HP MSA 2040 SAN Storage - Manual (Page 17)

Cache 17
SFF drive enclosure
MSA 2040 controllers support the D2700 6 Gb drive enclosure for adding storage. For information about
this product, visit http://www.hp.com/support . Pictorial representations of this drive enclosure are also
provided in the MSA 2040 Quick Start Instructions and MSA 2040 Cable Configuration Guide.
Cache
To enable faster data access from disk storage, the following types of caching are performed:
• Write-back or write-through caching. The controller writes user data in the cache memory on the
module rather than directly to the drives. Later, when the storage system is either idle or aging—and
continuing to receive new I/O data—the controller writes the data to the drive array.
• Read-ahead caching. The controller detects sequential array access, reads ahead into the next
sequence of data, and stores the data in the read-ahead cache. Then, if the next read access is for
cached data, the controller immediately loads the data into the system memory, avoiding the latency of
a disk access.
NOTE: See HP MSA 2040 SMU Reference Guide for more information about volume cache options.
Transportable CompactFlash
During a power loss or array controller failure, data stored in cache is saved off to non-volatile memory
(CompactFlash). The data is then written to disk after the issue is corrected. To protect against writing
incomplete data to disk, the image stored on the CompactFlash is verified before committing to disk.
The CompactFlash card is located at the midplane-facing end of the controller module as shown below.
Figure 8 MSA 2040 CompactFlash card
In single-controller configurations, if the controller has failed or does not start, and the Cache Status LED is
on or blinking, the CompactFlash will need to be transported to a replacement controller to recover data
not flushed to disk (see "Controller failure in a single-controller configuration" (page 57) for more
information).
Do not remove
Used for cache recovery only
Controller module pictorial
CompactFlash card
(Midplane-facing rear view)