====================
$ bonnie++ -d tmp/ -c 10 -s 2048
Writing a byte at a time...done
Writing intelligently...done
Rewriting...done
Reading a byte at a time...done
Reading intelligently...done
start 'em...done...done...done...done...done...
Create files in sequential order...done.
Stat files in sequential order...done.
Delete files in sequential order...done.
Create files in random order...done.
Stat files in random order...done.
Delete files in random order...done.
Version  1.96       ------Sequential Output------ --Sequential Input- --Random-
Concurrency  10     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
prasad-virtual-m 2G  1043  99 555406  74 227605  55  5360  99 489080  68 +++++ +++
Latency             24646us   48544us   57893us    6686us    3595us   21026us
Version  1.96       ------Sequential Create------ --------Random Create--------
prasad-virtual-mach -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                 16 +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++
Latency               343us    1175us     327us     555us      48us      82us
1.96,1.96,prasad-virtual-machine,10,1307043085,2G,,1043,99,555406,74,227605,55,
5360,99,489080,68,+++++,+++,16,,,,,+++++,+++,+++++,+++,+++++,+++,+++++,+++,
+++++,+++,+++++,+++,24646us,48544us,57893us,6686us,3595us,21026us,343us,1175us,
327us,555us,48us,82us

With QCOW caching
=================
$ bonnie++ -d tmp/ -c 10 -s 2048
Writing a byte at a time...done
Writing intelligently...done
Rewriting...done
Reading a byte at a time...done
Reading intelligently...done
start 'em...done...done...done...done...done...
Create files in sequential order...done.
Stat files in sequential order...done.
Delete files in sequential order...done.
Create files in random order...done.
Stat files in random order...done.
Delete files in random order...done.
Version  1.96       ------Sequential Output------ --Sequential Input- --Random-
Concurrency  10     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
prasad-virtual-m 2G  1033  99 467899  64 182387  41  5422 100 338294  48 +++++ +++
Latency             21549us   60585us   65723us    6331us   30014us   19994us
Version  1.96       ------Sequential Create------ --------Random Create--------
prasad-virtual-mach -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                 16 +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++
Latency               478us    1142us     344us     402us      72us      98us
1.96,1.96,prasad-virtual-machine,10,1307042839,2G,,1033,99,467899,64,182387,41,
5422,100,338294,48,+++++,+++,16,,,,,+++++,+++,+++++,+++,+++++,+++,+++++,+++,
+++++,+++,+++++,+++,21549us,60585us,65723us,6331us,30014us,19994us,478us,1142us,
344us,402us,72us,98us

Summary of performance numbers
==============================
There is not much difference with sequential character operations are
performed, the code with caching performed better by small margin. The caching
code performance raised by 18% to 24% with sequential block output and by 44%
for sequentail block input. Which is understandable as the Level2 table will
always be cached after a write operation. Random seek operation worked slower
with caching code.

Signed-off-by: Prasad Joshi <prasadjoshi124@gmail.com>
---
 tools/kvm/disk/qcow.c        |  216 ++++++++++++++++++++++++++++++++++++++----
 tools/kvm/include/kvm/qcow.h |   17 ++++
 2 files changed, 216 insertions(+), 17 deletions(-)

diff --git a/tools/kvm/disk/qcow.c b/tools/kvm/disk/qcow.c
index b52b734..4fa3850 100644
--- a/tools/kvm/disk/qcow.c
+++ b/tools/kvm/disk/qcow.c
@@ -16,6 +16,140 @@
 #include <linux/kernel.h>
 #include <linux/types.h>
 
