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Btrfs: don't read leaf blocks containing only checksums during truncate
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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>
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Chris Mason committed Oct 1, 2008
1 parent cf74982 commit 323ac95
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Showing 3 changed files with 193 additions and 32 deletions.
69 changes: 38 additions & 31 deletions fs/btrfs/ctree.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1388,7 +1388,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root
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));
Expand Down Expand Up @@ -3186,6 +3186,36 @@ static int del_ptr(struct btrfs_trans_handle *trans, struct btrfs_root *root,
return ret;
}

/*
* 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
Expand Down Expand Up @@ -3251,17 +3281,8 @@ int btrfs_del_items(struct btrfs_trans_handle *trans, struct btrfs_root *root,
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);
Expand Down Expand Up @@ -3296,24 +3317,10 @@ int btrfs_del_items(struct btrfs_trans_handle *trans, struct btrfs_root *root,
}

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
Expand Down Expand Up @@ -3418,8 +3425,8 @@ int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
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;
Expand Down
4 changes: 3 additions & 1 deletion fs/btrfs/ctree.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1649,7 +1649,9 @@ void btrfs_free_path(struct btrfs_path *p);
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)
Expand Down
152 changes: 152 additions & 0 deletions fs/btrfs/inode.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1389,6 +1389,154 @@ static int btrfs_rmdir(struct inode *dir, struct dentry *dentry)
return err;
}

/*
* 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
Expand Down Expand Up @@ -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) {
Expand Down

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