Install guide

Asymmetrical RAC topologies
RAC clusters are often configured to be symmetrical; the type of workload presented to the nodes
is similar. In this topology, the servers are also of the same relative computing strength. Typically,
the servers are over-configured by 50% in order for the failover node to handle the work of both
nodes. However, this assumes that the business requirements for degraded operation are
identical to normal operation, which is not always the case.
An alternative configuration is to build asymmetric topologies. In our simple two-node case, one
node might be used to update the database (ETL - Extract, T ransform and Load), and the other
node may be used to run queries. Some nodes could be significantly larger in order to be
dedicated to just one form of processing (e.g., Parallel Queries). Oracle RAC is not universally
transparent to SQL workloads; awareness of when and where writes occur (SQL inserts, updates
and deletes) can dramatically improve scalability, even in the two-node case.
Asymmetrical RAC topologies do have implications for failover, as a set of nodes might be tuned
for queries and now must handle other work on behalf of the failed node. This topology is more
common with higher node counts.
1.3. Storage Considerations
A database does only a couple of things: it reads a lot of data and it writes a lot of data. It produces and
consumes I/O, and with few exceptions most of those I/O operations are in the form of small, random
reads and writes. A well-tuned application (they do exist) will access most of the data in the most
efficient way possible. This means extensive use of indexes and that translates into random IOPS, or
I/Os per second.
Disk drives are physical media and are at the mercy of the law of physics. A disk drive (or spindle) must
deliver as many IOPS as possible to make it a good candidate for database use. This usually means a
high RPM, and support for SCSI. Modern SAS drives (Serial Access SCSI) replaced the SCSI bus with a
cheaper, serial bus. Modern SATA (Serial ATA) replaced the ribbon cable in your PCI with a much
cheaper cable. SAS drives tend to be higher RPM, support something called tagged queuing and usually
have the best IOPS/spindle. However, disk drive technology changes often, so insist on the highest
IOPS/spindle/$; regardless of the technology. It is not possible to buy too many spindles.
The storage layer must absolutely preserve the persistency of the data, so the data is still there when
the lights go out. Be more aware of what hardware actually fails in a typical no single point of failure
configuration. Drives fail, the grid fails, power supplies fail, in that order. Most other components outlast
the lifetime of the deployed cluster.
RAM disks versus Flash RAM cards
Drive technology has not kept up with CPU and memory technology, and much of this has to do
with basic physics. A recent trend is the use of Flash technology in a disk form factor (Solid State
Drives or SSD). The other trend is the use of large Flash RAM cards (connected by 8-16 lanes of
PCI-e) to operate as a coherent, write cache, either in the storage array or somewhere between
you and the physical disks. Both Flash cards and SSDs are very fast, but must be just as
persistent. Since Red Hat Cluster Suite Oracle HA requires shared storage (in either case), the
storage vendor tends to have both options. Either can work well for a given workload, but it is
always the workload adaptability that will determine the success of these technologies (or any
disk technology).
Chapter 1. Overview
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