Information Technology Reference
In-Depth Information
Think of it this way: Disks are mechanical, spinning, rust-colored surfaces. The read/write
heads are l ying microns above the surface while reading minute magnetic i eld variations and
writing data by affecting surface areas also only microns in size.
The “Magic” of Disk Drive Technology
It really is a technological miracle that magnetic disks work at all. What a disk does all day long is
analogous to a pilot fl ying a 747 at 600 miles per hour 6 inches off the ground and reading pages
in a book while doing it!
In spite of the technological wonder of hard disks, they have unbelievable reliability statis-
tics. But they do fail—and fail predictably, unlike other elements of a system. RAID schemes
address this by leveraging multiple disks together and using copies of data to support I/O until
the drive can be replaced and the RAID protection can be rebuilt. Each RAID coni guration
tends to have different performance characteristics and different capacity overhead impact.
We recommend that you view RAID choices as a signii cant factor in your design. Most
arrays layer additional constructs on top of the basic RAID protection. (These constructs have
many different names, but common ones are
metas
,
virtual
pools
,
aggregates
, and
volumes
.)
Remember, all the RAID protection in the world won't protect you from an outage if the con-
nectivity to your host is lost, if you don't monitor and replace failed drives and allocate drives
as hot spares to automatically replace failed drives, or if the entire array is lost. It's for these
reasons that it's important to design the storage network properly, to coni gure hot spares as
advised by the storage vendor, and to monitor for and replace failed elements. Always consider
a disaster-recovery plan and remote replication to protect from complete array failure.
Let's examine the RAID choices:
RAID 0
This RAID level offers no redundancy and no protection against drive failure (see
Figure 6.2). In fact, it has a
higher
aggregate risk than a single disk because any single disk
failing affects the whole RAID group. Data is spread across all the disks in the RAID group,
which is often called a
stripe
. Although it delivers fast performance, this is the only RAID
type that is usually not appropriate for any production vSphere use because of the availabil-
ity proi le.
Figure 6.2
In a R AID 0 con-
fi guration, the data
is striped across
all the disks in the
R AID set, providing
very good perfor-
mance but very
poor availability.
Data: 1011
Write: 1
Write: 0
Write: 1
Write: 1
RAID 1, 1+0, 0+1
These mirrored RAID levels offer high degrees of protection but at the
cost of 50 percent loss of usable capacity (see Figure 6.3). This is versus the raw aggregate
capacity of the sum of the capacity of the drives. RAID 1 simply writes every I/O to two
