FIBRE-to-SAS/SATA RAID SUBSYSTEM
User Manual
69
6.1.2 RAID Caching
The Raid controller comes with a cache memory, 512MB, 1GB or 2GB cache size.
The cache memory is used for both reading and writing data. There are several
kinds of caching implemented by the system firmware:
Read Cached Data
Streaming Read Data
Write Cached Data
Disk Caching
Disable Sync Cache Command
6.1.2.1 Read Cache Data
The system records the number of read and write commands that occur in the
same physical stripe. When the number of commands exceeds a preset threshold,
the system will allocate storage space to cache this data. This increases
performance because subsequent read operations can be serviced from memory
instead of reading from the disk.
6.1.2.2 Streaming Read Data
The system records the starting block and block size of each command received.
When the start block of a new command matches the last block of a previous
command the system detects that there is a stream of commands in progress and
pre-fetches data up to 16 times the size of the original command. This increases
performance by lowering the latency of subsequent commands in the stream.
6.1.2.3 Write Cache Data
When write caching is enabled, performance is increased in two ways. Firstly it
lowers command latency to the host. Secondly, by coalescing contiguous write data
into larger segments before it is written to disk, seeks are reduced and bandwidths
increased.
If you do not have a BBU or UPS installed, you must carefully consider whether you
want to implement Write Caching. If power is lost to the system with Write Cache
enabled you will loose the data stored in the Cache memory. Therefore, you must
decide whether the advantages of Write Caching exceed the risks involved.
NOTE: If write caching is enabled, and the system is not protected
by a built-in BBU or an external UPS, data may be lost if there is an
AC power failure or the AC power is turned off prior to the cache
being written to disk.
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