SQL Server Backup and Restore - Simple Talk

Chapter 2: Planning, Storage and Documentation
For the added cost and complexity of SAN storage, you have access to storage space far
in excess of what a traditional DAS system could offer. This space is easily expandable
(up to the SAN limit) simply by adding more disk array enclosures (DAEs), and doesn't
take up any room in the physical server. Multiple database servers can share a single SAN,
and most SANs offer many additional features (multiple RAID configurations, dynamic
snapshots, and so on).
SAN storage is typically provided over a fiber optic network that is separated from
your other network traffic in order to minimize any network performance or latency
issues; you don't have to worry about any other type of network activity interfering
with your disks.
RAID 5 vs. RAID 10
There is some debate over which of these two High Availability RAID configurations is
best for use when storing a relational database. The main point of contention concerns
the write penalty of recalculating the parity data after a write in a RAID 5 disk array.
This was a much larger deal for RAID disks a few years ago than it is in most of today's
implementations. The parity recalculation is no longer an inline operation and is done by
the controller. This means that, instead of the parity recalculation happening before you
can continue with I/O operations, the controller takes care of this work separately and no
longer holds up the I/O queue. You do still see some overhead when performing certain
types of write, but for the most part this drawback has been obfuscated by improved
hardware design.
Nevertheless, my general advice, where a choice has to be made, is to go for a RAID 10
configuration for a database that is expected to be subject to "heavy writes" and RAID 5
for read-heavy databases. However, in a lot of cases, the performance gain you will see
from choosing one of these RAID configurations over the other will be relatively small.
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