IDS提供的“分片”技术在做了RAID5的磁盘阵列上还有意义吗?

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有意义是肯定的，只是作用的大小而已。
你可以看看PDQ的资料，再有，详细调节存放位置、以及其他的分片的好处（多看看资料，分片的好处多多），会对你数据库的性能、安全性等有很大帮助的。

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Fragmentation Goals
Consider fragmenting your tables if you have at least one of the following
goals:
n Improved single-user response time
To improve the performance of individual queries, use fragmentation
with parallel database query (PDQ) to scan in parallel
fragments that are spread across multiple disks.
n Improved concurrency
Fragmentation can reduce contention for data that is located in large
tables that are used by multiple queries and OLTP applications.
Fragmentation reduces contention because each fragment resides on
a separate I/O device, and the database server directs queries to the
appropriate fragment.
n Improved availability
If a fragment becomes unavailable, the database server can still
access the remaining fragments.
n . Improved ALTER FRAGMENT performance
You can also use the ALTER FRAGMENT TABLE with the ATTACH
clause to add data quickly to a very large table.
n Improved backup-and-restore characteristics
Fragmentation gives you a finer backup-and-restore granularity.
This granularity can reduce the time that is required for backup-andrestore
operations. In addition, you can improve the performance of
backup-and-restore operations if you use ON-Bar to perform these
operations in parallel.

摘自 管理员指南
关于PDQ的资料，还是建议你看这本书吧，其他的书也很多的，如果你有education service的教师用书最好了。