simple question: where is internal bitmap stored?

simple question: where is internal bitmap stored?

[Eyal Lebedinsky]

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?

If it carves extra space from the device which already has data
(e.g. a fs) then surely this will be problematic.

-- 
Eyal Lebedinsky	(eyal@eyal.emu.id.au)

Re: simple question: where is internal bitmap stored?

[Neil Brown]

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