Re: [ANNOUNCE][PATCH 2.6] md: persistent (file-backed) bitmap and async writes

[Paul Clements <paul.clements@steeleye.com>]

OK, here we go again. The patch has been updated to 2.6.9-rc2 and mdadm 
1.7.0, and I've implemented the suggestions you've made below...

Patches: http://parisc-linux.org/~jejb/md_bitmap/

Thanks,
Paul

----

Neil Brown wrote:

> This is definitely heading in the right direction.  However I think
> you have missed some of the subtleties of plugging.

[snip]

> That should then have the bitmap updates happening before the
> data updates, and should cluster bitmap updates better.

OK, I've fixed that.

> The next step would be to stop write_page from being synchronous - I'm
> not comfortable about ->unplug-fn having to wait for disk io.

Well, it still does have to wait for the bitmap writes to complete 
before letting the regular writes go, but I see your point about the 
writes being submitted simultaneously (nearly) rather than being 
completely serialized.

> This looks to be quite awkward as there is not call-back that happens
> when writeback on a page is completed.  You need to have a kernel
> thread to wait for the pages to complete.  I would get the
> bitmap_daemon to do this.

OK, this is what I've done...there didn't seem to be a better place to 
do it.

> Does that make sense?

Certainly does, and seems to work...