Date: Tue, 26 Sep 2006 11:39:30 -0500 From: Eric Anderson <anderson@centtech.com> To: freebsd-performance@freebsd.org Subject: Re: Why are disk writes so slow? Message-ID: <451957C2.90701@centtech.com> In-Reply-To: <20060926154643.GA3064@rabbit> References: <20060926154643.GA3064@rabbit>
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On 09/26/06 10:46, Mark Bucciarelli wrote: > 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 > disk. > > 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? It's probably because of caching on the disk. The normal write goes in/out of the on-disk cache, the O_SYNC may be forced to go to the platters. Also, if you didn't already, you should run the test many times, umounting/mounting the filesystem in question in between each test. Also, I recommend using a block device, instead of a file on a filesystem, since the filesystem could allocate blocks for the file differently each time, causing varying results. Eric -- ------------------------------------------------------------------------ Eric Anderson Sr. Systems Administrator Centaur Technology Anything that works is better than anything that doesn't. ------------------------------------------------------------------------
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