Re: Unix File System versus ROW device

In a note on this thread was this quote from an Oracle rep> given by a guy from Oracle US who
specialized in storage-related issues. He said that full table scans perform significantly less well on raw devices because they don't write to the oracle buffer cache - which normally acts as a cache for raw device access. <<

This is not exactly true. Oracle data blocks read via a full table scan are read into the Oracle buffer pool (cache)! However, they are placed on the least recently used end of the LRU chain so they are flushed first under the assumption that they are unlikely to be reused. If the query will need to access the data block only once then this will not harm performance, but where Oracle will need to access the block again later (like in a series of nested loops) then this hurts performance big time. There is a cache hint that can be used in these situations.

Data blocks read by randon read and index blocks are normally placed on the most recently used end of the LRU chain. A physical raw data block read is often 50% faster than a standard Unix read. And if you are using OS level stripping which tends to distribute your system i/o to appear to be randon i/o then the difference can be noticable.

Still, ever system needs to be judged on its own merits.

Just more stuff to complicate our decisions, but I felt the clarification was important.   

Mark Powell -- The only advise that counts is the advise that you follow so follow your own advise Received on Wed Jun 25 1997 - 00:00:00 CDT