* [PATCH] ext4: allow inode_readahead_blks=0 (linux-2.6.37)
@ 2011-02-08 6:39 Alexander V. Lukyanov
2011-02-22 2:32 ` Ted Ts'o
0 siblings, 1 reply; 3+ messages in thread
From: Alexander V. Lukyanov @ 2011-02-08 6:39 UTC (permalink / raw)
To: linux-ext4; +Cc: Theodore Ts'o, Andreas Dilger, linux-kernel
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.
--- 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;
Signed-off-by: Alexander V. Lukyanov <lav@netis.ru>
--
Alexander.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] ext4: allow inode_readahead_blks=0 (linux-2.6.37)
2011-02-08 6:39 [PATCH] ext4: allow inode_readahead_blks=0 (linux-2.6.37) Alexander V. Lukyanov
@ 2011-02-22 2:32 ` Ted Ts'o
2011-02-24 8:18 ` Alexander V. Lukyanov
0 siblings, 1 reply; 3+ messages in thread
From: Ted Ts'o @ 2011-02-22 2:32 UTC (permalink / raw)
To: Alexander V. Lukyanov; +Cc: linux-ext4, Andreas Dilger, linux-kernel
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
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] ext4: allow inode_readahead_blks=0 (linux-2.6.37)
2011-02-22 2:32 ` Ted Ts'o
@ 2011-02-24 8:18 ` Alexander V. Lukyanov
0 siblings, 0 replies; 3+ messages in thread
From: Alexander V. Lukyanov @ 2011-02-24 8:18 UTC (permalink / raw)
To: Ted Ts'o, linux-ext4, Andreas Dilger, linux-kernel
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.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2011-02-08 6:39 [PATCH] ext4: allow inode_readahead_blks=0 (linux-2.6.37) Alexander V. Lukyanov
2011-02-22 2:32 ` Ted Ts'o
2011-02-24 8:18 ` Alexander V. Lukyanov
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