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Ext3 overhead vs Raw


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 .



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