[PATCH] FAT cluster chain cache per inode (5/5)

This adds a cache of lastest accessed cluster to inode for sequential
access.

The following is 500M file of FAT-to-FAT copy test, this may be a most
different case in usual operations, because maximum readahead window
flush the all caches.

512 bytes blocksize, 4096 bytes cluster size.

linux-2.5.69-bk12
	root@devron (a)[1232]# time cp file file1

	real    7m58.900s
	user    0m0.267s
	sys     6m44.258s

linux-2.5.69-bk12+patch
	root@devron (a)[1576]# time cp file file1

	real    2m44.309s
	user    0m0.270s
	sys     0m28.631s

fs/fat/cache.c

@@ -152,11 +152,20 @@ void fat_cache_lookup(struct inode *inode, int cluster, int *f_clu, int *d_clu)
 	struct fat_cache *walk;
 	int first;
 
+	BUG_ON(cluster == 0);
+	
 	first = MSDOS_I(inode)->i_start;
 	if (!first)
 		return;
 
 	spin_lock(&sbi->cache_lock);
+
+	if (MSDOS_I(inode)->disk_cluster &&
+	    MSDOS_I(inode)->file_cluster <= cluster) {
+		*d_clu = MSDOS_I(inode)->disk_cluster;
+		*f_clu = MSDOS_I(inode)->file_cluster;
+	}
+
 	for (walk = sbi->cache; walk; walk = walk->next) {
 		if (walk->start_cluster == first
 		    && walk->file_cluster <= cluster
@@ -203,6 +212,7 @@ static void __fat_cache_inval_inode(struct inode *inode)
 {
 	struct fat_cache *walk;
 	int first = MSDOS_I(inode)->i_start;
+	MSDOS_I(inode)->file_cluster = MSDOS_I(inode)->disk_cluster = 0;
 	for (walk = MSDOS_SB(inode->i_sb)->cache; walk; walk = walk->next)
 		if (walk->start_cluster == first)
 			walk->start_cluster = 0;
@@ -220,7 +230,7 @@ void fat_cache_add(struct inode *inode, int f_clu, int d_clu)
 {
 	struct msdos_sb_info *sbi = MSDOS_SB(inode->i_sb);
 	struct fat_cache *walk, *last;
-	int first;
+	int first, prev_f_clu, prev_d_clu;
 
 	first = MSDOS_I(inode)->i_start;
 	if (!first)
@@ -228,6 +238,20 @@ void fat_cache_add(struct inode *inode, int f_clu, int d_clu)
 
 	last = NULL;
 	spin_lock(&sbi->cache_lock);
+
+	if (MSDOS_I(inode)->file_cluster == f_clu)
+		goto out;
+	else {
+		prev_f_clu = MSDOS_I(inode)->file_cluster;
+		prev_d_clu = MSDOS_I(inode)->disk_cluster;
+		MSDOS_I(inode)->file_cluster = f_clu;
+		MSDOS_I(inode)->disk_cluster = d_clu;
+		if (prev_f_clu == 0)
+			goto out;
+		f_clu = prev_f_clu;
+		d_clu = prev_d_clu;
+	}
+	
 	for (walk = sbi->cache; walk->next; walk = (last = walk)->next) {
 		if (walk->start_cluster == first &&
 		    walk->file_cluster == f_clu) {

fs/fat/inode.c

@@ -1101,6 +1101,7 @@ static int fat_fill_inode(struct inode *inode, struct msdos_dir_entry *de)
 	struct msdos_sb_info *sbi = MSDOS_SB(sb);
 	int error;
 
+	MSDOS_I(inode)->file_cluster = MSDOS_I(inode)->disk_cluster = 0;
 	MSDOS_I(inode)->i_pos = 0;
 	inode->i_uid = sbi->options.fs_uid;
 	inode->i_gid = sbi->options.fs_gid;

include/linux/msdos_fs_i.h

@@ -8,6 +8,10 @@
  */
 
 struct msdos_inode_info {
+	/* cache of lastest accessed cluster */
+	int file_cluster;	/* cluster number in the file. */
+	int disk_cluster;	/* cluster number on disk. */
+
 	loff_t mmu_private;
 	int i_start;	/* first cluster or 0 */
 	int i_logstart;	/* logical first cluster */