Re: simple question: where is internal bitmap stored?

Neil Brown
2009-05-13T10:42:34Z

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