Btrfs: Do not use data_alloc_cluster in ssd mode

    In the first year of btrfs development, around early 2008, btrfs
gained a mount option which enables specific functionality for
filesystems on solid state devices. The first occurance of this
functionality is in commit e18e4809, labeled "Add mount -o ssd, which
includes optimizations for seek free storage".

The effect on allocating free space for doing (data) writes is to
'cluster' writes together, writing them out in contiguous space, as
opposed to a 'tetris' way of putting all separate writes into any free
space fragment that fits (which is what the -o nossd behaviour does).

A somewhat simplified explanation of what happens is that, when for
example, the 'cluster' size is set to 2MiB, when we do some writes, the
data allocator will search for a free space block that is 2MiB big, and
put the writes in there. The ssd mode itself might allow a 2MiB cluster
to be composed of multiple free space extents with some existing data in
between, while the additional ssd_spread mount option kills off this
option and requires fully free space.

The idea behind this is (commit 536ac8ae): "The [...] clusters make it
more likely a given IO will completely overwrite the ssd block, so it
doesn't have to do an internal rwm cycle."; ssd block meaning nand erase
block. So, effectively this means applying a "locality based algorithm"
and trying to outsmart the actual ssd.

Since then, various changes have been made to the involved code, but the
basic idea is still present, and gets activated whenever the ssd mount
option is active. This also happens by default, when the rotational flag
as seen at /sys/block/<device>/queue/rotational is set to 0.

    However, there's a number of problems with this approach.

    First, what the optimization is trying to do is outsmart the ssd by
assuming there is a relation between the physical address space of the
block device as seen by btrfs and the actual physical storage of the
ssd, and then adjusting data placement. However, since the introduction
of the Flash Translation Layer (FTL) which is a part of the internal
controller of an ssd, these attempts are futile. The use of good quality
FTL in consumer ssd products might have been limited in 2008, but this
situation has changed drastically soon after that time. Today, even the
flash memory in your automatic cat feeding machine or your grandma's
wheelchair has a full featured one.

Second, the behaviour as described above results in the filesystem being
filled up with badly fragmented free space extents because of relatively
small pieces of space that are freed up by deletes, but not selected
again as part of a 'cluster'. Since the algorithm prefers allocating a
new chunk over going back to tetris mode, the end result is a filesystem
in which all raw space is allocated, but which is composed of
underutilized chunks with a 'shotgun blast' pattern of fragmented free
space. Usually, the next problematic thing that happens is the
filesystem wanting to allocate new space for metadata, which causes the
filesystem to fail in spectacular ways.

Third, the default mount options you get for an ssd ('ssd' mode enabled,
'discard' not enabled), in combination with spreading out writes over
the full address space and ignoring freed up space leads to worst case
behaviour in providing information to the ssd itself, since it will
never learn that all the free space left behind is actually free.  There
are two ways to let an ssd know previously written data does not have to
be preserved, which are sending explicit signals using discard or
fstrim, or by simply overwriting the space with new data.  The worst
case behaviour is the btrfs ssd_spread mount option in combination with
not having discard enabled. It has a side effect of minimizing the reuse
of free space previously written in.

Fourth, the rotational flag in /sys/ does not reliably indicate if the
device is a locally attached ssd. For example, iSCSI or NBD displays as
non-rotational, while a loop device on an ssd shows up as rotational.

The combination of the second and third problem effectively means that
despite all the good intentions, the btrfs ssd mode reliably causes the
ssd hardware and the filesystem structures and performance to be choked
to death. The clickbait version of the title of this story would have
been "Btrfs ssd optimizations condsidered harmful for ssds".

The current nossd 'tetris' mode (even still without discard) allows a
pattern of overwriting much more previously used space, causing many
more implicit discards to happen because of the overwrite information
the ssd gets. The actual location in the physical address space, as seen
from the point of view of btrfs is irrelevant, because the actual writes
to the low level flash are reordered anyway thanks to the FTL.

    So what now...?

The changes in here do the following:

1. Throw out the current ssd_spread behaviour.
2. Move the current ssd behaviour to the ssd_spread option.
3. Make ssd mode data allocation identical to tetris mode, like nossd.
4. Adjust and clean up filesystem mount messages so that we can easily
identify if a kernel has this patch applied or not, when providing
support to end users.

Instead of directly cutting out all code related to the data cluster, it
makes sense to take a gradual approach and allow users who are still
able to find a valid reason to prefer the current ssd mode the means to
do so by specifiying the additional ssd_spread option.

Since there are other uses of the ssd mode, we keep the difference
between nossd and ssd mode. However, the usage of the rotational
attribute warrants some reconsideration in the future.

