Re: The old raw devices chestnut.

Raw disk has the potential for best performance with large tablespace and index scans in a data warehouse environment. During sequential prefetch, DB2 can make sure that the extents are contiguous on the disk if the space is DMS raw disk. It makes no difference for OLTP systems. Decision support systems that do not frequently use large tablespace scans, also will show very little benefit. To the extent that data is already in the buffer pool, it makes no difference.

Software RAID does not perform well. If striping is desired, superior performance can usually be achieved by placing DB2 containers on different physical drives (or different hardware arrays) and letting DB2 stripe the data across the containers.

If disks are striped by the OS or hardware using some form of RAID, you must make sure the DB2 parameter for Container Tag Size is set properly. For version 7 that means that DB2_STRIPED_CONTAINERS must be set ON or YES if RAID is used. This sets the container tag equal to the extent size (instead of using a single page). For version 8, make sure that DB2_USE_PAGE_CONTAINER_TAG is set OFF or NULL for RAID disks. This will make sure that extents after the container tag will align properly with the RAID stripes.

Also be sure that the extent size is equal to or a multiple of the stripe size, and the prefetch size should be a multiple of the extent size. If data is striped by DB2 (by placing the containers on different physical drives) then the prefetch size should be n times the extent size, where n is the number of containers that are placed on different physical disks (or different physical RAID arrays).

These comments relate to DB2 and are different with other DBMS'. Received on Mon Apr 12 2004 - 23:25:30 CDT