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Utilising
Cache Memory
Many
RAID controllers incorporate a cache memory to speed up the servicing
of I/O requests. Usually this cache takes the form of standard SIMM
or DIMM sockets in which either special or standard memory chips
can be installed. Maximum memory varies from controller to controller
but may be as high as 128MB, 256MB or more.
Remember that this memory simply serves as a read/write buffer -
no system functions, large operating system, or applications to
run. If you imagine a simjple bare hard drive carrying around 512K
or so of cache, suddenly making 128MB of memory available just for
I/O requests and parity calculations looks impressive.
Optimisation
for Sequential or Random I/O
Some controllers allow additional parameters beyond simply setting
memory speed and RAID Level with the ability to be configured for
either Random or Sequential read/write. Sequential I/O is ideal
for environments where large files or data blocks are used constantly,
for example when video or image orientated applications. Sequential
I/O will set the controller to efficiently work with the large-block,
sequential files and write/read to the array in large sequential
stripes also known as Block or Chunk size.
When set to use Random I/O the controller will be efficiently deal
with small-block, random access files more suited to high transaction
processing environments such as database servers.
Related
topics:
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