Re: simple question: where is internal bitmap stored?

[Neil Brown <neilb@suse.de>]

On Wednesday May 13, eyal@eyal.emu.id.au wrote:
> If I have a live raid and want to enable bitmap then I do
> 	mdadm --grow /dev/md0 --bitmap=internal
> Does this bitmap use space in the already present header? If so then
> does this mean that the headers have enough reserve for this? Why
> not always use the bitmap then?

1/ Yes.
2/ Yes.
3/ because there are other reasons for not wanting a bitmap.
   Using a write-intent bitmap can cause a reduction in throughput.

   It is conceivable that having a bitmap should be the default, but
   it should definitely be options.

   In fact, ever array does has a default bitmap.  However that bitmap
   contains exactly one bit, and it cover the whole array.  It is the
   clean/active flag.  Defaulting to a few more bits is possibly a
   good idea...
> 
> If it carves extra space from the device which already has data
> (e.g. a fs) then surely this will be problematic.

Yes.  It surely would be.

NeilBrown