+static int insert(struct rb_root *root, struct qcow_l2_cache *new)
+{
+	struct rb_node **link = &(root->rb_node), *parent = NULL;
+	u64 offset = new->offset;
+
+	/* search the tree */
+	while (*link) {
+		struct qcow_l2_cache *t;
+
+		t = rb_entry(*link, struct qcow_l2_cache, node);
+		if (!t)
+			goto error;
+
+		parent = *link;
+
+		if (t->offset > offset)
+			link = &(*link)->rb_left;
+		else if (t->offset < offset)
+			link = &(*link)->rb_right;
+		else
+			goto out;
+	}
+
+	/* add new node */
+	rb_link_node(&new->node, parent, link);
+	rb_insert_color(&new->node, root);
+out:
+	return 0;
+error:
+	return -1;
+}
+
+static struct qcow_l2_cache *search(struct rb_root *root, u64 offset)
+{
+	struct rb_node *link = root->rb_node;
+
+	while (link) {
+		struct qcow_l2_cache *t;
+
+		t = rb_entry(link, struct qcow_l2_cache, node);
+		if (!t)
+			goto out;
+
+		if (t->offset > offset)
+			link = link->rb_left;
+		else if (t->offset < offset)
+			link = link->rb_right;
+		else
+			return t;
+	}
+out:
+	return NULL;
+}
+
+static void free_cache(struct qcow *q)
+{
+	struct list_head *pos, *n;
+	struct qcow_l2_cache *t;
+	struct rb_root *r = &q->root;
+
+	list_for_each_safe(pos, n, &q->lru_list) {
+		/* Remove cache table from the list and RB tree */
+		list_del(pos);
+		t = list_entry(pos, struct qcow_l2_cache, list);
+		rb_erase(&t->node, r);
+
+		/* Free the cached node */
+		free(t->table);
+		free(t);
+	}
+}
+
+static int cache_table(struct qcow *q, u64 *table, u64 offset)
+{
+	struct qcow_l2_cache *n;
+	struct rb_root *r = &q->root;
+	struct qcow_l2_cache *lru;
+
+	n = calloc(1, sizeof(struct qcow_l2_cache));
+	if (!n)
+		goto error;
+
+	n->offset = offset;
+	n->table  = table;
+	n->qcow   = q;
+	RB_CLEAR_NODE(&n->node);
+	INIT_LIST_HEAD(&n->list);
+
+	if (q->no_cached == MAX_CACHE_NODES) {
+		/* Find the node for LRU replacement */
+		lru = list_first_entry(&q->lru_list, struct qcow_l2_cache, list);
+
+		/* Remove the node from the cache */
+		rb_erase(&lru->node, r);
+		list_del_init(&lru->list);
+		q->no_cached--;
+
+		/* Free the LRUed node */
+		free(lru->table);
+		free(lru);
+	}
+
+	/* Add new node in RB Tree: Helps in searching faster */
+	if (insert(r, n) < 0)
+		goto error;
+
+	/* Add in LRU replacement list */
+	list_add_tail(&n->list, &q->lru_list);
+	q->no_cached++;
+
+	return 0;
+error:
+	free(n);
+	return -1;
+}
+
+static int search_table(struct qcow *q, u64 **table, u64 offset)
+{
+	struct qcow_l2_cache *c;
+
+	*table = NULL;
+
+	c = search(&q->root, offset);
+	if (!c)
+		return -1;
+
+	/* Update the LRU state */
+	list_del_init(&c->list);
+	list_add_tail(&c->list, &q->lru_list);
+
+	*table = c->table;
+	return 0;
+}
+
 static inline u64 get_l1_index(struct qcow *q, u64 offset)
 {
 	struct qcow_header *header = q->header;
@@ -37,6 +171,43 @@ static inline u64 get_cluster_offset(struct qcow *q, u64 offset)
 	return offset & ((1 << header->cluster_bits)-1);
 }
 
+static int qcow_read_l2_table(struct qcow *q, u64 **table, u64 offset)
+{
+	struct qcow_header *header = q->header;
+	u64 size;
+	u64 i;
+	u64 *t;
+
+	*table = NULL;
+	size   = 1 << header->l2_bits;
+
+	/* search an entry for offset in cache */
+	if (search_table(q, table, offset) >= 0)
+		return 0;
+
+	t = calloc(size, sizeof(*t));
+	if (!t)
+		goto error;
+
+	/* table not cached: read from the disk */
+	if (pread_in_full(q->fd, t, size * sizeof(u64), offset) < 0)
+		goto error;
+
+	/* cache the table */
+	if (cache_table(q, t, offset) < 0)
+		goto error;
+
+	/* change cached table to CPU's byte-order */
+	for (i = 0; i < size; i++)
+		be64_to_cpus(&t[i]);
+
+	*table = t;
+	return 0;
+error:
+	free(t);
+	return -1;
+}
+
 static ssize_t qcow_read_cluster(struct qcow *q, u64 offset, void *dst, u32 dst_len)
 {
 	struct qcow_header *header = q->header;
@@ -51,7 +222,6 @@ static ssize_t qcow_read_cluster(struct qcow *q, u64 offset, void *dst, u32 dst_
 	u64 l1_idx;
 	u64 l2_idx;
 
-	l2_table = NULL;
 
 	cluster_size = 1 << header->cluster_bits;
 
@@ -72,18 +242,16 @@ static ssize_t qcow_read_cluster(struct qcow *q, u64 offset, void *dst, u32 dst_
 		goto zero_cluster;
 
 	l2_table_size = 1 << header->l2_bits;
-	l2_table = calloc(l2_table_size, sizeof(u64));
-	if (!l2_table)
-		goto out_error;
 
-	if (pread_in_full(q->fd, l2_table, l2_table_size * sizeof(u64), l2_table_offset) < 0)
+	/* read and cache level 2 table */
+	if (qcow_read_l2_table(q, &l2_table, l2_table_offset) < 0)
 		goto out_error;
 
 	l2_idx = get_l2_index(q, offset);
 	if (l2_idx >= l2_table_size)
 		goto out_error;
 
-	clust_start = be64_to_cpu(l2_table[l2_idx]) & ~header->oflag_mask;
+	clust_start = l2_table[l2_idx] & ~header->oflag_mask;
 	if (!clust_start)
 		goto zero_cluster;
 
