HP P2000 G3 MSA Array Systems - Manual (Page 21)

HP P2000 G3 iSCSI MSA System User Guide 21
MSA2000 3Gb 3.5" 12-drive enclosure
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 P2000 G3 MSA System SMU Reference Guide for more information about volume cache
options.
Transportable CompactFlash
During a power loss or array controller failure, data stored in cache are saved off to non-volatile memory
(CompactFlash). These data are 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.
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 on page 57 for more
information).
1 Power supplies
2 Power switches (not applicable to newer models)
3 I/O module A
4 SAS In port
5 Service port (used by service personnel only)
6 SAS Out port
7 I/O module B
0 0
1 5 6 2 1 2
7
3
4
Controller Module
CompactFla s h