Cisco NSS2000 - Gigabit Storage System Chassis Administration Manual page 51

Configuring your Storage
Choosing a RAID Array Level
Cisco Small Business NSS2000 Series Administration Guide
RAID, referred to as RAID levels. Some RAID levels increase the performance of
the array, some increase the reliability, and others do a mixture of both. The NSS
supports the following RAID levels: 0 and 1. The NSS also supports JBOD (Just a
Bunch of Disks), which is technically not a RAID level.
The following variables are used in the formulas used to calculate the total
capacity of each RAID level:
•
m – capacity of the smallest disk in the array
•
n – number of disks in the array
Stripe (RAID0): RAID0 stripes the data written to the array across the component
disks. The data is broken into chunks and each chunk is written to a different disk.
Reads and writes to each disk occur in parallel, speeding up the total read and
write performance of the array.
•
Minimum Number of Disks: 2
•
Total capacity: m * n
•
Advantages: Increased read and write performance.
•
Disadvantages: Decreased reliability. A failure of any component disk in the
array causes the entire array to fail.
Mirror (RAID1): RAID1 writes the same data to each disk in the array. The disks are
referred to as "mirrors" because each one mirrors the data stored on the others. As
long as one disk in the array is intact, all data can be read back from the array. If a
disk fails in the array and is then replaced, the array must copy the entire contents
of a good disk to the new disk. This process is referred to as "resyncing". During a
resync, the array continues to be available for reads and writes. When an array
contains a failed disk, it is said to be operating in "degraded" mode. This reflects
the decreased performance and reliability of the array when it is missing disks.
•
Minimum Number of Disks: 2
•
Total capacity: m
•
Advantages: Increased reliability. The array can sustain the loss of all but
one disk without any data loss. Each mirror disk added to the array
increases the reliability (for example, a two-disk RAID1 is half as likely to fail,
a three-disk RAID1 is one-third as likely to fail, and so on). Increased read
performance.
•
Disadvantages: Decreased aggregate storage capacity (each mirror disk
does not contribute to the total capacity of the array). Decreased write
performance. I/O intensive when resyncing mirrors.
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