Disk Configurations - US Robotics USR8700 User Manual

Disk Configurations

Your storage system supports the following types of disk configurations:
• Linear—A linear configuration is similar to using multiple hard disks in a regular computer. Each disk is
an independent entity, and the data on it is self-contained. You can add or remove the disks without
affecting the other disks. All the available disk space is used for data.
If your storage system has only one disk, you must use a linear configuration. However, you can use a
linear configuration for two, three, or four disks as well.
• RAID 0—Instead of writing all the data to one disk in a linear fashion, some bytes are written to one
disk, and other bytes are written to another. Performance is faster because reading and writing activi-
ties can occur on multiple disks simultaneously. All the available disk space is used for data.
For RAID 0, your storage system must have at least two disks. However, you can use RAID 0 with
three or four disks as well, and the disks can be any size.
• RAID 1—In this configuration, all the data written to one disk is duplicated on the other disk. This offers
greater data protection since if one disk fails, all your data is still intact on the other disk. However,
using RAID 1 means only half your available disk space is used for data; the other half is used for a
duplicate (mirror) of that data.
You can use RAID 1 only if your storage system has only two disks. If the disks are not the same size,
the smaller of the two disks is used for data, and the larger of the two disks is used as the mirror.
If one disk fails, the other disk continues to make its data available.
• RAID 5—Like RAID 0, RAID 5 offers increased performance by distributing the data across multiple
disks. But unlike RAID 0, RAID 5 also offers data protection. If your storage system has three disks of
equal size, two thirds of each disk are used for data, and the remaining third contains the parity infor-
mation needed to reconstruct either of the other two. In this way, if any of the three disks fails, it can be
reconstructed when a new disk is installed in the storage system.
If your storage system has four disks of equal size, three fourths of each disk are used for data, and
the remaining fourth contains the parity information needed to reconstruct any of the other three. If any
of the four disks fails, it can be reconstructed when a new disk is installed.
You can use RAID 5 only if your storage system has at least three disks. If the disks are not the same
size, the smallest of the disks determines how much disk space is available for data. For example, if
one disk is 300 GB, one is 400 GB, and one is 500 GB, only 300 GB from each disk can be used. Two
thirds of each disk (200 GB) is used for storage space, and the remaining third is used for parity infor-
mation. As a result, for all three disks, only 600 GB of disk space would be available for data.
• RAID 5 + spare—In this configuration, three of the disks use RAID 5, and the fourth is empty. If any of
the three disks fails, it is immediately rebuilt using the fourth spare disk. As a result, you can remove
the failed disk and still have the ongoing fast performance and data protection offered by RAID 5.
When the failed disk is repaired or replaced and re-installed into the storage system, it automatically
becomes the spare for the other functioning three.
You can use RAID 5 + spare only if your storage system has four disks. If the disks are not the same
size, the smallest of the disks determines how much disk space is available for data, similar to RAID 5.
Serial ATA 4-Drive NAS
User Guide
Disk Configurations - 125