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>

1 files changed, 152 insertions, 0 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index f3abecc2d14c..e5c9261dcbaa 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1390,6 +1390,154 @@ fail:
 }
 
 /*
+ * 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 @@ noinline int btrfs_truncate_inode_items(struct btrfs_trans_handle *trans,
         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) {