Re: Extent sizes

In article <2p0snc$cil@maicgn.SoftwareEngineering> ar@mai.de (Achim Reiners) writes: >Are there any experiences or guidelines of configuring the first and >next extent size the best way? Use the space calculation formulas provided in the OnLine Programmers Reference Manual to determine the space required for each table. This is an arduous process, but well worth the effort. I mechanized the process by writing a small ESQL/C program which reads the system catalogs to determine for each already extant table a first extent size large enough to hold that table. Additional manual calculations must be performed to determine "tblspace tblspace" overhead for the dbspace size(s) you are using. You must estimate future table growth to determine an optimum next extent size in order to prevent at all costs tables housed in more than 8 extents. >How big can the performance degradation be if configured in a bad way ? >( access is n times slower ? Which n ) Informix explained the performance degradation to me as follows: Only the first eight pointers to table extents remain memory-resident. For extents 9 through N, each table access operation requires "significant" additional I/O. I am not clear as to the exact nature of this I/O (how many), but it could conceivably involve as many as one read for *each* extent past 8 for *each* table access operation, or as many as 42 extra disk reads for your 50-extent table! (Someone clarify if you know - I myself am curious about this one.) In any event, performance degradation will be significant but not factorable without additional info. >Is it possible to change these for the system tables as well? These >tables were the first ones exceeding the warning level of 8 extents! Assume you are referring to systables, sysusers, etc. - this is *serious*. To the best of my knowledge, the fextsize and nextsize of these tables cannot be modified. On my 4.10 OnLine engine on our HP9000/855S machine, the default sizes are 16KB, which should be large enough to hold info on an enormous number of tables. Any ideas, anybody? Bob Sorrells Process Specialist- Information Services Southwestern Bell Telephone One Bell Center, Room 16-N-01 St. Louis, MO 63101-3099 (Voice) 314-235-3963 (Fax) 314-235-1397 (E-Mail) rs4498@swuts.sbc.com