btrfs: quasi-round-robin for chunk allocation

In a multi device setup, the chunk allocator currently always allocates
chunks on the devices in the same order. This leads to a very uneven
distribution, especially with RAID1 or RAID10 and an uneven number of
devices.
This patch always sorts the devices before allocating, and allocates the
stripes on the devices with the most available space, as long as there
is enough space available. In a low space situation, it first tries to
maximize striping.
The patch also simplifies the allocator and reduces the checks for
corner cases.
The simplification is done by several means. First, it defines the
properties of each RAID type upfront. These properties are used afterwards
instead of differentiating cases in several places.
Second, the old allocator defined a minimum stripe size for each block
group type, tried to find a large enough chunk, and if this fails just
allocates a smaller one. This is now done in one step. The largest possible
chunk (up to max_chunk_size) is searched and allocated.
Because we now have only one pass, the allocation of the map (struct
map_lookup) is moved down to the point where the number of stripes is
already known. This way we avoid reallocation of the map.
We still avoid allocating stripes that are not a multiple of STRIPE_SIZE.

fs/btrfs/volumes.c

@@ -2268,262 +2268,204 @@ static int btrfs_add_system_chunk(struct btrfs_trans_handle *trans,
 	return 0;
 }
 
-static noinline u64 chunk_bytes_by_type(u64 type, u64 calc_size,
-					int num_stripes, int sub_stripes)
+/*
+ * sort the devices in descending order by max_avail, total_avail
+ */
+static int btrfs_cmp_device_info(const void *a, const void *b)
 {
-	if (type & (BTRFS_BLOCK_GROUP_RAID1 | BTRFS_BLOCK_GROUP_DUP))
-		return calc_size;
-	else if (type & BTRFS_BLOCK_GROUP_RAID10)
-		return calc_size * (num_stripes / sub_stripes);
-	else
-		return calc_size * num_stripes;
+	const struct btrfs_device_info *di_a = a;
+	const struct btrfs_device_info *di_b = b;
+
+	if (di_a->max_avail > di_b->max_avail)
+		return -1;
+	if (di_a->max_avail < di_b->max_avail)
+		return 1;
+	if (di_a->total_avail > di_b->total_avail)
+		return -1;
+	if (di_a->total_avail < di_b->total_avail)
+		return 1;
+	return 0;
 }
 
-static int __btrfs_calc_nstripes(struct btrfs_fs_devices *fs_devices, u64 type,
-				 int *num_stripes, int *min_stripes,
-				 int *sub_stripes)
+static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
+			       struct btrfs_root *extent_root,
+			       struct map_lookup **map_ret,
+			       u64 *num_bytes_out, u64 *stripe_size_out,
+			       u64 start, u64 type)
 {
-	*num_stripes = 1;
-	*min_stripes = 1;
-	*sub_stripes = 0;
+	struct btrfs_fs_info *info = extent_root->fs_info;
+	struct btrfs_fs_devices *fs_devices = info->fs_devices;
+	struct list_head *cur;
+	struct map_lookup *map = NULL;
+	struct extent_map_tree *em_tree;
+	struct extent_map *em;
+	struct btrfs_device_info *devices_info = NULL;
+	u64 total_avail;
+	int num_stripes;	/* total number of stripes to allocate */
+	int sub_stripes;	/* sub_stripes info for map */
+	int dev_stripes;	/* stripes per dev */
+	int devs_max;		/* max devs to use */
+	int devs_min;		/* min devs needed */
+	int devs_increment;	/* ndevs has to be a multiple of this */
+	int ncopies;		/* how many copies to data has */
+	int ret;
+	u64 max_stripe_size;
+	u64 max_chunk_size;
+	u64 stripe_size;
+	u64 num_bytes;
+	int ndevs;
+	int i;
+	int j;
 
-	if (type & (BTRFS_BLOCK_GROUP_RAID0)) {
-		*num_stripes = fs_devices->rw_devices;
-		*min_stripes = 2;
-	}
-	if (type & (BTRFS_BLOCK_GROUP_DUP)) {
-		*num_stripes = 2;
-		*min_stripes = 2;
-	}
-	if (type & (BTRFS_BLOCK_GROUP_RAID1)) {
-		if (fs_devices->rw_devices < 2)
-			return -ENOSPC;
-		*num_stripes = 2;
-		*min_stripes = 2;
-	}
-	if (type & (BTRFS_BLOCK_GROUP_RAID10)) {
-		*num_stripes = fs_devices->rw_devices;
-		if (*num_stripes < 4)
-			return -ENOSPC;
-		*num_stripes &= ~(u32)1;
-		*sub_stripes = 2;
-		*min_stripes = 4;
+	if ((type & BTRFS_BLOCK_GROUP_RAID1) &&
+	    (type & BTRFS_BLOCK_GROUP_DUP)) {
+		WARN_ON(1);
+		type &= ~BTRFS_BLOCK_GROUP_DUP;
 	}
 
