Re: How to initialize "composite" RAID

[Neil Brown <neilb@suse.de>]

On Sat, 11 Sep 2010 00:58:21 -0400
Mike Hartman <mike@hartmanipulation.com> wrote:

> > If you add a bitmap (mdadm --grow /dev/md0 --bitmap=internal) after the
> > current resync finished, then any subsequent resync due to an unclean
> > shutdown will be much faster.
> 
> I read somewhere (I think in the wiki) that an intent bitmap only
> works properly on ext2 and ext3 and can cause trouble on other file
> systems. Can I use one on ext4 (what I'm using)? I'm hoping/assuming
> what I read just predates the common use of ext4.

That is completely wrong.  An intent bitmap works properly no matter what
filesystem is on top.
It does impose a small performance penalty for writes which is very
work-load-dependant, and it is not impossible that different filesystems
would suffer more or less from this, but I doubt you would notice.

If you can find it again let me know and I'll try to get it fixed.

> 
> Will I need to remove the bitmap before adding another disk and
> growing the array to use it? If I don't, will it speed up that
> operation any?

You, you will need to remove the bitmap before growing the array.  I really
should fix that but it doesn't seem to rise to the top of my to-do list...

And no, a bitmap would have no effect on a reshape operation even if it were
allowed to be present.

NeilBrown