26 Components
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 into the cache memory in the
controller module rather than directly to the disks. 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 disks.
• Read-ahead caching. The controller detects sequential data access, reads ahead into the next
sequence of data—based upon settings—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.
TIP: See the Storage Management Guide for more information about cache options and settings.
CompactFlash
During a power loss or controller failure, data stored in cache is saved off to non-volatile memory
(CompactFlash). The data is restored to cache, and 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 memory card is located at the midplane-facing end of the controller module as shown
below. Do not remove the card; it is used for cache recovery only.
Figure 17 CompactFlash memory card
In single-controller configurations (supported with 2U24/2U12 chassis), 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 on page 89 for more information).
Do not remove
Used for cache recovery only
Controller module pictorial
CompactFlash memory card
(Midplane-facing rear view)
Host interface
USB - Type A
(shown covered)
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