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[dm-devel] Re: clustered snapshots



On Mon, Oct 05 2009 at 11:57am -0400,
Mikulas Patocka <mpatocka redhat com> wrote:

> 
> 
> On Mon, 5 Oct 2009, Mike Snitzer wrote:
...
> > Lesson I learned is that the in-core dm_snap_exception's consecutive
> > chunk accounting is _really_ subtle.  And while I agree there should be
> > no relation between the proper cleanup of on-disk and in-core
> > exceptions; doing the cleanup of each in lock-step appears to be the
> > only way forward (that I found).
> 
> Looking closer at it, all that the patch changes is that before the patch, 
> exceptions were cleaned last-to-first and after the patch they are cleaned 
> first-to-last. This is likely a reason that the patch works around a real 
> bug somewhere else. Not the callback.

Callback aside; the code didn't change the processing order.  You're
reading the code wrong.  persistent_commit_merge() processes the on-disk
exceptions in reverse with the following loop:

	   for (i = 0; i < n; i++)
	       clear_exception(ps, ps->current_committed - 1 - i);

The in-core exception cleanup in merge_callback() also cleans up in
reverse:

	  /*
	   * Must process chunks (and associated exceptions) in reverse
	   * so that dm_consecutive_chunk_count_dec() accounting works
	   */
	   for (i = s->merge_write_interlock_n - 1; i >= 0; i--) {
	   ...

So the fact that I added a callback from persistent_commit_merge()
did _not_ change the last-to-first exception cleanup ordering (for
either on-disk or in-core).

Regardless, I'm not using the callback.. I have simpler code to handle
back-merges within the merge_callback() for-loop.  I emailed that patch
in my previous reply to this thread...

> > > I think this patch just hides other bug that you have made. Do you set and 
> > > test the interlock correctly? If yes, it will block any I/Os to the chunks 
> > > being merged.
> > 
> > What bug have I made?  Pretty bold assertion only to back it up with
> > hand-waving and bravado.  I've worked hard to preserve your original work
> > as closely as possible; yet add the ability to support insertion of
> > chunks before existing chunks.
> 
> If it worked before without this patch and stopped working after you've 
> made some changes, than you have likely made a bug. So find it.

I'm not going to go round and round on this.  Backmerges never worked
with your snapshot-merge because you never took them into account (you
disabled them!).

In the interest of moving forward please test that the following code
works on your sparc64 system:
http://people.redhat.com/msnitzer/patches/snapshot-merge/kernel/2.6.31/

You'll also need to update LVM2 2.02-54-cvs to use these patches:
http://people.redhat.com/msnitzer/patches/snapshot-merge/lvm2/LVM2-2.02.54/

Mike


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