-	return 0;
-}
+	if (list_empty(&fs_devices->alloc_list))
+		return -ENOSPC;
 
-static u64 __btrfs_calc_stripe_size(struct btrfs_fs_devices *fs_devices,
-				    u64 proposed_size, u64 type,
-				    int num_stripes, int small_stripe)
-{
-	int min_stripe_size = 1 * 1024 * 1024;
-	u64 calc_size = proposed_size;
-	u64 max_chunk_size = calc_size;
-	int ncopies = 1;
+	sub_stripes = 1;
+	dev_stripes = 1;
+	devs_increment = 1;
+	ncopies = 1;
+	devs_max = 0;	/* 0 == as many as possible */
+	devs_min = 1;
 
-	if (type & (BTRFS_BLOCK_GROUP_RAID1 |
-		    BTRFS_BLOCK_GROUP_DUP |
-		    BTRFS_BLOCK_GROUP_RAID10))
+	/*
+	 * define the properties of each RAID type.
+	 * FIXME: move this to a global table and use it in all RAID
+	 * calculation code
+	 */
+	if (type & (BTRFS_BLOCK_GROUP_DUP)) {
+		dev_stripes = 2;
+		ncopies = 2;
+		devs_max = 1;
+	} else if (type & (BTRFS_BLOCK_GROUP_RAID0)) {
+		devs_min = 2;
+	} else if (type & (BTRFS_BLOCK_GROUP_RAID1)) {
+		devs_increment = 2;
 		ncopies = 2;
+		devs_max = 2;
+		devs_min = 2;
+	} else if (type & (BTRFS_BLOCK_GROUP_RAID10)) {
+		sub_stripes = 2;
+		devs_increment = 2;
+		ncopies = 2;
+		devs_min = 4;
+	} else {
+		devs_max = 1;
+	}
 
 	if (type & BTRFS_BLOCK_GROUP_DATA) {
-		max_chunk_size = 10 * calc_size;
-		min_stripe_size = 64 * 1024 * 1024;
+		max_stripe_size = 1024 * 1024 * 1024;
+		max_chunk_size = 10 * max_stripe_size;
 	} else if (type & BTRFS_BLOCK_GROUP_METADATA) {
-		max_chunk_size = 256 * 1024 * 1024;
-		min_stripe_size = 32 * 1024 * 1024;
+		max_stripe_size = 256 * 1024 * 1024;
+		max_chunk_size = max_stripe_size;
 	} else if (type & BTRFS_BLOCK_GROUP_SYSTEM) {
-		calc_size = 8 * 1024 * 1024;
-		max_chunk_size = calc_size * 2;
-		min_stripe_size = 1 * 1024 * 1024;
+		max_stripe_size = 8 * 1024 * 1024;
+		max_chunk_size = 2 * max_stripe_size;
+	} else {
+		printk(KERN_ERR "btrfs: invalid chunk type 0x%llx requested\n",
+		       type);
+		BUG_ON(1);
 	}
 
 	/* we don't want a chunk larger than 10% of writeable space */
 	max_chunk_size = min(div_factor(fs_devices->total_rw_bytes, 1),
 			     max_chunk_size);
 
-	if (calc_size * num_stripes > max_chunk_size * ncopies) {
-		calc_size = max_chunk_size * ncopies;
-		do_div(calc_size, num_stripes);
-		do_div(calc_size, BTRFS_STRIPE_LEN);
-		calc_size *= BTRFS_STRIPE_LEN;
-	}
+	devices_info = kzalloc(sizeof(*devices_info) * fs_devices->rw_devices,
+			       GFP_NOFS);
+	if (!devices_info)
+		return -ENOMEM;
 
-	/* we don't want tiny stripes */
-	if (!small_stripe)
-		calc_size = max_t(u64, min_stripe_size, calc_size);
+	cur = fs_devices->alloc_list.next;
 
