Why are disk writes so slow?

Mark Bucciarelli mark at gaiahost.coop
Tue Sep 26 08:48:05 PDT 2006

I am reading Richard Stevens' "Advanced Programming in the UNIX
Environment," a most excellent book.

Out of curiosity, I tried his I/O efficiency program on my IBM
A30 Thinkpad, running 6.0-RELEASE with default tuning parameters.
The test program reads file on stdin and writes to stdout, and
you modify bufsize to watch how time changes.

As in his example (with a bufsize of 8192), 

    time ./a.out < 1.5M-testfile > /dev/null

runs five times faster than (clock time)
    time ./a.out < 1.5M-testfile > /a.out.out

Can someone explain to me why writing is five times as slow as
writing?  What's going on in the computer?  

The file is not O_SYNC, so it can't be validating the data on the

Later in the same chapter, he shows the impact of O_SYNC flag. I
re-ran this experiment too, and while everything is two orders of
magnitude faster than his times in the book, the relative speed
of writing with O_SYNC is three times slower.

                     1993       2006
                    -----       ----
    normal write     2.3s       .023s
    O_SYNC          13.4s       .364s
    slowdow factor   5.8        15.8

Is this all b/c disks are so much larger?


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