Ext3 overhead vs Raw

Stephen C. Tweedie sct at redhat.com
Fri Jul 14 21:47:51 UTC 2006


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?


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