| Message ID | 20110208063925.GA13619@lw.yar.ru |
|---|---|
| State | Accepted, archived |
| Headers | show |
On Tue, Feb 08, 2011 at 09:39:25AM +0300, Alexander V. Lukyanov wrote: > Hello! > > I cannot disable inode-read-ahead feature of ext4 (on 2.6.37): > > # echo 0 > /sys/fs/ext4/sda2/inode_readahead_blks > bash: echo: write error: Invalid argument > > On a server with lots of small files and random access this read-ahead makes > performance worse, and I'd like to disable it. I work around this problem > by using value of 1, but it still reads an extra block. So I'm curious --- have you actually benchmarked a performance decrease? What sort of hardware are you using? The readahead should be changing a 4k read to a 8k read with a value of 1, which shouldn't take a much of a difference to a HDD. I can apply this patch, but is it really making a difference for you? - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Mon, Feb 21, 2011 at 09:32:11PM -0500, Ted Ts'o wrote: > On Tue, Feb 08, 2011 at 09:39:25AM +0300, Alexander V. Lukyanov wrote: > > Hello! > > > > I cannot disable inode-read-ahead feature of ext4 (on 2.6.37): > > > > # echo 0 > /sys/fs/ext4/sda2/inode_readahead_blks > > bash: echo: write error: Invalid argument > > > > On a server with lots of small files and random access this read-ahead makes > > performance worse, and I'd like to disable it. I work around this problem > > by using value of 1, but it still reads an extra block. > > So I'm curious --- have you actually benchmarked a performance > decrease? What sort of hardware are you using? Yes, with the default value of inode_readahead_blks LA went from 4 to 30 (if I remember correctly). The problem was the increased load on HDD. The hardware is: Core2duo CPU, 4GB RAM, 4x80GB SATA disks without NCQ, the load is evenly distributed on the disks. At that time each disk contained 1 million files, randomly accessed for read/create-write, 10MB/s read and 10MB/s write (rate sum of 4 disks). > The readahead should be changing a 4k read to a 8k read with a value > of 1, which shouldn't take a much of a difference to a HDD. Sure, with inode_readahead_blks=1 it works acceptably. But I'd like to disable the inode read-ahead completely. > I can apply this patch, but is it really making a difference for you? I think it is logical to be able to disable an unneeded feature. Besides, there is a code already to check s_inode_readahead_blks!=0 (fs/ext4/inode.c:4737): /* * If we need to do any I/O, try to pre-readahead extra * blocks from the inode table. */ if (EXT4_SB(sb)->s_inode_readahead_blks) { -- Alexander. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
--- a/fs/ext4/super.c.0 2010-11-16 10:48:33.418629215 +0300 +++ b/fs/ext4/super.c 2010-11-16 10:46:07.739753246 +0300 @@ -1657,7 +1657,7 @@ set_qf_format: return 0; if (option < 0 || option > (1 << 30)) return 0; - if (!is_power_of_2(option)) { + if (option && !is_power_of_2(option)) { ext4_msg(sb, KERN_ERR, "EXT4-fs: inode_readahead_blks" " must be a power of 2"); @@ -2274,7 +2274,7 @@ static ssize_t inode_readahead_blks_stor if (parse_strtoul(buf, 0x40000000, &t)) return -EINVAL; - if (!is_power_of_2(t)) + if (t && !is_power_of_2(t)) return -EINVAL; sbi->s_inode_readahead_blks = t;
Hello! I cannot disable inode-read-ahead feature of ext4 (on 2.6.37): # echo 0 > /sys/fs/ext4/sda2/inode_readahead_blks bash: echo: write error: Invalid argument On a server with lots of small files and random access this read-ahead makes performance worse, and I'd like to disable it. I work around this problem by using value of 1, but it still reads an extra block. This patch fixes the problem by checking for zero explicitly. Signed-off-by: Alexander V. Lukyanov <lav@netis.ru>