Re: Buffer Waits

> hmm, could you go into a bit detail on the light-scan? I have a DSS system > but I am not sure how to make it use light scan. I read something on the > book about it, but it wasn't very clear on exactly how to utilize it or > how to check it. thanks yan Here's some more information I dug up on light scans: Light scans are used for sequential scans of tables larger than the buffer pool. Use onstat -g lsc to monitor light scans (I don't think it's available in 8.21, though). Light scans bypass the buffer cache and read pages from disk directly into light scan buffers located in the virtual segment of shared memory. Scan threads then read the pages in parallel from the light scan buffers. Light scan are managed at the session level. Since light scan buffers are located on the virtual segment of shared memory, be sure to configure a large SHMVIRTSIZE (which should be at least 75% of available memory for a DSS system anyway). Light scans are invoked when the following conditions apply: * Read-only or DDL operations * Sequential scans of tables larger than the buffer pool. This is determined by systables.npused column > buffer cache size. * Static table types * Dirty or repeatable read isolation level. * Shared table lock (committed read isolation level) The performance of light scans are based on: * Correct database update statistics * RA_PAGES and RA_THRESHOLD * SHMVIRTSIZE Light scan buffers are calculated: read_ahead_pages = RA_PAGES + RA_THRESHOLD light_scan_buffers = roundup (read_ahead_pages / (MAXAIOSIZE / PAGESIZE)) MAXAIOSIZE usually defaults to 16 and is defined as a constant. Hope that helps, Chuck