Commit de7180ff authored by Mike Snitzer's avatar Mike Snitzer
Browse files

dm cache: add support for discard passdown to the origin device 



DM cache now defaults to passing discards down to the origin device.
User may disable this using the "no_discard_passdown" feature when
creating the cache device.

If the cache's underlying origin device doesn't support discards then
passdown is disabled (with warning).  Similarly, if the underlying
origin device's max_discard_sectors is less than a cache block discard
passdown will be disabled (this is required because sizing of the cache
internal discard bitset depends on it).

Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent f87e033b

Documentation/device-mapper/cache.txt

+3 −0
Original line number Diff line number Diff line
@@ -206,6 +206,9 @@ Optional feature arguments are: 
                  in a separate btree, which improves speed of shutting

		  down the cache.



   no_discard_passdown	: disable passing down discards from the cache

			  to the origin's data device.



A policy called 'default' is always registered.  This is an alias for

the policy we currently think is giving best all round performance.

drivers/md/dm-cache-target.c

+100 −26
Original line number Diff line number Diff line
@@ -353,6 +353,7 @@ struct cache_features { 
	enum cache_metadata_mode mode;

	enum cache_io_mode io_mode;

	unsigned metadata_version;

	bool discard_passdown:1;

};



struct cache_stats {
@@ -1899,6 +1900,10 @@ static bool process_discard_bio(struct cache *cache, struct bio *bio) 
		b = to_dblock(from_dblock(b) + 1);

	}



	if (cache->features.discard_passdown) {

		remap_to_origin(cache, bio);

		generic_make_request(bio);

	} else

		bio_endio(bio);



	return false;
@@ -2233,13 +2238,14 @@ static void init_features(struct cache_features *cf) 
	cf->mode = CM_WRITE;

	cf->io_mode = CM_IO_WRITEBACK;

	cf->metadata_version = 1;

	cf->discard_passdown = true;

}



static int parse_features(struct cache_args *ca, struct dm_arg_set *as,

			  char **error)

{

	static const struct dm_arg _args[] = {

		{0, 2, "Invalid number of cache feature arguments"},

		{0, 3, "Invalid number of cache feature arguments"},

	};



	int r, mode_ctr = 0;
@@ -2274,6 +2280,9 @@ static int parse_features(struct cache_args *ca, struct dm_arg_set *as, 
		else if (!strcasecmp(arg, "metadata2"))

			cf->metadata_version = 2;



		else if (!strcasecmp(arg, "no_discard_passdown"))

			cf->discard_passdown = false;



		else {

			*error = "Unrecognised cache feature requested";

			return -EINVAL;
@@ -3119,6 +3128,39 @@ static void cache_resume(struct dm_target *ti) 
	do_waker(&cache->waker.work);

}



static void emit_flags(struct cache *cache, char *result,

		       unsigned maxlen, ssize_t *sz_ptr)

{

	ssize_t sz = *sz_ptr;

	struct cache_features *cf = &cache->features;

	unsigned count = (cf->metadata_version == 2) + !cf->discard_passdown + 1;



	DMEMIT("%u ", count);



	if (cf->metadata_version == 2)

		DMEMIT("metadata2 ");



	if (writethrough_mode(cache))

		DMEMIT("writethrough ");



	else if (passthrough_mode(cache))

		DMEMIT("passthrough ");



	else if (writeback_mode(cache))

		DMEMIT("writeback ");



	else {

		DMEMIT("unknown ");

		DMERR("%s: internal error: unknown io mode: %d",

		      cache_device_name(cache), (int) cf->io_mode);

	}



	if (!cf->discard_passdown)

		DMEMIT("no_discard_passdown ");



	*sz_ptr = sz;

}



/*

 * Status format:

 *
@@ -3185,25 +3227,7 @@ static void cache_status(struct dm_target *ti, status_type_t type, 
		       (unsigned) atomic_read(&cache->stats.promotion),

		       (unsigned long) atomic_read(&cache->nr_dirty));



		if (cache->features.metadata_version == 2)

