Hi=20 (Sorry for the delay... but I'm offline by a customer...) >This kind of brings up an interesting thought. There is the script on >Ixora to test the largest MBR size and then you are suppose to set the >value to that, Setting it to the largest value is not good on all I/O sub-systems and = sometimes very poor for the optimizer. Usually a value that gives good = performance (let's say 90-95% of the maximum) lead to much better = execution plans. Notice that if system statistics are used, then they = automatically "compensate" large values with much better one (e.g. on a = real system it happens not very often that you can read 50-60 contiguous = blocks with a FTS...). >maybe it would be a better practice to generate a huge >table, run tests at different sizes then set. For my tests I took a table of at least 1GB (on small systems) or 10GB = (on big systems), i.e. I just reused a table with real data. >In theory the largest >size possible would be fastest but it would be interesting to find out >if this always was true. This is not true! You should really do some tests!!!! Chris >-----Original Message----- >From: oracle-l-bounce@xxxxxxxxxxxxx >[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Christian Antognini >Sent: Tuesday, December 07, 2004 2:18 AM >To: ryan_gaffuri@xxxxxxxxxxx >Cc: Oracle-L@xxxxxxxxxxxxx >Subject: RE: db_file_multiblock_read_count and performance > >Hi Ryan >>I have been testing this extensively over the last few months. I do a >full table scan with a >>db_file_multiblock_read_count =3D 1 and then one =3D 128( i check the = 10046 >trace to verify i am >>getting this much) and I see absolutely no difference whatsoever in >response time. > >Attached you find some results that I get during some tests that I >performed on different servers by different customers (notice that I = had >no influence on the setup, I just run a test script...). > >As you can see many different behaviors are to be expected. > >System 1: higher values are better, of course they are "technical" >limits... (notice that 55MB/s is the maximum throughput measured on = this >system, i.e. with DFMRC=3D32). > >System 2: values higher than 16 give bad performance, i.e. the optimal >value is 16. > >System 3: values less than 17 are useless, i.e. at least 17 should be >used to have "correct" performance. > >System 4: no performance difference was measured. Notice that this >system, with 230MB/s, is also the faster I tested... > > >HTH >Chris > > > > >-- >//www.freelists.org/webpage/oracle-l -- //www.freelists.org/webpage/oracle-l