Bug#598721: mdadm: internal bitmap uses way too large chunks

[Paul Slootman]

On Sat 02 Oct 2010, Neil Brown wrote:
> > 
> > I noticed that when adding internal bitmaps to md devices,
> > that the chunks used were far too large:
> 
> That depends on what you mean by 'too large'.

I was used to the prior behaviour as in 3.0.3 :-)

> I find that the ideal chunk size relates to how much of the array can be
> resynced in about second or a bit less.  I suspect I might be able to do an
> IO test and see how fast the devices are, but that is messy and error prone.
> So I simply choose a default of 64M as that seems to be the right ball-park
> for modern hardware.

OK, that reasoning makes sense. Perhaps having that reasoning available
in the changelog or manpage would be helpful. I'm sure that when squeeze
gets released more people will wonder about this change.


> > I mean, using 64M chunks on an md device that's just 208MB is silly.
> 
> It might also be said that having a bitmap on a 208MB array is a bit silly as
> it would only take about 5 seconds to resync it without a bitmap.

I see your point :)

> > After downgrading to mdadm 3.0.3:
> 
> You don't need to down grade.  If you don't like the default that mdadm

That was just for demonstration purposes.


Thanks,
Paul