Re: RA, bugs, and performance

>>> most of the pages that were read-ahead. >>> >> >>Wow, that's way too high for even a DSS environment. Rule of thumb I use >>isto set these to about 32, 16 for a good sized DSS and to about 12,8 for an >>OLTP envrionment. > >Hmm, what you say can only be true on a system very low on memory. >Our values are RA_PAGES 256 and RA_THRESHOLD 96 and we get very >good values with this. On the contrary, for DSS I consider these values still >rather low. > > The optimum values for RA_PAGES/RA_THRESHOLD is very dependent on the environment. If the disk bus width is a bottleneck, then the extra bytes being transfered could be a problem. This is also true if the site is a little tight on memory. Generally what I recommend is that RA_THRESHOLD be placed fairly close to RA_PAGES. For instant, maybe RA_PAGES at 32 and RA_THRESHOLD at 30, or maybe 64/62. The object is to try to get the next buffer in memory before the query thread reaches the end of the current buffer. Yes - this may cause an additional read, but in the total scan, that additional read is not that big of an expense. This will take a bit of testing to see what is optimal on any given system. Madison Pruet