Choosing and Configuring Hardware - Professional SQL Server 2005 Performance Tuning - page 155

the drive. The same also applies to Fibre Channel (FC) drives, which are just SCSI disks with an FC
interface instead of SAS.

Due to the huge capacity SATA disks available today at very low prices, you might start to see them
appearing in organizations as storage for infrequently accessed data like archives. As I write this the
largest SAS disk I can find to buy is 300 GB, and the largest SATA disk is 1 TB and costs 80 percent less.
If you can find the right use for them, SATA disks can bring substantial cost savings.

Additionally, a new disk interface card that supports SATA is emerging on the market, that claims a
sustained transfer rate of 1.2 GB/s. For smaller organizations, using SATA is definitely a possibility,
especially if on a budget.

Best Practice
Use SAS for most if not of all of your storage requirements if you can afford it and
consider SATA for infrequently accessed data like archives if you're on a budget.

Direct-Attached Storage
Direct-attached storage (DAS) simply means that one server is connected directly to a disk array without
a storage network in between. A few years ago it was the predominant choice for storage and still in many
cases yields the best performance because it's a direct relationship. The problem with DAS becomes
apparent in enterprise organizations where they have dozens or hundreds of servers. Each server has
different disk space requirements but you're limited to the disk size in what you can provision. The
result is that TBs of storage space throughout the organization sit unused and wasted.

Storage Area Network
A storage area network (SAN) provides a common storage pool for an organization providing highly
robust and dynamic storage without the wasted disk space you see in an enterprise deployment of DAS.
A common mistake is to refer to the box that holds all the disks as the SAN; that is just the disk array.
The SAN is the entire network that supports the storage platform. Figure 6-5 shows a simple SAN to help
understand some of the terminology.

The host bus adapter (HBA) is the storage equivalent of an Ethernet network card.

❑

The switches operate in much the same way as network switches routing traffic.

❑

A fabric refers to a collection of switches that together provide the link between the server and the
disk array.

❑

There is a common perception that SANs give greater performance than DAS and although that can be
true, it isn't the key driver for moving to a SAN. Storage consolidation is the value proposition but of
course it would be useless without a good performance baseline. To help offset the overhead inevitable
with the complexity and concurrency requirements of a SAN they can be installed with a very large
cache. I worked with a banking customer in London on one of their trading platforms that used a large
SAN. They had two 32 GB caches installed, which meant that I/O requests were never served from disk
99 percent of the time. The I/O performance was incredible.

Having said that it's worth clarifying that the example above came at a huge price (millions of dollars)
both in the cost of the hardware and the specialist skills required to design and maintain the solution.