[v2] btrfs: prefetch chunk tree leaves at mount
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Message ID 20200713121324.3667-1-dsterba@suse.com
State New
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Series
  • [v2] btrfs: prefetch chunk tree leaves at mount
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Commit Message

David Sterba July 13, 2020, 12:13 p.m. UTC
The whole chunk tree is read at mount time so we can utilize readahead
to get the tree blocks to memory before we read the items. The idea is
from Robbie, but instead of updating search slot readahead, this patch
implements the chunk tree readahead manually from nodes on level 1.

We've decided to do specific readahead optimizations and then unify them
under a common API so we don't break everything by changing the search
slot readahead logic.

Higher chunk trees grow on large filesystems (many terabytes), and
prefetching just level 1 seems to be sufficient. Provided example was
from a 200TiB filesystem with chunk tree level 2.

CC: Robbie Ko <robbieko@synology.com>
Signed-off-by: David Sterba <dsterba@suse.com>
---

v2:
- make readahead_tree_node_children static
- add comment about node locks

 fs/btrfs/volumes.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

Comments

Josef Bacik July 13, 2020, 1:55 p.m. UTC | #1
On 7/13/20 8:13 AM, David Sterba wrote:
> The whole chunk tree is read at mount time so we can utilize readahead
> to get the tree blocks to memory before we read the items. The idea is
> from Robbie, but instead of updating search slot readahead, this patch
> implements the chunk tree readahead manually from nodes on level 1.
> 
> We've decided to do specific readahead optimizations and then unify them
> under a common API so we don't break everything by changing the search
> slot readahead logic.
> 
> Higher chunk trees grow on large filesystems (many terabytes), and
> prefetching just level 1 seems to be sufficient. Provided example was
> from a 200TiB filesystem with chunk tree level 2.
> 
> CC: Robbie Ko <robbieko@synology.com>
> Signed-off-by: David Sterba <dsterba@suse.com>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef

Patch
diff mbox series

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index c7a3d4d730a3..ce01e44f8134 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -7013,6 +7013,19 @@  bool btrfs_check_rw_degradable(struct btrfs_fs_info *fs_info,
 	return ret;
 }
 
+static void readahead_tree_node_children(struct extent_buffer *node)
+{
+	int i;
+	const int nr_items = btrfs_header_nritems(node);
+
+	for (i = 0; i < nr_items; i++) {
+		u64 start;
+
+		start = btrfs_node_blockptr(node, i);
+		readahead_tree_block(node->fs_info, start);
+	}
+}
+
 int btrfs_read_chunk_tree(struct btrfs_fs_info *fs_info)
 {
 	struct btrfs_root *root = fs_info->chunk_root;
@@ -7023,6 +7036,7 @@  int btrfs_read_chunk_tree(struct btrfs_fs_info *fs_info)
 	int ret;
 	int slot;
 	u64 total_dev = 0;
+	u64 last_ra_node = 0;
 
 	path = btrfs_alloc_path();
 	if (!path)
@@ -7048,6 +7062,8 @@  int btrfs_read_chunk_tree(struct btrfs_fs_info *fs_info)
 	if (ret < 0)
 		goto error;
 	while (1) {
+		struct extent_buffer *node;
+
 		leaf = path->nodes[0];
 		slot = path->slots[0];
 		if (slot >= btrfs_header_nritems(leaf)) {
@@ -7058,6 +7074,17 @@  int btrfs_read_chunk_tree(struct btrfs_fs_info *fs_info)
 				goto error;
 			break;
 		}
+		/*
+		 * The nodes on level 1 are not locked but we don't need to do
+		 * that during mount time as nothing else can access the tree
+		 */
+		node = path->nodes[1];
+		if (node) {
+			if (last_ra_node != node->start) {
+				readahead_tree_node_children(node);
+				last_ra_node = node->start;
+			}
+		}
 		btrfs_item_key_to_cpu(leaf, &found_key, slot);
 		if (found_key.type == BTRFS_DEV_ITEM_KEY) {
 			struct btrfs_dev_item *dev_item;