Ext3 users - Jul 2006 - Ext3 overhead vs Raw

Hi,

I am trying to find way to speed up read access on ext3 filesystem.
I did some tests using dd, with different block sizes, directio and 
none, etc. The test file is about 1Gig in size, and spread across 25 
fragments (found using filefrag). Block size is 4k. I have also tried 
setting readahead buffer using blockdev , from 256 to 32767.

time /root/dd conv=nocreat  ibs=4096 obs=4096 if=/sam/cache/test/test3 
of=/dev/null
The best real elapsed time I get is about 23.5s.

If I dd the same amount of data from the disk device itself, I get about 
18.5s, which matches what hdparm -tT gives me.

Comparing strace outputs, I can see the read system calls reading from 
ext3 takes 30-35% longer to complete compare to raw device. Is this 
something  expected or  can I expect better performance?

I am running kernel.org kernel 2.6.12 .

Thanks,

...
ling

Hi,

On Thu, 2006-07-13 at 15:13 -0500, Ling C. Ho wrote:
> If I dd the same amount of data from the disk device itself, I get about 
> 18.5s, which matches what hdparm -tT gives me.
Be aware, disks typically have different performance depending on where
the data is, with data on the outermost cylinders getting higher
throughput than data on innermost cylinders (there's constant rotational
velocity for the surface, but the outer tracks are longer so each
rotation carries more data past the heads.)

So all sorts of things like the exact data placement can come into
effect.  Are you sure you're using the same bits of the disk for the raw
and filesystem cases?

--Stephen