Sorry, I'm a bit non-clued up on this "read ahead algorithm". Could I be a pain and ask for more details? Does the OS return one OS block if exactly one is requested, but if 2 are requested it thinks "aha! sequential scan" and goes and gets 4 or 8 or something?
The follow on is, does this mean you should use a (minimal) 2k block size on UFS, 512 bytes blocks, or is this read-ahead overhead a smaller performance hit than that of using a database block size which is too small for the application? Thanks - Bill. At 08:48 26/04/02 -0800, you wrote: >All, > >You always want to ensure that your DB_BLOCK_SIZE = >File System Block Size. This is to avoid wasted I/O >and also the case where the "read ahead algorithm" is >triggered accidentally, when 1 Database Block results >in multiple file system blocks being read from disk. > >If your application performs range scans, there is a >high possibility that multiple "single database block" >read requests to a set of contiguous blocks, may >result in the "read ahead algorithm" performing 128K >or 256K pre-fetches, even though your application may >have not required all 128K or 256K. > >This problem is rampant on ufs file systems where the >default block size is 512 bytes, and with a 8K >DB_BLOCK_SIZE, it takes 16 file system blocks to store >1 DB block on disk. However, even if you have advanced >file systems and have a 1-is-to-2 ratio of DB block >is-to FS blocks, you are still in danger of >overloading your I/O sub-system, "under the right >conditions". -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Bill Buchan INET: [EMAIL PROTECTED] Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 San Diego, California -- Public Internet access / Mailing Lists -------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).