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
> > >
> > >
> > >
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> > >
> >
> >
>
>
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