			DMEMIT("2 metadata2 ");

		else

			DMEMIT("1 ");



		if (writethrough_mode(cache))

			DMEMIT("writethrough ");



		else if (passthrough_mode(cache))

			DMEMIT("passthrough ");



		else if (writeback_mode(cache))

			DMEMIT("writeback ");



		else {

			DMERR("%s: internal error: unknown io mode: %d",

			      cache_device_name(cache), (int) cache->features.io_mode);

			goto err;

		}

		emit_flags(cache, result, maxlen, &sz);



		DMEMIT("2 migration_threshold %llu ", (unsigned long long) cache->migration_threshold);
@@ -3432,14 +3456,62 @@ static int cache_iterate_devices(struct dm_target *ti, 
	return r;

}



static void set_discard_limits(struct cache *cache, struct queue_limits *limits)

static bool origin_dev_supports_discard(struct block_device *origin_bdev)

{

	struct request_queue *q = bdev_get_queue(origin_bdev);



	return q && blk_queue_discard(q);

}



/*

	 * FIXME: these limits may be incompatible with the cache device

 * If discard_passdown was enabled verify that the origin device

 * supports discards.  Disable discard_passdown if not.

 */

static void disable_passdown_if_not_supported(struct cache *cache)

{

	struct block_device *origin_bdev = cache->origin_dev->bdev;

	struct queue_limits *origin_limits = &bdev_get_queue(origin_bdev)->limits;

	const char *reason = NULL;

	char buf[BDEVNAME_SIZE];



	if (!cache->features.discard_passdown)

		return;



	if (!origin_dev_supports_discard(origin_bdev))

		reason = "discard unsupported";



	else if (origin_limits->max_discard_sectors < cache->sectors_per_block)

		reason = "max discard sectors smaller than a block";



	if (reason) {

		DMWARN("Origin device (%s) %s: Disabling discard passdown.",

		       bdevname(origin_bdev, buf), reason);

		cache->features.discard_passdown = false;

	}

}



static void set_discard_limits(struct cache *cache, struct queue_limits *limits)

{

	struct block_device *origin_bdev = cache->origin_dev->bdev;

	struct queue_limits *origin_limits = &bdev_get_queue(origin_bdev)->limits;



	if (!cache->features.discard_passdown) {

		/* No passdown is done so setting own virtual limits */

		limits->max_discard_sectors = min_t(sector_t, cache->discard_block_size * 1024,

						    cache->origin_sectors);

		limits->discard_granularity = cache->discard_block_size << SECTOR_SHIFT;

		return;

	}



	/*

	 * cache_iterate_devices() is stacking both origin and fast device limits

	 * but discards aren't passed to fast device, so inherit origin's limits.

	 */

	limits->max_discard_sectors = origin_limits->max_discard_sectors;

	limits->max_hw_discard_sectors = origin_limits->max_hw_discard_sectors;

	limits->discard_granularity = origin_limits->discard_granularity;

	limits->discard_alignment = origin_limits->discard_alignment;

	limits->discard_misaligned = origin_limits->discard_misaligned;

}



static void cache_io_hints(struct dm_target *ti, struct queue_limits *limits)
@@ -3456,6 +3528,8 @@ static void cache_io_hints(struct dm_target *ti, struct queue_limits *limits) 
		blk_limits_io_min(limits, cache->sectors_per_block << SECTOR_SHIFT);

		blk_limits_io_opt(limits, cache->sectors_per_block << SECTOR_SHIFT);

	}



	disable_passdown_if_not_supported(cache);

	set_discard_limits(cache, limits);

}
@@ -3463,7 +3537,7 @@ static void cache_io_hints(struct dm_target *ti, struct queue_limits *limits) 


static struct target_type cache_target = {

	.name = "cache",

	.version = {2, 0, 0},

	.version = {2, 1, 0},

	.module = THIS_MODULE,

	.ctr = cache_ctr,

	.dtr = cache_dtr,

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