Re: disk cache useful?

Scott Winger (swinger_at_mailsrvr.bussvc.wisc.edu) wrote:
: We currently have an IBM RS/6000 C20 (Power PC 604, 64 Mb RAM, 2.2Gb disk)
: which is running AIX V4.1.4 and Oracle 7.1.6.
 

: We are debating whether an L2 cache would be worth purchasing for this machine.
: At the moment, the database is pre-production but eventually we plan to have as
: many as 150 concurrent users each doing about 1 database transaction (lookup
: and/or update) per minute.
 

: I maintain that the L2 cache would have almost no impact on performance in this
: scenario. My reasoning is that:
: 1. Any data that's in the L2 cache would also be in the oracle cache.
: (Because Oracle maintains its own cache for data blocks and this cache
: is larger than the L2 cache,)
 

: 2. Any data that wouldn't be in the Oracle cache would also not be in the L2 cache.
: (Because A. the Oracle cache is larger, and B. the L2 cache is used for all disk
: I/O not just disk I/O related to the database.)

: My guess is that if we want to improve response time, more RAM and/or a
: RAID disk system would be a better place to invest than in an L2 cache.
 

: Have I missed something here?

Yes, an L2 cache is between the CPU and the Memory subsystem. An Oracle SGA is in memory. Therefore access to the L2 cache is considerably faster. The L2 cache also caches instructions and data and exists so that the CPU does not stall waiting for memory access.

I have no performance figures on L2 cache usage, because every modern system which I have used for some years has implemented an L2 cache of some type. This in itself should tell you something.

The "probable" answer to your question is that both the L2 cache and a large SGA are required.

Nick

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+    Nick Price                               Technical Consultant             +
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