IBM i series Handbook page 54

iSeries Architecture: Fundamental Strength of the iSeries
Management of the cache is performed automatically within the I/O adapter. It is designed to
cache data by using a predictive algorithm. The algorithm considers how recently and how
frequently the host has accessed a predetermined range of data.
The design of Extended Adaptive Cache is based on specific data management strategies of
the iSeries server. To explain, whether the disks are device parity protected, mirrored, or
unprotected, the data stored on the disks has a tendency to occur in bands. This means that
there are physically contiguous areas of disk storage that fall under one of the following
categories:
• Areas where the data is actively read
• Areas of data that are both actively read from and written to
• Areas that are frequently written to
• Areas of storage that are not frequently accessed
The goal is to cache bands characterized as read or write and read-only. A band that is
characterized as write-only, while cached in the storage subsystem write cache, remains
largely unaffected by Extended Adaptive Cache. Extended Adaptive Cache is designed to not
harm the performance of large blocks of data that are either sequentially written or
sequentially read. In this case, the pre-fetch capability of the disks, as well as other caches in
the system, ensures a quick response time.
Using Extended Adaptive Cache improves the performance of database-read actions and all
read actions. This includes read actions that are generated by other system components
such as the Integrated xSeries Server. It also works effectively in storage subsystems that
have device parity protection or mirroring.
Extended Adaptive Cache Simulator Restrictions and Considerations
• A storage controller capable of supporting Extended Adaptive Cache is required for
Extended Adaptive Cache Simulator. This includes the #2748, #4748, #2778, or
#4778.
• The Simulator is enabled and disabled by the user through AS/400 Operations
Navigator, Management Central, or Collection Services.
• Performance Tools LPP is required for Extended Adaptive Cache Simulator.
• Activating the Extended Adaptive Cache Simulator does not actually improve your
system's performance. It gathers statistical information to predict the performance
improvement that Extended Adaptive Cache could offer.
• Extended Adaptive Cache Simulator and Extended Adaptive Cache cannot be active
at the same time on the same I/O adapter.
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iSeries Handbook