Oracle Essentials [Electronic resources] : Oracle Database 10g, 3rd Edition نسخه متنی

11.8 Disk Technology

The discussion of hardware
architectures and performance in this chapter so far has centered on
ways of increasing performance by increasing available system
resources such as CPUs, memory, and I/O subsystems and on parallelism
that can take advantage of these resources.

Another classic way to increase hardware performance is to tune for
I/O, which includes tuning for disk layout. Because disk access has
the greatest latency, the main focus of I/O tuning is typically
keeping what has been retrieved from disk in memory. The actual
performance of retrieving that data from disks can also be improved
by spreading the data evenly across multiple disks and by making sure
there are enough disk controllers to transfer the data from disk onto
the I/O bus and into memory. As noted previously, Oracle Database
10g now includes automatic data rebalancing
across disks, greatly simplifying the day-to-day management. However,
working with your hardware vendor regarding disk controller selection
and configuration is still important.

11.8.1 Disk Deployment Strategies

Disks have typically been directly
attached to systemsmore expensive systems offer fastest disk
controllers and I/O. Disks are also configured in a variety of ways
for redundancy, eliminating the possibility of single points of disk
failure resulting in loss of access to data. As network bandwidth
improved, deployments that include Network Attached Storage (NAS) and
Storage Area Networks (SAN) have appeared as cost-effective
alternatives.

Disk is commonly deployed in arrays, the industry standard being RAID
(Redundant Array of Inexpensive/Independent Disks). You can use RAID
as a part of any of the configurations we've
discussed to provide higher performance and reliability. RAID disk
arrays were introduced in Chapter 6 and discussed in the context of
their use in high-availability scenarios in Chapter 10. Please refer
to those chapters for more information about RAID disk arrays. In
addition, Oracle Database 10g's
Automatic Storage Management (ASM) capabilities can deliver much of
the functionality of a RAID array, such as striping and mirroring,
with a collection of commodity disks. ASM is further described in
Chapter 5.

Keep in mind that newer higher capacity drives may allow you to store
more and more data cheaply, but may not provide the access paths
(e.g. spindles) necessary for demanding multiuser query performance.
Proper disk configuration is a balance of capacity, availability, and
performance considerations that meet your business application needs.

Oracle9i introduced table compression in the database as
a means of decreasing disk storage requirements. Duplicate values in
a data block are eliminated because values that are duplicated are
stored in a symbol table at the beginning of the block, and all
additional occurrences are replaced with a short reference to the
symbol table. Compression ratios vary, but typical reductions
observed in storage required in real implementations is about half
that of uncompressed data.

In addition to reducing disk storage, compressed data can also be
advantageous to query speed when it fits entirely into cache (instead
of requiring disk access). Loading data as compressed can take about
twice as long, so many companies load new data uncompressed in a
partition, then compress that partitioned data as it ages.