[dm-devel] Improving dm-mirror as a final year project

[Miklos Vajna]

On Tue, Feb 15, 2011 at 04:13:15PM -0600, Jonathan Brassow <jbrassow at redhat.com> wrote:
> > Oh, do I read the code correctly that rhel6/upstream always reads from
> > the first mirror and switches only in case there is a read failure?
> 
> yes

Hm, was the was reason for dropping the round robin feature? I thought
that round robin causes better performance, given that IO reads are done
in an async way. Did I miss something or just nobody ported the patch to
RHEL6/upstream?

> Perhaps, but if you don't encode this in the LVM metadata, you will  
> have to perform the action every time you reboot.  Instead, you could  
> reorder the devices in userspace and reload the table.

I was not aware such a reorder is possible. That patch then does not
make too much sense, agreed. ;) (Do you have a pointer to some
documentation on how that reorder can be done? I can't find anything
about reorder in the lvconvert/dmsetup manpage.

> The basic component that covers RAID456 is available upstream, as you  
> saw.  I have an additional set of ~12 (reasonably small) patches that  
> add RAID1 and superblock/bitmap support.  These patches are not yet  
> upstream nor are they in any RHEL product.

Then what is the recommended platform to hack dm-raid? I have RHEL6 at
the moment. Is it OK to try to cherry-pick the single commit from
upstream + apply your patches or is it better to install rawhide where
the kernel is already 2.6.38rc5 (as far as I see) and only apply your
patches there?

> Yes, I have a script called 'gime_raid.pl' that creates the device- 
> mapper tables for dm-raid.  Eventually, this will be pushed into LVM,  
> but it was much easier (for testing purposes) to start with a perl  
> script.

Sure. :)

> For convenience, I've attached the patches I'm working on (quilt  
> directory) and the latest gime_raid.pl script.

Thanks!