Re: [PATCH] FAT: Readahead FAT table to reduce the read request of FAT tabl

[ykzhao <yakui.zhao@intel.com>]

On Thu, 2010-11-25 at 16:46 +0800, ykzhao wrote:
> >From badc27e6b5f732191fb3b7e964f68da030144c76 Mon Sep 17 00:00:00 2001
> From: Zhao Yakui <yakui.zhao@intel.com>
> Date: Fri, 19 Nov 2010 10:29:33 +0800
> Subject: [PATCH] FAT: Readahead FAT table to reduce the read request of FAT table
> 
> On FAT filesystem the FAT table will be accessed very frequently when
> reading/writing the file. Now it reads only block sector from FAT table every
> time and it will have to wait for the completion of read-access. In fact
> as the FAT table is located sequentially on the disk, maybe we can readahead
> the following sectors to reduce the read access of FAT table.
> 
> Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>

Hi, Hirofumi
    Does this patch make sense to you?

thanks
> ---
> The following is the test result on SD card/U-disk.
> a. Read/Write a large file by using direct mode on SD card
>    dd if=sda.bin of=sda.bin2 bs=64k iflag=direct
> 	The average copy time can be reduced from 470 second to 452 seconds(The file
> of sda.bin is about 1.8G).
> 
> a. Read/Write a large file by using direct mode on U-disk
>    dd if=sda.bin of=sda.bin2 bs=64k iflag=direct
> 	The average copy time can be reduced from about 880 to 855 seconds.(The file
> of sda.bin is about 1.8G).
> 
>  fs/fat/fat.h    |    1 +
>  fs/fat/fatent.c |   26 ++++++++++++++++++++++++++
>  fs/fat/inode.c  |    4 ++++
>  3 files changed, 31 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/fat/fat.h b/fs/fat/fat.h
> index d75a77f..caf5449 100644
> --- a/fs/fat/fat.h
> +++ b/fs/fat/fat.h
> @@ -84,6 +84,7 @@ struct msdos_sb_info {
>  	struct inode *fat_inode;
>  
>  	struct ratelimit_state ratelimit;
> +	int fat_readahead_cnt; /* block number for readahead FAT table every time */
>  
>  	spinlock_t inode_hash_lock;
>  	struct hlist_head inode_hashtable[FAT_HASH_SIZE];
> diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
> index b47d2c9..1dfd73f 100644
> --- a/fs/fat/fatent.c
> +++ b/fs/fat/fatent.c
> @@ -67,6 +67,26 @@ static void fat32_ent_set_ptr(struct fat_entry *fatent, int offset)
>  	fatent->u.ent32_p = (__le32 *)(fatent->bhs[0]->b_data + offset);
>  }
>  
> +static void fat_table_readahead(struct super_block *sb, sector_t iphys)
> +{
> +	struct msdos_sb_info *sbi = MSDOS_SB(sb);
> +	struct buffer_head *bh;
> +	int sec, sec_count;
> +
> +	sec_count = sbi->fat_readahead_cnt;
> +	/*
> +	 * It will check whether the current block is already obtained from the
> +	 * disk. If not, we will try to readahead more sectors when
> +	 * reading the FAT table
> +	 */
> +	bh = sb_find_get_block(sb, iphys);
> +	if (bh == NULL || !buffer_uptodate(bh)) {
> +		for (sec = 0; sec < sec_count; sec++)
> +			sb_breadahead(sb, iphys + sec);
> +	}
> +	brelse(bh);
> +}
> +
>  static int fat12_ent_bread(struct super_block *sb, struct fat_entry *fatent,
>  			   int offset, sector_t blocknr)
>  {
> @@ -75,6 +95,7 @@ static int fat12_ent_bread(struct super_block *sb, struct fat_entry *fatent,
>  	WARN_ON(blocknr < MSDOS_SB(sb)->fat_start);
>  	fatent->fat_inode = MSDOS_SB(sb)->fat_inode;
>  
> +	fat_table_readahead(sb, blocknr);
>  	bhs[0] = sb_bread(sb, blocknr);
>  	if (!bhs[0])
>  		goto err;
> @@ -84,6 +105,7 @@ static int fat12_ent_bread(struct super_block *sb, struct fat_entry *fatent,
>  	else {
>  		/* This entry is block boundary, it needs the next block */
>  		blocknr++;
> +		fat_table_readahead(sb, blocknr);
>  		bhs[1] = sb_bread(sb, blocknr);
>  		if (!bhs[1])
>  			goto err_brelse;
> @@ -105,6 +127,10 @@ static int fat_ent_bread(struct super_block *sb, struct fat_entry *fatent,
>  	struct fatent_operations *ops = MSDOS_SB(sb)->fatent_ops;
>  
>  	WARN_ON(blocknr < MSDOS_SB(sb)->fat_start);
> +
> +	/* readahead the content of FAT table */
> +	fat_table_readahead(sb, blocknr);
> +
>  	fatent->fat_inode = MSDOS_SB(sb)->fat_inode;
>  	fatent->bhs[0] = sb_bread(sb, blocknr);
>  	if (!fatent->bhs[0]) {
> diff --git a/fs/fat/inode.c b/fs/fat/inode.c
> index ad6998a..fa00b6f 100644
> --- a/fs/fat/inode.c
> +++ b/fs/fat/inode.c
> @@ -34,6 +34,8 @@
>  #define CONFIG_FAT_DEFAULT_IOCHARSET	""
>  #endif
>  
> +
> +#define FAT_MAX_READAHEAD	32768
>  static int fat_default_codepage = CONFIG_FAT_DEFAULT_CODEPAGE;
>  static char fat_default_iocharset[] = CONFIG_FAT_DEFAULT_IOCHARSET;
>  
> @@ -1406,6 +1408,8 @@ int fat_fill_super(struct super_block *sb, void *data, int silent,
>  	sbi->dir_per_block = sb->s_blocksize / sizeof(struct msdos_dir_entry);
>  	sbi->dir_per_block_bits = ffs(sbi->dir_per_block) - 1;
>  
> +
> +	sbi->fat_readahead_cnt = FAT_MAX_READAHEAD / sb->s_blocksize;
>  	sbi->dir_start = sbi->fat_start + sbi->fats * sbi->fat_length;
>  	sbi->dir_entries = get_unaligned_le16(&b->dir_entries);
>  	if (sbi->dir_entries & (sbi->dir_per_block - 1)) {