Btrfs: don't read leaf blocks containing only checksums during truncate
Checksum items take up a significant portion of the metadata for large files.
It is possible to avoid reading them during truncates by checking the keys in
the higher level nodes.
If a given leaf is followed by another leaf where the lowest key is a checksum
item from the same file, we know we can safely delete the leaf without
reading it.
For a 32GB file on a 6 drive raid0 array, Btrfs needs 8s to delete
the file with a cold cache. It is read bound during the run.
With this change, Btrfs is able to delete the file in 0.5s
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index f3abecc..e5c9261 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1390,6 +1390,154 @@
}
/*
+ * when truncating bytes in a file, it is possible to avoid reading
+ * the leaves that contain only checksum items. This can be the
+ * majority of the IO required to delete a large file, but it must
+ * be done carefully.
+ *
+ * The keys in the level just above the leaves are checked to make sure
+ * the lowest key in a given leaf is a csum key, and starts at an offset
+ * after the new size.
+ *
+ * Then the key for the next leaf is checked to make sure it also has
+ * a checksum item for the same file. If it does, we know our target leaf
+ * contains only checksum items, and it can be safely freed without reading
+ * it.
+ *
+ * This is just an optimization targeted at large files. It may do
+ * nothing. It will return 0 unless things went badly.
+ */
+static noinline int drop_csum_leaves(struct btrfs_trans_handle *trans,
+ struct btrfs_root *root,
+ struct btrfs_path *path,
+ struct inode *inode, u64 new_size)
+{
+ struct btrfs_key key;
+ int ret;
+ int nritems;
+ struct btrfs_key found_key;
+ struct btrfs_key other_key;
+
+ path->lowest_level = 1;
+ key.objectid = inode->i_ino;
+ key.type = BTRFS_CSUM_ITEM_KEY;
+ key.offset = new_size;
+again:
+ ret = btrfs_search_slot(trans, root, &key, path, -1, 1);
+ if (ret < 0)
+ goto out;
+
+ if (path->nodes[1] == NULL) {
+ ret = 0;
+ goto out;
+ }
+ ret = 0;
+ btrfs_node_key_to_cpu(path->nodes[1], &found_key, path->slots[1]);
+ nritems = btrfs_header_nritems(path->nodes[1]);
+
+ if (!nritems)
+ goto out;
+
+ if (path->slots[1] >= nritems)
+ goto next_node;
+
+ /* did we find a key greater than anything we want to delete? */
+ if (found_key.objectid > inode->i_ino ||
+ (found_key.objectid == inode->i_ino && found_key.type > key.type))
+ goto out;
+
+ /* we check the next key in the node to make sure the leave contains
+ * only checksum items. This comparison doesn't work if our
+ * leaf is the last one in the node
+ */
+ if (path->slots[1] + 1 >= nritems) {
+next_node:
+ /* search forward from the last key in the node, this
+ * will bring us into the next node in the tree
+ */
+ btrfs_node_key_to_cpu(path->nodes[1], &found_key, nritems - 1);
+
+ /* unlikely, but we inc below, so check to be safe */
+ if (found_key.offset == (u64)-1)
+ goto out;
+
+ /* search_forward needs a path with locks held, do the
+ * search again for the original key. It is possible
+ * this will race with a balance and return a path that
+ * we could modify, but this drop is just an optimization
+ * and is allowed to miss some leaves.
+ */
+ btrfs_release_path(root, path);
+ found_key.offset++;
+
+ /* setup a max key for search_forward */
+ other_key.offset = (u64)-1;
+ other_key.type = key.type;
+ other_key.objectid = key.objectid;
+
+ path->keep_locks = 1;
+ ret = btrfs_search_forward(root, &found_key, &other_key,
+ path, 0, 0);
+ path->keep_locks = 0;
+ if (ret || found_key.objectid != key.objectid ||
+ found_key.type != key.type) {
+ ret = 0;
+ goto out;
+ }
+
+ key.offset = found_key.offset;
+ btrfs_release_path(root, path);
+ cond_resched();
+ goto again;
+ }
+
+ /* we know there's one more slot after us in the tree,
+ * read that key so we can verify it is also a checksum item
+ */
+ btrfs_node_key_to_cpu(path->nodes[1], &other_key, path->slots[1] + 1);
+
+ if (found_key.objectid < inode->i_ino)
+ goto next_key;
+
+ if (found_key.type != key.type || found_key.offset < new_size)
+ goto next_key;
+
+ /*
+ * if the key for the next leaf isn't a csum key from this objectid,
+ * we can't be sure there aren't good items inside this leaf.
+ * Bail out
+ */
+ if (other_key.objectid != inode->i_ino || other_key.type != key.type)
+ goto out;
+
+ /*
+ * it is safe to delete this leaf, it contains only
+ * csum items from this inode at an offset >= new_size
+ */
+ ret = btrfs_del_leaf(trans, root, path,
+ btrfs_node_blockptr(path->nodes[1],
+ path->slots[1]));
+ BUG_ON(ret);
+
+next_key:
+ btrfs_release_path(root, path);
+
+ if (other_key.objectid == inode->i_ino &&
+ other_key.type == key.type && other_key.offset > key.offset) {
+ key.offset = other_key.offset;
+ cond_resched();
+ goto again;
+ }
+ ret = 0;
+out:
+ /* fixup any changes we've made to the path */
+ path->lowest_level = 0;
+ path->keep_locks = 0;
+ btrfs_release_path(root, path);
+ return ret;
+}
+
+/*
* this can truncate away extent items, csum items and directory items.
* It starts at a high offset and removes keys until it can't find
* any higher than new_size
@@ -1436,6 +1584,10 @@
key.type = (u8)-1;
btrfs_init_path(path);
+
+ ret = drop_csum_leaves(trans, root, path, inode, new_size);
+ BUG_ON(ret);
+
search_again:
ret = btrfs_search_slot(trans, root, &key, path, -1, 1);
if (ret < 0) {