Re: [HACKERS] Sequential scan speed, mmap, disk i/o

>
> >
> > Someone was complaining about sequential scan speed, so I decided to run
> > a test.
>
> > wc                            41 sec
> > wc -l                            31 sec
> > dd if=/u/pg/data/base/test/testv of=/dev/null bs=512    32 sec
> > dd if=/u/pg/data/base/test/testv of=/dev/null bs=8k    31 sec
> > dd if=/u/pg/data/base/test/testv of=/dev/null bs=256k    31 sec
> > dd if=/u/pg/data/base/test/testv of=/dev/null bs=1m    30 sec
> > mmap() of file in 8k chunks                99 sec
> > mmap() of file in 8mb chunks                40 sec
> > mmap() of file in 32mb chunks                56 sec
> >
> > PostgreSQL sequential scan                37 sec
>
> Let me add, these times are on a PP200, with SCSI Ultra Barracuda
> drives, BSD/OS 3.1, 64MB RAM.

Also, the table was very small, with two ints, a char(10), and a
varchar(50), so PostgreSQL was processing most of the 177MB of data in
terms of having to read most of each block.

--
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