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2 raid implementation – Promise Technology 66 Pro User Manual

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SuperTrak66™ User's Manual

Chapter 6

- 105 -

Largest Storage Capacity Required

In the event that the largest capacity possible is required, one of two possible
configurations are suggested:

1) Spanning - This method effectively takes advantage of the total

capacity of all attached drives. It essentially links all drives together to
form one large drive regardless of the size of any individual drive (i.e. a
14.0GB drive and a 16.8GB drive will yield a 30.8GB storage array).
This method offers no other RAID performance or data redundant
features. It simply offers storage capacity.

2) Striping (RAID 0) - This method may be used with up to four (4)

identical drives and may provide an additional performance gain over
Spanning. If you connect four 16GB drives , you will get a 64GB drive
array.

6.2

RAID Implementation

This section describes the advantages and disadvantages of the various RAID
levels under particular workloads. This information can be used to select a RAID
level appropriate for a system with known performance requirements which do not
match any of the application scenarios in section 5.1.

Striping (RAID 0)

Striping is an effective method of gaining performance, but offers no fault tolerance.
The measurable performance gain from using a striped array may vary depending
on the typical application usage.

For applications which tend to perform random read/writes of varying sizes to the
drive, a large stripe block size is recommended. With a large block size, a higher
percentage of random read/write I/Os will be completed with just a single physical
drive I/O. In such a scenario, four drives striped may provide up to four times the
performance in multi-threaded applications versus a single drive.

For applications that tend to perform sequential/streaming read/writes (such as
working with large database or project files, or editing audio/video), a small stripe
block size is recommended. With a small block size, the extended sequential
read/writes will assure that all physical drives are feeding data simultaneously. In
such a scenario, four drives striped will provide much higher performance than a
single drive. With the same small stripe block size, random multi-threaded
read/writes of small-to-medium sized files may exhibit poorer performance when
compared with a large block size.

Depending on the typical usage of the array, performance gains are directly
affected by the stripe block size. Note that whatever block size you do choose, it
will always be a compromise.