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Re: [dm-devel] [PATCH 4/4] dm-exception-store-dont-read-metadata-if-going-to-handover

I don't think the extra parameter is necessary... Couldn't you use the functionality of your future "re-read metadata" patch (or my old "allow metadata re-read" patch) to achieve the same thing here? (It is possible that idea won't work, because when re-reading the metadata, I like to read the COW header to ensure someone else didn't invalidate the exception store.)

Reviewed-by: Jonathan Brassow <jbrassow redhat com>


On Oct 5, 2009, at 2:00 PM, Mike Snitzer wrote:

From: Mikulas Patocka <mpatocka redhat com>

Don't read metadata if we are going to do handover.
This saves memory (without the patch, the exception store would consume twice
more memory while doing handover).

If the handover doesn't happen (due to user abusing dmsetup), the snapshot is
marked as invalid.

Signed-off-by: Mikulas Patocka <mpatocka redhat com>
Reviewed-by: Mike Snitzer <snitzer redhat com>
drivers/md/dm-exception-store.h |    6 +++++-
drivers/md/dm-snap-persistent.c |    5 +++--
drivers/md/dm-snap-transient.c  |    2 +-
drivers/md/dm-snap.c            |    2 +-
4 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/drivers/md/dm-exception-store.h b/drivers/md/dm- exception-store.h
index 4e8086f..5737796 100644
--- a/drivers/md/dm-exception-store.h
+++ b/drivers/md/dm-exception-store.h
@@ -54,11 +54,15 @@ struct dm_exception_store_type {
	 * The target shouldn't read the COW device until this is
	 * called.  As exceptions are read from the COW, they are
	 * reported back via the callback.
+	 *
+	 * will_handover means that there is another snapshot active;
+	 * chunk size must be setup but no exceptions need to be read
+	 * because they will be handed over from the active snapshot.
	int (*read_metadata) (struct dm_exception_store *store,
			      int (*callback)(void *callback_context,
					      chunk_t old, chunk_t new),
-			      void *callback_context);
+			      void *callback_context, int will_handover);

	 * Find somewhere to store the next exception.
diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap- persistent.c
index 746602b..fbcedc3 100644
--- a/drivers/md/dm-snap-persistent.c
+++ b/drivers/md/dm-snap-persistent.c
@@ -518,7 +518,7 @@ static void persistent_dtr(struct dm_exception_store *store)
static int persistent_read_metadata(struct dm_exception_store *store,
				    int (*callback)(void *callback_context,
						    chunk_t old, chunk_t new),
-				    void *callback_context)
+				    void *callback_context, int will_handover)
	int r, uninitialized_var(new_snapshot);
	struct pstore *ps = get_info(store);
@@ -575,7 +575,8 @@ static int persistent_read_metadata(struct dm_exception_store *store,
	 * Read the metadata.
-	r = read_exceptions(ps, callback, callback_context);
+	if (!will_handover)
+		r = read_exceptions(ps, callback, callback_context);

	return r;
diff --git a/drivers/md/dm-snap-transient.c b/drivers/md/dm-snap- transient.c
index 3d9fd91..7f4f825 100644
--- a/drivers/md/dm-snap-transient.c
+++ b/drivers/md/dm-snap-transient.c
@@ -30,7 +30,7 @@ static void transient_dtr(struct dm_exception_store *store)
static int transient_read_metadata(struct dm_exception_store *store,
				   int (*callback)(void *callback_context,
						   chunk_t old, chunk_t new),
-				   void *callback_context)
+				   void *callback_context, int will_handover)
	return 0;
diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 8d3efa0..275ae33 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -708,7 +708,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)

	/* Metadata must only be loaded into one table at once */
	r = s->store->type->read_metadata(s->store, dm_add_exception,
-					  (void *)s);
+					  (void *)s, s->handover);
	if (r < 0) {
		ti->error = "Failed to read snapshot metadata";
		goto bad_load_and_register;

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