@@ -91,7 +259,6 @@ static ssize_t qcow_read_cluster(struct qcow *q, u64 offset, void *dst, u32 dst_
 		goto out_error;
 
 out:
-	free(l2_table);
 	return length;
 
 zero_cluster:
@@ -221,15 +388,16 @@ static ssize_t qcow_write_cluster(struct qcow *q, u64 offset, void *buf, u32 src
 	if (len > src_len)
 		len = src_len;
 
-	l2t = calloc(l2t_sz, sizeof(u64));
-	if (!l2t)
-		goto error;
-
 	l2t_off		= table->l1_table[l1t_idx] & ~header->oflag_mask;
 	if (l2t_off) {
-		if (pread_in_full(q->fd, l2t, l2t_sz * sizeof(u64), l2t_off) < 0)
-			goto free_l2;
+		/* cache level2 table */
+		if (qcow_read_l2_table(q, &l2t, l2t_off) < 0)
+			goto error;
 	} else {
+		l2t = calloc(l2t_sz, sizeof(*l2t));
+		if (!l2t)
+			goto error;
+
 		/* Capture the state of the consistent QCOW image */
 		f_sz		= file_size(q->fd);
 		if (!f_sz)
@@ -251,6 +419,13 @@ static ssize_t qcow_write_cluster(struct qcow *q, u64 offset, void *buf, u32 src
 			goto free_l2;
 		}
 
+		if (cache_table(q, l2t, l2t_off) < 0) {
+			if (ftruncate(q->fd, f_sz) < 0)
+				goto free_l2;
+
+			goto free_l2;
+		}
+
 		/* Update the in-core entry */
 		table->l1_table[l1t_idx] = l2t_off;
 	}
@@ -258,17 +433,15 @@ static ssize_t qcow_write_cluster(struct qcow *q, u64 offset, void *buf, u32 src
 	/* Capture the state of the consistent QCOW image */
 	f_sz		= file_size(q->fd);
 	if (!f_sz)
-		goto free_l2;
+		goto error;
 
-	clust_start	= be64_to_cpu(l2t[l2t_idx]) & ~header->oflag_mask;
+	clust_start	= l2t[l2t_idx] & ~header->oflag_mask;
 	if (!clust_start) {
 		clust_start	= ALIGN(f_sz, clust_sz);
 		update_meta	= true;
 	} else
 		update_meta	= false;
 
-	free(l2t);
-
 	/* Write actual data */
 	if (pwrite_in_full(q->fd, buf, len, clust_start + clust_off) < 0)
 		goto error;
@@ -282,9 +455,13 @@ static ssize_t qcow_write_cluster(struct qcow *q, u64 offset, void *buf, u32 src
 
 			goto error;
 		}
+
+		/* Update the cached level2 entry */
+		l2t[l2t_idx] = clust_start;
 	}
 
 	return len;
+
 free_l2:
 	free(l2t);
 error:
@@ -336,6 +513,7 @@ static int qcow_disk_close(struct disk_image *disk)
 
 	q = disk->priv;
 
+	free_cache(q);
 	free(q->table.l1_table);
 	free(q->header);
 	free(q);
@@ -428,6 +606,8 @@ static struct disk_image *qcow2_probe(int fd, bool readonly)
 		goto error;
 
 	q->fd = fd;
+	q->root = RB_ROOT;
+	INIT_LIST_HEAD(&q->lru_list);
 
 	h = q->header = qcow2_read_header(fd);
 	if (!h)
@@ -525,6 +705,8 @@ static struct disk_image *qcow1_probe(int fd, bool readonly)
 		goto error;
 
 	q->fd = fd;
+	q->root = RB_ROOT;
+	INIT_LIST_HEAD(&q->lru_list);
 
 	h = q->header = qcow1_read_header(fd);
 	if (!h)
diff --git a/tools/kvm/include/kvm/qcow.h b/tools/kvm/include/kvm/qcow.h
index b6e7493..1663819 100644
--- a/tools/kvm/include/kvm/qcow.h
+++ b/tools/kvm/include/kvm/qcow.h
@@ -3,6 +3,8 @@
 
 #include <linux/types.h>
 #include <stdbool.h>
+#include <linux/rbtree.h>
+#include <linux/list.h>
 
 #define QCOW_MAGIC		(('Q' << 24) | ('F' << 16) | ('I' << 8) | 0xfb)
 
@@ -17,6 +19,16 @@
 #define QCOW2_OFLAG_COMPRESSED	(1LL << 62)
 #define QCOW2_OFLAG_MASK	(QCOW2_OFLAG_COPIED|QCOW2_OFLAG_COMPRESSED)
 
+#define MAX_CACHE_NODES         128
+
+struct qcow_l2_cache {
+	u64                     offset;
+	u64                     *table;
+	struct rb_node          node;
+	struct list_head        list;
+	struct qcow             *qcow;
+};
+
 struct qcow_table {
 	u32			table_size;
 	u64			*l1_table;
@@ -26,6 +38,11 @@ struct qcow {
 	void			*header;
 	struct qcow_table	table;
 	int			fd;
+
+	/* Level2 caching data structures */
+	struct rb_root          root;
+	struct list_head        lru_list;
+	int                     no_cached;
 };
 
 struct qcow_header {
