Date: Tue, 5 Mar 2002 17:03:28 -0500 (EST) From: Zhihui Zhang <zzhang@cs.binghamton.edu> To: "Rogier R. Mulhuijzen" <drwilco@drwilco.net> Cc: Julian Elischer <julian@elischer.org>, freebsd-hackers@FreeBSD.ORG Subject: Re: A weird disk behaviour Message-ID: <Pine.SOL.4.21.0203051659350.12061-100000@onyx> In-Reply-To: <5.1.0.14.0.20020305225819.01c3a678@mail.drwilco.net>
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The machine has 128M memory. I am doing physical I/O one block at a time, so there should be no memory copy. -Zhihui On Tue, 5 Mar 2002, Rogier R. Mulhuijzen wrote: > At 16:03 5-3-2002 -0500, Zhihui Zhang wrote: > > > >On Tue, 5 Mar 2002, Julian Elischer wrote: > > > > > > > > more writes fit in the disk's write cache? > > > >For (1), it writes 15000 * 8192 bytes in all. For (2), it writes 15000 * > >4096 bytes in all (assuming the random number distributes evenly between 0 > >and 8192). So your suggestion does not make sense to me. > > How large is your buffercache? it might be that the 15000 * ~4096 roughly > matches with your cache, and 15000 * 8912 doesn't. > > Case (1) would require a lot more physical IO in that case than case (2) > would require. > > Doc > > > >-Zhihui > > > > > On Tue, 5 Mar 2002, Zhihui Zhang wrote: > > > > > > > > > > > I am doing some raw I/O test on a seagate SCSI disk running FreeBSD 4.5. > > > > This situation is like this: > > > > > > > > +-----+----+----+----+----+----+----+----+----+----+---+------ > > > > | | | | | | | | | | | | .... > > > > +-----+----+----+----+----+----+----+----+----+----+---+------ > > > > > > > > Each block is of fixed size, say 8192 bytes. Now I have a user program > > > > writing each contiguously laid out block sequentially using /dev/daxxx > > > > interface. There are a lot of them, say 15000. I write the blocks in two > > > > ways (the data used in writing are garbage): > > > > > > > > (1) Write each block fully and sequentially, ie. 8192 bytes. > > > > > > > > (2) I still write these blocks sequentially, but for each block I only > > > > write part of it. Exactly how many bytes are written inside each > > block is > > > > determinted by a random number between 512 .. 8192 bytes (rounded up a > > > > to multiple of 512 bytes). > > > > > > > > I find out the the performance of (2) is several times better than the > > > > performance of (1). Can anyone explain to me why this is the case? > > > > > > > > Thanks for any suggestions or hints. > > > > > > > > -Zhihui > > > > > > > > > > > > > > > > To Unsubscribe: send mail to majordomo@FreeBSD.org > > > > with "unsubscribe freebsd-hackers" in the body of the message > > > > > > > > > > > > > > > >To Unsubscribe: send mail to majordomo@FreeBSD.org > >with "unsubscribe freebsd-hackers" in the body of the message > > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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