> 
> Yes, as far as I remember. However, it seems a bit unfortunate to have any
> significant difference between adding the bitmap right when creating the
> array, and adding it later. For that reason, I'd suggest reserving more space
> in the metadata than 3K, even if the bitmap isn't requested - and even if it
> won't be added later, then that space could prove useful for something else
> that might require it later. Maybe 64, 128 or 256K - still miniscule compared
> to the array size, and could provide some nice flexibility for the future.
> 

Yes. An I'm fairly sure mdadm does always reserve space.
The point is that (until very recently) it wasn't possible to tell the kernel
where to add a bitmap to an active array - just that it should add one.
So it could  only add it at a place that it was certain would be usable.
That is the place I described.
It is now possible to give the kernel more details of the bitmap to add.  I
just need to teach mdadm how to choose the best space and how to tell the
kernel about it.

NeilBrown