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Database applications with memory-sensitive workloads that require working
data sets larger than 4 GB to be loaded in memory will benefit from the larger
memory support of the 64-bit platform.
The following is an example from the field. Microsoft SQL Server Enterprise
Edition uses AWE memory only for the buffer pool. The AWE (Advanced
Windowing Extensions) API allows applications that are written to use the AWE
API to access more than 4 GB of RAM (basically anything between 4 GB and 64
GB). However, due to the AWE mapping overhead, it is not practical to try to use
it for sort areas, procedure cache, or any other type of work area. Many
applications do make heavy use of these areas and may not benefit by having
the large buffer pool. The most efficient solution in such cases is to move the
applications on to a 64-bit database server, which can access memory area
above 4 GB much more efficiently without AWE's overhead.
Users will see a reasonable performance improvement on a 64-bit
implementation over a 32-bit implementation. Through the 64-bit memory paths,
all memory operations move twice as much data at the same speed as the 32-bit
platform.
The database server will also benefit from a larger 3 MB third-level and 64 MB
XceL4 cache. With such large cache, the need to go to memory or disk for
database transaction elements is greatly reduced and this directly implies a
performance increase, faster access to data, and improved throughput. Itanium 2
systems are likely to be able to hold database transaction records in cache
during the entire transaction, which enables the I/O portion of the transaction to
occur at speeds faster than memory access.
In-memory databases
Architectures with 64-bit addresses can store reasonably large databases in
memory and access them with little or no paging overhead. This is often done for
databases that are constantly being accessed and for databases that serve as
the basis for complex analysis. The theoretical maximum of 16 Exabytes for
memory has not yet been tested, but multi-Gigabyte databases are frequently
run on 64-bit machines.
A major challenge to providing high-performance access to database information
is the time it takes to access disk drives. When disk access is required, disk
access times add what can be an intolerable delay to efficient information access
and utilization. Access to disk is typically hundreds to thousands times slower
than access to memory.
Today, the disk access time challenge can be overcome. The price of random
access memory has come down to affordable levels for many systems. This price
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Chapter 2. Positioning
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