seq vs random, disk performance when all of the io_u->offset=0

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Hi.

I have a question about the disk performance/response time, needing
some helps :). Thanks in advance.

For the sequential case, the offset increments accordingly to the
buflen, i.e., every 4096 (4k fixed block).
I run this test, and I get expectedly high iops and low ms, i.e., 6x
HDDs disks (HW RAID), the seq-write, I get 850MB/s+, 2.4ms (libaio,
500 depth, direct=1, counting the clat).
and for rand-write, I get 7MB/s, 287ms. So far so good.

As I want to know about basic io depth RTT, and what I don't
understand is that, if I forcedly just change the io_u->offset to a
constant fixed number, e.g., 0 in fill_io_u function, so as to
observed what-should-be the lowest ms.
instead, it returns much higher ms, i.e., 140MB/s, 14ms. I'd expect
the result to be lower than 2.4ms as the offset is not moving at all,
thus making the disk doing virtually no head movements. Why?
One further test I did was to simply forcedly alternate the offset to
0 and 4096 every time, in this, I get about about double, to 280MB/s
and 7ms.

I've confirmed the same behavior with other disks and SSDs, it just
returns much lower ms for the sequential. The change of IO depths
(100, 500, 1000, etc) just scales the results.
SSD:
seq-write=230MB/s, 8.6ms,   rand-write=84MB/s, 23ms
offset0 = 104MB/s, 19ms,   offset-alternate=150MB/s, 13ms.  Again, no
methods beat the seq-write..any reasons?

I could assume that this is due to the internal disk device's
controller ability to predict some offsets, maybe optimized for the
sequential ?
Or I could just be missing out how the series of io flights are
calculated for the clat time?


Thanks
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