Date: Tue, 09 Nov 1999 23:22:58 -0500 From: Simon Shapiro <shimon@simon-shapiro.org> To: Bruce Evans <bde@zeta.org.au> Cc: freebsd-arch@freebsd.org Subject: Re: I/O Evaluation Questions (Long but interesting!) Message-ID: <3828F322.B62D174E@simon-shapiro.org> References: <Pine.BSF.4.10.9911101412490.12295-100000@alphplex.bde.org>
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Bruce Evans wrote:
>
> > And this, ladies and gentlemen is what I do not understand;
> >
> > Why is random WRITE to a block device about 10-11 times
> > slower than raw device?
> > Actually, sequential read is 1/3 of raw device too. Why?
>
> Block devices have to use a fixed block size. This size is
> normally BLKDEV_IOSIZE. For historical reasons, BLKDEV_IOSIZE
> is normally too small. On i386's, it is 2048 in RELENG_3 and
> 4096 in -current. -current has a sysctl to set the default
> size.
>
> Large i/o's are split up into blocks of the fixed size. Small
> blocks are very bad for sequential i/o's. They may actually be
> good for random i/o's if the original i/o's are small.
>
> Bruce
Thanx Bruce.
What I observed, is that ALL block device I/O is happening
in 8KB calls, except for the 512 bytes calls for slice
and partition management (5 of them, methinks).
The 10:1 random write problem may be mine; An ancient
and well hidden bug in st.d which made lock unlock calls
for every i/o.
We run circles around NT in the Random I/O department,
but take a beating in the sequential I/O arena;
For about the same hardware, they do 98 MB/Sec,
I cannot get more than 45.
Their test machine has 64bit PCI. Mine does not.
--
Sincerely Yours, Shimon@Simon-Shapiro.ORG
404.664.6401
Simon Shapiro
Unwritten code has no bugs and executes at twice the speed of mouth
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