unable to produce sequential writes to redo log

From: Paul M. Aoki <aoki_at_faerie.CS.Berkeley.EDU>
Date: 4 Oct 1999 20:21:25 GMT
Message-ID: <7tb285$2k5$1@agate-ether.berkeley.edu>

i have a oracle 8.0.3 installation on a sun ultra 30 running solaris 2.5.1. i am running a homebrew microbenchmark that measures the ability of the system to absorb insertions, and i have tuned my system and schema so that the system is fully bottlenecked on the redo log. for testing purposes, i've turned off ARCHIVELOG at the database level. i have observed the following curious behavior:

% iostat -D -l 1 sd8 5

          sd8
 rps wps util

  23 115 35.0 
  26 123 39.6 
  25 113 37.9 
  26 128 40.1 

^C
%

that is, the redo log disk is getting a significant number of READS in the steady state in spite of the fact that archiving is turned off! obviously, this is killing performance - it forces a large number of disk seeks (meaning that the disk can't do sequential writes to the log tail - hence the low disk utilization).

my installation has two redo log files, each of which is by itself on a separate disk (literally - there's one unix file system on each disk, and there are no other files in either of the file systems). as mentioned, archiving is turned off; the system is set up to checkpoint at log switches (only), and the block checksumming options are not enabled.

does anyone have any idea why oracle would need to perform ongoing reads from its redo log files? for example, is there some kind of bizarre read-ahead it needs to do to scrub redo log blocks before they are reused?
--

  Paul M. Aoki         | Computer Science Division, EECS #1776
  aoki_at_CS.Berkeley.EDU | University of California
                       | Berkeley, CA 94720-1776 USA

Received on Mon Oct 04 1999 - 15:21:25 CDT