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Chapter 7: Technology Background
191
Stripe Size
Stripe Size, also called “Stripe Block Size”, refers to the size of the data blocks
written to, and read from, the physical drives. Stripe Size is specified when you
create a disk array. In order to change the Stripe Size of an existing disk array,
you must delete the disk array and create a new one. You can select Stripe Size
directly when you use the Advanced function to create a disk array. If you use the
Express function to create a disk array, WebPAM PRO selects the Stripe Size
when you choose an Application Type.
The available Stripe Sizes are 8, 16, 32, 64, 128, 256, 512 KB, and 1 MB. 64 KB
is the default. There are two issues to consider when selecting the Stripe Size.
First, you should choose a Stripe Size equal to, or smaller than, the smallest
cache buffer found on any physical drive in the disk array. Selecting a larger
value slows read/write performance because physical drives with smaller cache
buffers need more time for multiple accesses to fill their buffers.
Second, if your data retrieval consists of fixed data blocks, such as with some
database or video applications, then you should choose that size as your Stripe
Size.
If you do not know the cache buffer or fixed data block sizes, Promise suggests
you select 64 KB as your Stripe Size. Generally speaking, email, POS, and
webservers prefer smaller stripe sizes. Video and database applications prefer
larger stripe sizes.
Sector Size
A sector is the smallest addressable area on a physical disk drive. Sector Size
refers to the size of sector measured by the number of bytes of data it can hold.
The most common sector size is 512 bytes (512 B). Depending on its capacity,
there can be up to 4,000,000,000 sectors on a single disk drive. The number of
sectors is limited by the addressing method of the computer's operating system.
Sector size is important for two reasons. First, data is written to a disk drive in
units called blocks. If a data block is smaller than the disk drive's sector size, part
of the storage space in the sector goes unused. This is why a smaller sector size
results in a more efficient use of a disk drive’s capacity.
Second, while a logical drive can be expanded by adding more physical disk
drives, the number of addresses cannot be increased above 4,000,000,000, as
noted above. For example, your logical drive has 512 byte sectors, multiplied by
4 billion addresses. The result is 2,048,000,000,000 bytes or 2 terabytes (TB) of
data storage capacity. If you simply add more disk drives, your operating system
will not recognize the additional capacity and you will not be able to use it.