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dm snapshot: avoid allocating exceptions in merge 

The snapshot-merge target should not allocate new exceptions because the
intent is to merge all of its exceptions as quickly and safely as
possible.

This patch introduces the snapshot-merge mapping function and updates
__origin_write() so that it doesn't allocate exceptions on any snapshots
that are being merged.

If a write request to a merging snapshot device is to be dispatched
directly to the origin (because the chunk is not remapped or was already
merged), snapshot_merge_map() must make exceptions in other snapshots so
calls do_origin().

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
This commit is contained in:
Mikulas Patocka 2009-12-10 23:52:31 +00:00 committed by Alasdair G Kergon
parent 515ad66cc4
commit 3452c2a1eb
1 changed files with 56 additions and 1 deletions
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57
drivers/md/dm-snap.c
View File

@ -1308,6 +1308,54 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
return r;
}
/*
* A snapshot-merge target behaves like a combination of a snapshot
* target and a snapshot-origin target. It only generates new
* exceptions in other snapshots and not in the one that is being
* merged.
*
* For each chunk, if there is an existing exception, it is used to
* redirect I/O to the cow device. Otherwise I/O is sent to the origin,
* which in turn might generate exceptions in other snapshots.
*/
static int snapshot_merge_map(struct dm_target *ti, struct bio *bio,
union map_info *map_context)
{
struct dm_exception *e;
struct dm_snapshot *s = ti->private;
int r = DM_MAPIO_REMAPPED;
chunk_t chunk;
chunk = sector_to_chunk(s->store, bio->bi_sector);
down_read(&s->lock);
/* Full snapshots are not usable */
if (!s->valid) {
r = -EIO;
goto out_unlock;
}
/* If the block is already remapped - use that */
e = dm_lookup_exception(&s->complete, chunk);
if (e) {
remap_exception(s, e, bio, chunk);
goto out_unlock;
}
bio->bi_bdev = s->origin->bdev;
if (bio_rw(bio) == WRITE) {
up_read(&s->lock);
return do_origin(s->origin, bio);
}
out_unlock:
up_read(&s->lock);
return r;
}
static int snapshot_end_io(struct dm_target *ti, struct bio *bio,
int error, union map_info *map_context)
{
@ -1465,6 +1513,13 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
/* Do all the snapshots on this origin */
list_for_each_entry (snap, snapshots, list) {
/*
* Don't make new exceptions in a merging snapshot
* because it has effectively been deleted
*/
if (dm_target_is_snapshot_merge(snap->ti))
continue;
down_write(&snap->lock);
/* Only deal with valid and active snapshots */
@ -1697,7 +1752,7 @@ static struct target_type merge_target = {
.module = THIS_MODULE,
.ctr = snapshot_ctr,
.dtr = snapshot_dtr,
.map = snapshot_map,
.map = snapshot_merge_map,
.end_io = snapshot_end_io,
.postsuspend = snapshot_postsuspend,
.preresume = snapshot_preresume,