Cache Module Benefits; Read-Ahead Caching; Write-Back Caching - HP 273914-B21 - Smart Array 6404/256 RAID Controller Technology Brief

Cache module benefits

With advanced read-ahead and write-back caching capabilities, the Smart Array controller cache
module produces significant performance improvements for I/O operations.

Read-ahead caching

The HP Smart Array controller family uses an intelligent read-ahead algorithm that can anticipate data
needs and reduce wait time. It can detect sequential read activity on single or multiple I/O threads
and predict when sequential read requests will follow. The algorithm then "reads ahead," or pre-
fetches data, from the disk drives before the data is actually requested. When the read request
occurs, the controller retrieves the data from high-speed cache memory in microseconds rather than
from the disk drive in milliseconds.
This adaptive read-ahead scheme provides excellent performance for sequential small block read
requests. At the same time, the system is not penalized by random read patterns because read-ahead
functionality is disabled when non-sequential read activity is detected. Thus, Smart Array controller
technology overcomes the problem with some array controllers in the industry today that use fixed
read-ahead schemes to increase sequential read performance but degrade random read
performance.

Write-back caching

HP Smart Array controllers also use a caching scheme that allows host applications to continue
without waiting for write operations to complete to the disk. This technique is also called posted-
writes, or write-back caching. Without this type of caching, the controller must wait until write data is
actually written to disk before returning completion status to the operating system. With write-back
caching, the controller can "post" write data to high-speed cache memory and immediately return
"back" completion status to the operating system. The write operation is completed in microseconds
rather than milliseconds.
Once write data is located in the cache, subsequent reads to the same disk location will be sourced
from the cache. Subsequent writes to the same disk location will replace the data held in cache. This
technique is called a "read cache hit." This feature improves bandwidth and latency for applications
that frequently write and read the same area of the disk.
After write data is located in the cache, the controller finds opportunities to combine adjacent write
transfers into a larger transfer that can efficiently move to the disk drive. This technique is called "write
coalescing." Disk drives achieve higher throughput when processing many small transfers rather than
fewer large transfers. This feature improves write bandwidth whenever data is sequentially written to
the controller using write command sizes smaller than the stripe size of the logical drive.
With write data located in the cache, logical drives in RAID 5 and RAID 6 configurations achieve
higher write performance by combining adjacent write requests to form a full stripe of data. This
technique is called "full-stripe writes." Write operation for RAID 5 and RAID 6 normally requires extra
disk reads to compute the data for the parity drives. However, if all the data required for a full stripe
is available in the cache, the controller does not require the extra disk reads. This feature improves
write bandwidth whenever data is sequentially written to a logical drive in a RAID 5 or RAID 6
configuration.
Data in the controller's write cache is written to disk later, at an optimal time for the controller. While
the data is in cache, it is protected against memory chip failure by error checking and correction
(ECC) DRAM technology, and against system power loss by the integrated battery backup
mechanism. Smart Array controllers avoid the risk of data loss by ensuring that the battery backup is
present before enabling write-back cache. Disk drives provide an option to enable write-caching that
is not battery backed. HP advises against enabling disk drive write cache because a power or
equipment outage could result in data loss.
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