Notes for whoever wants to backport this patch on their 4.9 LTS kernel:
* First apply commit 951e7966 "btrfs: drop the nossd flag when
  remounting with -o ssd", or fixup the differences manually.
* The rest of the conflicts are because of the fs_info refactoring. So,
  for example, instead of using fs_info, it's root->fs_info in
  extent-tree.c

Signed-off-by: Hans van Kranenburg <hans.van.kranenburg@mendix.com>
Tested-by: Austin S. Hemmelgarn <ahferroin7@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

5 files changed, 23 insertions, 25 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 5bdd36664421..c4b26f147833 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -471,7 +471,7 @@ struct btrfs_block_rsv {
 /*
  * free clusters are used to claim free space in relatively large chunks,
  * allowing us to do less seeky writes.  They are used for all metadata
- * allocations and data allocations in ssd mode.
+ * allocations and data allocations in ssd_spread mode.
  */
 struct btrfs_free_cluster {
 	spinlock_t lock;
@@ -968,7 +968,7 @@ struct btrfs_fs_info {
 
 	struct reloc_control *reloc_ctl;
 
-	/* data_alloc_cluster is only used in ssd mode */
+	/* data_alloc_cluster is only used in ssd_spread mode */
 	struct btrfs_free_cluster data_alloc_cluster;
 
 	/* all metadata allocations go through this cluster */
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 075beedb4352..354f2b6bb9a4 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3058,11 +3058,9 @@ retry_root_backup:
 	if (IS_ERR(fs_info->transaction_kthread))
 		goto fail_cleaner;
 
-	if (!btrfs_test_opt(fs_info, SSD) &&
-	    !btrfs_test_opt(fs_info, NOSSD) &&
+	if (!btrfs_test_opt(fs_info, NOSSD) &&
 	    !fs_info->fs_devices->rotating) {
-		btrfs_info(fs_info, "detected SSD devices, enabling SSD mode");
-		btrfs_set_opt(fs_info->mount_opt, SSD);
+		btrfs_set_and_info(fs_info, SSD, "enabling ssd optimizations");
 	}
 
 	/*
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 375f8c728d91..7842a1e7069a 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -6668,19 +6668,20 @@ fetch_cluster_info(struct btrfs_fs_info *fs_info,
 		   struct btrfs_space_info *space_info, u64 *empty_cluster)
 {
 	struct btrfs_free_cluster *ret = NULL;
-	bool ssd = btrfs_test_opt(fs_info, SSD);
 
 	*empty_cluster = 0;
 	if (btrfs_mixed_space_info(space_info))
 		return ret;
 
-	if (ssd)
-		*empty_cluster = SZ_2M;
 	if (space_info->flags & BTRFS_BLOCK_GROUP_METADATA) {
 		ret = &fs_info->meta_alloc_cluster;
-		if (!ssd)
+		if (btrfs_test_opt(fs_info, SSD))
+			*empty_cluster = SZ_2M;
+		else
 			*empty_cluster = SZ_64K;
-	} else if ((space_info->flags & BTRFS_BLOCK_GROUP_DATA) && ssd) {
+	} else if ((space_info->flags & BTRFS_BLOCK_GROUP_DATA) &&
+		   btrfs_test_opt(fs_info, SSD_SPREAD)) {
+		*empty_cluster = SZ_2M;
 		ret = &fs_info->data_alloc_cluster;
 	}
 
diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index c5e6180cdb8c..8ebbb182ba94 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -3034,14 +3034,11 @@ int btrfs_find_space_cluster(struct btrfs_fs_info *fs_info,
 	int ret;
 
 	/*
-	 * Choose the minimum extent size we'll require for this
-	 * cluster.  For SSD_SPREAD, don't allow any fragmentation.
-	 * For metadata, allow allocates with smaller extents.  For
-	 * data, keep it dense.
+	 * Choose the minimum extent size we'll require for this cluster.  For
+	 * metadata, allow allocates with smaller extents.  For data, keep it
+	 * dense.
 	 */
-	if (btrfs_test_opt(fs_info, SSD_SPREAD)) {
-		cont1_bytes = min_bytes = bytes + empty_size;
-	} else if (block_group->flags & BTRFS_BLOCK_GROUP_METADATA) {
+	if (block_group->flags & BTRFS_BLOCK_GROUP_METADATA) {
 		cont1_bytes = bytes;
 		min_bytes = fs_info->sectorsize;
 	} else {
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 74e47794e63f..b66c152784df 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -548,20 +548,22 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
 			break;
 		case Opt_ssd:
 			btrfs_set_and_info(info, SSD,
-					   "use ssd allocation scheme");
+					   "enabling ssd optimizations");
 			btrfs_clear_opt(info->mount_opt, NOSSD);
 			break;
 		case Opt_ssd_spread:
+			btrfs_set_and_info(info, SSD,
+					   "enabling ssd optimizations");
 			btrfs_set_and_info(info, SSD_SPREAD,
-					   "use spread ssd allocation scheme");
-			btrfs_set_opt(info->mount_opt, SSD);
+					   "using legacy ssd allocation scheme");
 			btrfs_clear_opt(info->mount_opt, NOSSD);
 			break;
 		case Opt_nossd:
-			btrfs_set_and_info(info, NOSSD,
-					     "not using ssd allocation scheme");
-			btrfs_clear_opt(info->mount_opt, SSD);
-			btrfs_clear_opt(info->mount_opt, SSD_SPREAD);
+			btrfs_set_opt(info->mount_opt, NOSSD);
+			btrfs_clear_and_info(info, SSD,
+					     "not using ssd optimizations");
+			btrfs_clear_and_info(info, SSD_SPREAD,
+					     "not using legacy ssd allocation scheme");
 			break;
 		case Opt_barrier:
 			btrfs_clear_and_info(info, NOBARRIER,