Servicing of requests

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Hi,

Whenever I read data of 'x'k size using one read() system call, I find batches of some 'y' no. of make_requests calls followed by the same no. of end_io's. Something like :
make_req
make_req
make_req
end_io
end_io
end_io
make_req
make_req
make_req
end_io
end_io
end_io

The output above gives me an idea that 3(hypothetical no.) buffer_heads above form a request.
(since 1 make_request corresponds to 1 buffer_head) and maybe since 1 request is serviced at a time I can see 3 make_req's together. Is my understanding right ??

But, I have read that sd uses some optimization algorithm to club requests so that the disk seek time is reduced. In which case since all requests are to adjecents sectors it should create a single request of all 'x'k assuming 1 buffer_head is of size 1k.

Does this make sense ??

-subodh

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