 	/*
-	 * we're about to do_div by the BTRFS_STRIPE_LEN so lets make sure
-	 * we end up with something bigger than a stripe
+	 * in the first pass through the devices list, we gather information
+	 * about the available holes on each device.
 	 */
-	calc_size = max_t(u64, calc_size, BTRFS_STRIPE_LEN);
-
-	do_div(calc_size, BTRFS_STRIPE_LEN);
-	calc_size *= BTRFS_STRIPE_LEN;
-
-	return calc_size;
-}
-
-static struct map_lookup *__shrink_map_lookup_stripes(struct map_lookup *map,
-						      int num_stripes)
-{
-	struct map_lookup *new;
-	size_t len = map_lookup_size(num_stripes);
+	ndevs = 0;
+	while (cur != &fs_devices->alloc_list) {
+		struct btrfs_device *device;
+		u64 max_avail;
+		u64 dev_offset;
 
-	BUG_ON(map->num_stripes < num_stripes);
+		device = list_entry(cur, struct btrfs_device, dev_alloc_list);
 
-	if (map->num_stripes == num_stripes)
-		return map;
+		cur = cur->next;
 
-	new = kmalloc(len, GFP_NOFS);
-	if (!new) {
-		/* just change map->num_stripes */
-		map->num_stripes = num_stripes;
-		return map;
+		if (!device->writeable) {
+			printk(KERN_ERR
+			       "btrfs: read-only device in alloc_list\n");
+			WARN_ON(1);
+			continue;
 		}
 
-	memcpy(new, map, len);
-	new->num_stripes = num_stripes;
-	kfree(map);
-	return new;
-}
+		if (!device->in_fs_metadata)
+			continue;
 
-/*
- * helper to allocate device space from btrfs_device_info, in which we stored
- * max free space information of every device. It is used when we can not
- * allocate chunks by default size.
- *
- * By this helper, we can allocate a new chunk as larger as possible.
+		if (device->total_bytes > device->bytes_used)
+			total_avail = device->total_bytes - device->bytes_used;
+		else
+			total_avail = 0;
+		/* avail is off by max(alloc_start, 1MB), but that is the same
+		 * for all devices, so it doesn't hurt the sorting later on
 		 */
-static int __btrfs_alloc_tiny_space(struct btrfs_trans_handle *trans,
-				    struct btrfs_fs_devices *fs_devices,
-				    struct btrfs_device_info *devices,
-				    int nr_device, u64 type,
-				    struct map_lookup **map_lookup,
-				    int min_stripes, u64 *stripe_size)
-{
-	int i, index, sort_again = 0;
-	int min_devices = min_stripes;
-	u64 max_avail, min_free;
-	struct map_lookup *map = *map_lookup;
-	int ret;
 
-	if (nr_device < min_stripes)
-		return -ENOSPC;
+		ret = find_free_dev_extent(trans, device,
+					   max_stripe_size * dev_stripes,
+					   &dev_offset, &max_avail);
+		if (ret && ret != -ENOSPC)
+			goto error;
 
-	btrfs_descending_sort_devices(devices, nr_device);
+		if (ret == 0)
+			max_avail = max_stripe_size * dev_stripes;
 
-	max_avail = devices[0].max_avail;
-	if (!max_avail)
-		return -ENOSPC;
+		if (max_avail < BTRFS_STRIPE_LEN * dev_stripes)
+			continue;
 
-	for (i = 0; i < nr_device; i++) {
-		/*
-		 * if dev_offset = 0, it means the free space of this device
-		 * is less than what we need, and we didn't search max avail
-		 * extent on this device, so do it now.
-		 */
-		if (!devices[i].dev_offset) {
-			ret = find_free_dev_extent(trans, devices[i].dev,
-						   max_avail,
-						   &devices[i].dev_offset,
-						   &devices[i].max_avail);
-			if (ret != 0 && ret != -ENOSPC)
-				return ret;
-			sort_again = 1;
-		}
+		devices_info[ndevs].dev_offset = dev_offset;
+		devices_info[ndevs].max_avail = max_avail;
+		devices_info[ndevs].total_avail = total_avail;
+		devices_info[ndevs].dev = device;
+		++ndevs;
 	}
 
-	/* we update the max avail free extent of each devices, sort again */
-	if (sort_again)
-		btrfs_descending_sort_devices(devices, nr_device);
-
-	if (type & BTRFS_BLOCK_GROUP_DUP)
-		min_devices = 1;
-
-	if (!devices[min_devices - 1].max_avail)
-		return -ENOSPC;
-
-	max_avail = devices[min_devices - 1].max_avail;
-	if (type & BTRFS_BLOCK_GROUP_DUP)
-		do_div(max_avail, 2);
-
-	max_avail = __btrfs_calc_stripe_size(fs_devices, max_avail, type,
-					     min_stripes, 1);
-	if (type & BTRFS_BLOCK_GROUP_DUP)
-		min_free = max_avail * 2;
-	else
-		min_free = max_avail;
-
-	if (min_free > devices[min_devices - 1].max_avail)
-		return -ENOSPC;
+	/*
+	 * now sort the devices by hole size / available space
+	 */
+	sort(devices_info, ndevs, sizeof(struct btrfs_device_info),
+	     btrfs_cmp_device_info, NULL);
 
