If this function's purpose is what I'm assuming it to be, then
it will also be necessary for cluster-aware exceptions stores as well.
Using the normal method of preloading exceptions won't be good
enough in a cluster, because node1 will need to be aware of the fact
that node2 caused an exception.
brassow
On Nov 13, 2008, at 4:12 AM, Jonathan Brassow wrote:
After twiddling with phillips/tomonori's shared snapshot, I believe
the
addition of the 'lookup_completed_exception' function to the
exception store API will be useful. If I understand correctly, it
allows
us to defer populating a snapshot's exception cache up front. Instead
allowing us to look them up as necessary. This is nice, because when
the exception store is shared, we don't want 60 copies of all the
exceptions for each snapshot structure. We just want one copy in the
exceptions store that we can lookup.
- brassow
Index: linux-2.6/drivers/md/dm-exception-store.h
===================================================================
--- linux-2.6.orig/drivers/md/dm-exception-store.h
+++ linux-2.6/drivers/md/dm-exception-store.h
@@ -78,6 +78,18 @@ struct dm_exception_store_type {
void *callback_context);
/*
+ * An exception store implementation can forgo using the
+ * callbacks provided to read_metadata, which would populate
+ * all exception data upfront. Instead, it can choose to
+ * look them up as needed via 'lookup_completed_exception'.
+ * This is especially useful for shared exception store
+ * implementations.
+ */
+ struct dm_snap_exception *
+ (*lookup_completed_exception)(struct dm_exception_store *store,
+ chunk_t chunk);
+
+ /*
* The snapshot is invalid, note this in the metadata.
*/
void (*drop_snapshot) (struct dm_exception_store *store);
Index: linux-2.6/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-snap.c
+++ linux-2.6/drivers/md/dm-snap.c
@@ -417,6 +417,16 @@ static struct dm_snap_exception *lookup_
return NULL;
}
+static struct dm_snap_exception *
+lookup_completed_exception(struct dm_snapshot *s, chunk_t chunk)
+{
+ if (s->store->type->lookup_completed_exception)
+ return s->store->type->lookup_completed_exception(s->store,
+ chunk);
+ else
+ return lookup_exception(&s->complete, chunk);
+}
+
static struct dm_snap_exception *alloc_exception(void)
{
struct dm_snap_exception *e;
@@ -905,7 +915,7 @@ static struct bio *put_pending_exception
static void pending_complete(struct dm_snap_pending_exception *pe,
int success)
{
- struct dm_snap_exception *e;
+ struct dm_snap_exception *e = NULL;
struct dm_snapshot *s = pe->snap;
struct bio *origin_bios = NULL;
struct bio *snapshot_bios = NULL;
@@ -919,18 +929,21 @@ static void pending_complete(struct dm_s
goto out;
}
- e = alloc_exception();
- if (!e) {
- down_write(&s->lock);
- __invalidate_snapshot(s, -ENOMEM);
- error = 1;
- goto out;
+ if (!s->store->type->lookup_completed_exception) {
+ e = alloc_exception();
+ if (!e) {
+ down_write(&s->lock);
+ __invalidate_snapshot(s, -ENOMEM);
+ error = 1;
+ goto out;
+ }
+ *e = pe->e;
}
- *e = pe->e;
down_write(&s->lock);
if (!s->valid) {
- free_exception(e);
+ if (e)
+ free_exception(e);
error = 1;
goto out;
}
@@ -946,7 +959,8 @@ static void pending_complete(struct dm_s
* Add a proper exception, and remove the
* in-flight exception from the list.
*/
- insert_completed_exception(s, e);
+ if (e)
+ insert_completed_exception(s, e);
out:
remove_exception(&pe->e);
@@ -1113,7 +1127,7 @@ static int snapshot_map(struct dm_target
}
/* If the block is already remapped - use that, else remap it */
- e = lookup_exception(&s->complete, chunk);
+ e = lookup_completed_exception(s, chunk);
if (e) {
remap_exception(s, e, bio, chunk);
goto out_unlock;
@@ -1255,7 +1269,7 @@ static int __origin_write(struct list_he
* ref_count is initialised to 1 so pending_complete()
* won't destroy the primary_pe while we're inside this loop.
*/
- e = lookup_exception(&snap->complete, chunk);
+ e = lookup_completed_exception(snap, chunk);
if (e)
goto next_snapshot;
--
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