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/ctree.c b/fs/btrfs/ctree.c
index ff3261f..2eab464 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -1388,7 +1388,7 @@
struct btrfs_key prealloc_block;
lowest_level = p->lowest_level;
- WARN_ON(lowest_level && ins_len);
+ WARN_ON(lowest_level && ins_len > 0);
WARN_ON(p->nodes[0] != NULL);
WARN_ON(cow && root == root->fs_info->extent_root &&
!mutex_is_locked(&root->fs_info->alloc_mutex));
@@ -3187,6 +3187,36 @@
}
/*
+ * a helper function to delete the leaf pointed to by path->slots[1] and
+ * path->nodes[1]. bytenr is the node block pointer, but since the callers
+ * already know it, it is faster to have them pass it down than to
+ * read it out of the node again.
+ *
+ * This deletes the pointer in path->nodes[1] and frees the leaf
+ * block extent. zero is returned if it all worked out, < 0 otherwise.
+ *
+ * The path must have already been setup for deleting the leaf, including
+ * all the proper balancing. path->nodes[1] must be locked.
+ */
+noinline int btrfs_del_leaf(struct btrfs_trans_handle *trans,
+ struct btrfs_root *root,
+ struct btrfs_path *path, u64 bytenr)
+{
+ int ret;
+ u64 root_gen = btrfs_header_generation(path->nodes[1]);
+
+ ret = del_ptr(trans, root, path, 1, path->slots[1]);
+ if (ret)
+ return ret;
+
+ ret = btrfs_free_extent(trans, root, bytenr,
+ btrfs_level_size(root, 0),
+ path->nodes[1]->start,
+ btrfs_header_owner(path->nodes[1]),
+ root_gen, 0, 0, 1);
+ return ret;
+}
+/*
* delete the item at the leaf level in path. If that empties
* the leaf, remove it from the tree
*/
@@ -3251,17 +3281,8 @@
if (leaf == root->node) {
btrfs_set_header_level(leaf, 0);
} else {
- u64 root_gen = btrfs_header_generation(path->nodes[1]);
- wret = del_ptr(trans, root, path, 1, path->slots[1]);
- if (wret)
- ret = wret;
- wret = btrfs_free_extent(trans, root,
- leaf->start, leaf->len,
- path->nodes[1]->start,
- btrfs_header_owner(path->nodes[1]),
- root_gen, 0, 0, 1);
- if (wret)
- ret = wret;
+ ret = btrfs_del_leaf(trans, root, path, leaf->start);
+ BUG_ON(ret);
}
} else {
int used = leaf_space_used(leaf, 0, nritems);
@@ -3296,24 +3317,10 @@
}
if (btrfs_header_nritems(leaf) == 0) {
- u64 root_gen;
- u64 bytenr = leaf->start;
- u32 blocksize = leaf->len;
-
- root_gen = btrfs_header_generation(
- path->nodes[1]);
-
- wret = del_ptr(trans, root, path, 1, slot);
- if (wret)
- ret = wret;
-
+ path->slots[1] = slot;
+ ret = btrfs_del_leaf(trans, root, path, leaf->start);
+ BUG_ON(ret);
free_extent_buffer(leaf);
- wret = btrfs_free_extent(trans, root, bytenr,
- blocksize, path->nodes[1]->start,
- btrfs_header_owner(path->nodes[1]),
- root_gen, 0, 0, 1);
- if (wret)
- ret = wret;
} else {
/* if we're still in the path, make sure
* we're dirty. Otherwise, one of the
@@ -3418,8 +3425,8 @@
level = btrfs_header_level(cur);
sret = bin_search(cur, min_key, level, &slot);
- /* at level = 0, we're done, setup the path and exit */
- if (level == 0) {
+ /* at the lowest level, we're done, setup the path and exit */
+ if (level == path->lowest_level) {
if (slot >= nritems)
goto find_next_key;
ret = 0;
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index ded1643..94e0cdf 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1649,7 +1649,9 @@
void btrfs_init_path(struct btrfs_path *p);
int btrfs_del_items(struct btrfs_trans_handle *trans, struct btrfs_root *root,
struct btrfs_path *path, int slot, int nr);
-
+int btrfs_del_leaf(struct btrfs_trans_handle *trans,
+ struct btrfs_root *root,
+ struct btrfs_path *path, u64 bytenr);
static inline int btrfs_del_item(struct btrfs_trans_handle *trans,
struct btrfs_root *root,
struct btrfs_path *path)
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) {