-	map = __shrink_map_lookup_stripes(map, min_stripes);
-	*stripe_size = max_avail;
+	/* round down to number of usable stripes */
+	ndevs -= ndevs % devs_increment;
 
-	index = 0;
-	for (i = 0; i < min_stripes; i++) {
-		map->stripes[i].dev = devices[index].dev;
-		map->stripes[i].physical = devices[index].dev_offset;
-		if (type & BTRFS_BLOCK_GROUP_DUP) {
-			i++;
-			map->stripes[i].dev = devices[index].dev;
-			map->stripes[i].physical = devices[index].dev_offset +
-						   max_avail;
-		}
-		index++;
+	if (ndevs < devs_increment * sub_stripes || ndevs < devs_min) {
+		ret = -ENOSPC;
+		goto error;
 	}
-	*map_lookup = map;
 
-	return 0;
-}
-
-static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
-			       struct btrfs_root *extent_root,
-			       struct map_lookup **map_ret,
-			       u64 *num_bytes, u64 *stripe_size,
-			       u64 start, u64 type)
-{
-	struct btrfs_fs_info *info = extent_root->fs_info;
-	struct btrfs_device *device = NULL;
-	struct btrfs_fs_devices *fs_devices = info->fs_devices;
-	struct list_head *cur;
-	struct map_lookup *map;
-	struct extent_map_tree *em_tree;
-	struct extent_map *em;
-	struct btrfs_device_info *devices_info;
-	struct list_head private_devs;
-	u64 calc_size = 1024 * 1024 * 1024;
-	u64 min_free;
-	u64 avail;
-	u64 dev_offset;
-	int num_stripes;
-	int min_stripes;
-	int sub_stripes;
-	int min_devices;	/* the min number of devices we need */
-	int i;
-	int ret;
-	int index;
+	if (devs_max && ndevs > devs_max)
+		ndevs = devs_max;
+	/*
+	 * the primary goal is to maximize the number of stripes, so use as many
+	 * devices as possible, even if the stripes are not maximum sized.
+	 */
+	stripe_size = devices_info[ndevs-1].max_avail;
+	num_stripes = ndevs * dev_stripes;
 
-	if ((type & BTRFS_BLOCK_GROUP_RAID1) &&
-	    (type & BTRFS_BLOCK_GROUP_DUP)) {
-		WARN_ON(1);
-		type &= ~BTRFS_BLOCK_GROUP_DUP;
+	if (stripe_size * num_stripes > max_chunk_size * ncopies) {
+		stripe_size = max_chunk_size * ncopies;
+		do_div(stripe_size, num_stripes);
 	}
-	if (list_empty(&fs_devices->alloc_list))
-		return -ENOSPC;
 
-	ret = __btrfs_calc_nstripes(fs_devices, type, &num_stripes,
-				    &min_stripes, &sub_stripes);
-	if (ret)
-		return ret;
-
-	devices_info = kzalloc(sizeof(*devices_info) * fs_devices->rw_devices,
-			       GFP_NOFS);
-	if (!devices_info)
-		return -ENOMEM;
+	do_div(stripe_size, dev_stripes);
+	do_div(stripe_size, BTRFS_STRIPE_LEN);
+	stripe_size *= BTRFS_STRIPE_LEN;
 
 	map = kmalloc(map_lookup_size(num_stripes), GFP_NOFS);
 	if (!map) {
@@ -2532,85 +2474,12 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 	}
 	map->num_stripes = num_stripes;
 
