20
System overview
2
System overview
Enclosure configurations
The storage system supports two controller enclosure configurations.
•
2U (rack space) controller enclosure – see
holds up to 12 low profile
(1-inch high) 3.5"
form factor disk drive modules in a horizontal orientation.
•
2U (rack space) controller enclosure – see
: holds up to 24 low profile
(5/8 inch high) 2.5" form factor disk drive modules in a vertical orientation.
The same 2U chassis form factors are used for supported expansion enclosures; albeit with different I/O modules
(IOMs). Each individual disk drive is hot pluggable and replaceable on site. The 5U84 enclosure is also supported
as an expansion enclosure, and uses the same expansion IOMs used in 2U expansion enclosures.
The 5U (rack space) expansion enclosure (see
holds up to 84 low profile (1-inch high) 3.5" form
factor disk drive modules in a vertical orientation within the disk drawer. Two vertically-stacked drawers each hold
42 disks. If used, 2.5" disks require 3.5" adapters.
NOTE:
Throughout this guide—and the management interfaces documents used with this guide—I/O module
(IOM) is a general term denoting either a controller module (RAID canister) or an expansion module (expansion
canister).
The enclosure configurations—including chassis and CRUs—are described on the following pages.
Cache
To enable faster data access from disk storage, the following types of caching are performed:
•
Write-back 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. Do not remove the
card; it is used for cache recovery only.
NOTE:
In dual-controller configurations featuring one healthy partner controller, there is no need to transport
failed controller cache to a replacement controller because the cache is duplicated between the controllers,
provided that volume cache is set to standard on all volumes in the pool owned by the failed controller.
Supercapacitor pack
To protect controller module cache in case of power failure, each controller enclosure model is equipped with
supercapacitor technology—in conjunction with CompactFlash memory—built into each controller module to
provide extended cache memory backup time. The supercapacitor pack provides energy for backing up unwritten
