[dm-devel] Re: [PATCH v5 03/13] dm exception store: snapshot-merge usage accounting

[Mike Snitzer]

On Mon, Dec 07 2009 at  6:44pm -0500,
Alasdair G Kergon <agk at redhat.com> wrote:

> On Mon, Dec 07, 2009 at 04:10:53PM -0500, Mike Snitzer wrote:
> > Subject: dm exception store: snapshot-merge usage accounting
> 
> Comments updated a bit and folded into
> dm-exception-store-add-merge-specific-methods.patch # 64745

OK, I did the following based on our chat.. but haven't looked at your
version yet (ftp://sources.redhat.com appears to be down?).

Mike
---

From: Mike Snitzer <snitzer at redhat.com>
Subject: dm exception store: snapshot-merge usage accounting

Adjust the persistent exception store's next_free to the last chunk that
was processed in persistent_commit_merge().  prepare_merge() may change
ps->current_area but not before commit_merge() processes 'nr_merged'
chunks from it.

Signed-off-by: Mike Snitzer <snitzer at redhat.com>
Cc: Mikulas Patocka <mpatocka at redhat.com>
---
 drivers/md/dm-snap-persistent.c |   33 ++++++++++++++++++++++++++++-----
 1 file changed, 28 insertions(+), 5 deletions(-)

Index: linux-rhel6/drivers/md/dm-snap-persistent.c
===================================================================
--- linux-rhel6.orig/drivers/md/dm-snap-persistent.c
+++ linux-rhel6/drivers/md/dm-snap-persistent.c
@@ -119,7 +119,24 @@ struct pstore {
 	chunk_t current_area;
 
 	/*
-	 * The next free chunk for an exception.
+	 * 'next_free' can have two meanings.
+	 *
+	 * When creating exceptions:
+	 * The next free chunk for an exception.  'next_free' means all
+	 * the slots here and above are free.  Though there may be
+	 * holes in the exception store because chunks can be committed
+	 * out of order -- if the system halts abruptly it could
+	 * leave a hole.
+	 *
+	 * When merging exceptions:
+	 * The last chunk that was merged from a linear extent of
+	 * chunks [returned from persistent_prepare_merge()].
+	 * When merging 'next_free' does _not_ mean all the slots here
+	 * and above are free.  It holds the value it would have held
+	 * if all chunks in an area had been committed in order.  So
+	 * the value may occasionally be slightly inaccurate, because
+	 * chunks can be committed out of order, but since it's only
+	 * used for 'status' this doesn't matter.
 	 */
 	chunk_t next_free;
 
@@ -753,10 +770,16 @@ static int persistent_commit_merge(struc
 	ps->current_committed -= nr_merged;
 
 	/*
-	 * Note that we make no attempt to keep ps->next_free up-to-date
-	 * as exceptions may have been allocated out-of-order.
-	 * Once a snapshot has become merging, nothing further uses it.
-	 */
+	 * Update ps->next_free so persistent_usage() is accurate
+	 * - it may be less than the actual 'next_free' chunk but we
+	 *   will not allocate exceptions again so this doesn't matter
+	 * - must account for the first two metadata chunks
+	 * - prepare_merge() may change ps->current_area but not before
+	 *   commit_merge() processes 'nr_merged' from the current_area
+	 */
+	ps->next_free =
+		(area_location(ps, ps->current_area) - 1 +
+		 ps->current_committed + 2);
 
 	return 0;
 }