-	cur = fs_devices->alloc_list.next;
-	index = 0;
-	i = 0;
-
-	calc_size = __btrfs_calc_stripe_size(fs_devices, calc_size, type,
-					     num_stripes, 0);
-
-	if (type & BTRFS_BLOCK_GROUP_DUP) {
-		min_free = calc_size * 2;
-		min_devices = 1;
-	} else {
-		min_free = calc_size;
-		min_devices = min_stripes;
-	}
-
-	INIT_LIST_HEAD(&private_devs);
-	while (index < num_stripes) {
-		device = list_entry(cur, struct btrfs_device, dev_alloc_list);
-		BUG_ON(!device->writeable);
-		if (device->total_bytes > device->bytes_used)
-			avail = device->total_bytes - device->bytes_used;
-		else
-			avail = 0;
-		cur = cur->next;
-
-		if (device->in_fs_metadata && avail >= min_free) {
-			ret = find_free_dev_extent(trans, device, min_free,
-						   &devices_info[i].dev_offset,
-						   &devices_info[i].max_avail);
-			if (ret == 0) {
-				list_move_tail(&device->dev_alloc_list,
-					       &private_devs);
-				map->stripes[index].dev = device;
-				map->stripes[index].physical =
-						devices_info[i].dev_offset;
-				index++;
-				if (type & BTRFS_BLOCK_GROUP_DUP) {
-					map->stripes[index].dev = device;
-					map->stripes[index].physical =
-						devices_info[i].dev_offset +
-						calc_size;
-					index++;
-				}
-			} else if (ret != -ENOSPC)
-				goto error;
-
-			devices_info[i].dev = device;
-			i++;
-		} else if (device->in_fs_metadata &&
-			   avail >= BTRFS_STRIPE_LEN) {
-			devices_info[i].dev = device;
-			devices_info[i].max_avail = avail;
-			i++;
-		}
-
-		if (cur == &fs_devices->alloc_list)
-			break;
-	}
-
-	list_splice(&private_devs, &fs_devices->alloc_list);
-	if (index < num_stripes) {
-		if (index >= min_stripes) {
-			num_stripes = index;
-			if (type & (BTRFS_BLOCK_GROUP_RAID10)) {
-				num_stripes /= sub_stripes;
-				num_stripes *= sub_stripes;
-			}
-
-			map = __shrink_map_lookup_stripes(map, num_stripes);
-		} else if (i >= min_devices) {
-			ret = __btrfs_alloc_tiny_space(trans, fs_devices,
-						       devices_info, i, type,
-						       &map, min_stripes,
-						       &calc_size);
-			if (ret)
-				goto error;
-		} else {
-			ret = -ENOSPC;
-			goto error;
+	for (i = 0; i < ndevs; ++i) {
+		for (j = 0; j < dev_stripes; ++j) {
+			int s = i * dev_stripes + j;
+			map->stripes[s].dev = devices_info[i].dev;
+			map->stripes[s].physical = devices_info[i].dev_offset +
+						   j * stripe_size;
 		}
 	}
 	map->sector_size = extent_root->sectorsize;
@@ -2621,11 +2490,12 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 	map->sub_stripes = sub_stripes;
 
 	*map_ret = map;
-	*stripe_size = calc_size;
-	*num_bytes = chunk_bytes_by_type(type, calc_size,
-					 map->num_stripes, sub_stripes);
+	num_bytes = stripe_size * (num_stripes / ncopies);
+
+	*stripe_size_out = stripe_size;
+	*num_bytes_out = num_bytes;
 
-	trace_btrfs_chunk_alloc(info->chunk_root, map, start, *num_bytes);
+	trace_btrfs_chunk_alloc(info->chunk_root, map, start, num_bytes);
 
 	em = alloc_extent_map(GFP_NOFS);
 	if (!em) {
@@ -2634,7 +2504,7 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 	}
 	em->bdev = (struct block_device *)map;
 	em->start = start;
-	em->len = *num_bytes;
+	em->len = num_bytes;
 	em->block_start = 0;
 	em->block_len = em->len;
 
@@ -2647,20 +2517,21 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 
 	ret = btrfs_make_block_group(trans, extent_root, 0, type,
 				     BTRFS_FIRST_CHUNK_TREE_OBJECTID,
-				     start, *num_bytes);
+				     start, num_bytes);
 	BUG_ON(ret);
 
-	index = 0;
-	while (index < map->num_stripes) {
-		device = map->stripes[index].dev;
-		dev_offset = map->stripes[index].physical;
+	for (i = 0; i < map->num_stripes; ++i) {
+		struct btrfs_device *device;
+		u64 dev_offset;
+
+		device = map->stripes[i].dev;
+		dev_offset = map->stripes[i].physical;
 
 		ret = btrfs_alloc_dev_extent(trans, device,
 				info->chunk_root->root_key.objectid,
 				BTRFS_FIRST_CHUNK_TREE_OBJECTID,
-				start, dev_offset, calc_size);
+				start, dev_offset, stripe_size);
 		BUG_ON(ret);
-		index++;
 	}
 
 	kfree(devices_info);

fs/btrfs/volumes.h

@@ -144,6 +144,7 @@ struct btrfs_device_info {
 	struct btrfs_device *dev;
 	u64 dev_offset;
 	u64 max_avail;
+	u64 total_avail;
 };
 
 struct map_lookup {