OK, all the info I have received has been of great help and indeed the Technical Reference guide is very informative.
But what I still don't understand is this. I'm running expiration on the db at the moment and the cache hit it average at 98.5%. Not 99%, but not too bad. But the rest of the time, i.e. when running backups etc, it drops to as low as 95%. Should the cache hit rate consistently be above 98%? This is the answer I cannot find. Many thanks again to all who have provided help with this. Farren Minns - John Wiley & Sons Please respond to "ADSM: Dist Stor Manager" <[EMAIL PROTECTED]> Sent by: � � � �"ADSM: Dist Stor Manager" <[EMAIL PROTECTED]> To: � � � �[EMAIL PROTECTED] cc: Subject: � � � �Re: DB Cache Hit Rate Question Farren, I'm not sure exactly how the TSM database internals work, but typically a database cache is populated with "cache prefetch." �This isn't _exactly_ correct, but it'll do conceptually. �The db goes to read one row (check a file for expiration). �That one db row request loads one or more blocks/pages of db into the cache. Those blocks contain many other rows, so most of the next few thousand file checks get cache hits in the prefetched cached blocks. Hope this helps. Alex Paschal Freightliner, LLC (503) 745-6850 phone/vmail -----Original Message----- From: Farren Minns [mailto:[EMAIL PROTECTED]] Sent: Tuesday, February 18, 2003 8:25 AM To: [EMAIL PROTECTED] Subject: DB Cache Hit Rate Question Good afternoon all TSMers. I'm a little confused about the cache hit ratio information. If I run expiration on a database, surely the TSM server has to trawl through the entire thing on disk as it does not already reside in the buffer. So how would I expect to see hit ratios of 99% or higher. I won't go into details here as I'm more interested in the process of how the database and cache work than out particular performance problems. But, as we do very little in the way of restores, surely the amount of time that a requested database file is present in the cache must be very low, so a cache hit ratio of 96-98% would make sense. Please forgive my ignorance here as I'm not a expert in this field at all. Many thanks in advance All the best Farren Minns John Wiley & Sons Ltd
