RAU
Lester Policarpio's Read Ahead Utilization was 63.79%, with RA_THRESHOLD 30 and RA_PAGES 32, and asked how to reach near 100%. Eric Vercelletto said read ahead depends on access patterns and matters little for OLTP. Art Kagel said v11.50 changed read ahead so RAU is effectively worthless, and recommended his ratios.shr_ak package. No tuning change was identified.
Auto-generated by Claude from the posts below — may be imperfect; read the full thread.
Good Day, I saw a post regarding the computation of BR, BTR and RAU with below scripts select 'BR (Bufferwaits Ratio): [< 7%] ---> ' || trunc(100 * (select value from sysprofile where name='buffwts')/((select value from sysprofile where name='bufwrites')+(select value from sysmaster:sysprofile where name='pagreads')),2) || ' %' from systables where tabid = 1 ; select 'BTR (Buffer Turnover Ratio): [< 10] ---> '|| round(((( pagreads + bufwrites ) /nbuffs ) / ( select (ROUND ((( sh_curtime - sh_pfclrtime)/60)/60) ) from sysshmvals ) ),1) from sysmaster:sysbufpool; select 'RAU (Read Ahead Utilization): [near 100%] ---> ' || trunc(100 * (select value from sysmaster:sysprofile where name='rapgs_used')/((select value from sysmaster:sysprofile where name='btradata')+(select value from sysmaster:sysprofile where name='btraidx')+(select value from sysmaster:sysprofile where name='dpra')),2) || ' %' from sysmaster:systables where tabid = 1 ; the result for our production server is good except for RAU, does anyone know what to do so we can achieve a near 100% result? BR (Bufferwaits Ratio): [< 7%] ---> 1.55 % BTR (Buffer Turnover Ratio): [< 10] ---> 1.6 RAU (Read Ahead Utilization): [near 100%] ---> 63.79 % Thanks,
Hi, our current onconfig value for RA_THRESHOLD and RA_PAGES are 30 and 32 respectively. Do we need to increase/decrease both values? how can we measure the amount of increase/decrease we need to make? or is it a trial and error thing?
Lester, Read ahead is used only when relevant and totally depends on how your applications access the data. You expect to have a lot of read ahead in cases access your data sequentially, i.e more when running Decision Support Queries. Is this the case? If you app is mostly OLTP type of activity, you probably don't want to be worried with a bad RAU
The problem is that in v11.50 IBM changed how readahead works including additional structures being read ahead that were not being processed that way before. Unfortunately few of these new RA pages are being reported anywhere so the RAU calculation is effectively worthless <sigh>. Also, note that I have replaced the BTR calculation with the more accurate BTR3 calculation which is a bit more involved. The best thing is to download my ratios.shr_ak which includes a set of stored procedures that calculate the various metrics and a shell script to produce a formatted report.