Dell
DELL PERC H700 and H800 Technical Guide 17
Physical disk rebuild using hot spares
Enclosure affinity
Parity generation and checking (for RAID 5, 50, 6, and 60 only)
Battery backup of controller cache to protect data
Detection of batteries with low charge after boot up
4.5.1 Non-Volatile Cache Dell PERC controllers with non-volatile (NV) cache use the standard battery as contained in the Dell
PERC controllers with a battery back-up unit (BBU). The difference is in battery implementation:
The battery in the BBU offering retains the data in cache in the event of a power cycle for a
guaranteed period of 24 hours (typically up to 72 hours).
The battery in the NV cache offering will transfer the data from cache to flash in the event of
a power cycle, where the data will be retained for up to ten years.
4.5.2 Automatic Replace Member with Predicted Failure A Replace Member operation can occur when there is a SMART predictive failure reporting on a drive
in a virtual disk. The automatic Replace Member is initiated when the first SMART error occurs on a
physical disk that is part of a virtual disk. The target drive needs to be a hot spare that qualifies as a
rebuild drive. The physical disk with the SMART error is marked as failed only after the successful
completion of the Replace Member. This avoids putting the array in degraded status. If an automatic
Replace Member occurs using a source drive that was originally a hot spare (that was used in a
rebuild), and a new drive added for the Replace Member operation as the target drive, the hot spare
reverts to the hot spare state after a successful Replace Member operation. To enable the automatic
Replace Member, use the Dell OpenManage storage management application.
4.5.3 Redundant Path with Load Balancing Support The PERC H800 adapter can detect and use redundant paths to drives contained in enclosures. This
provides the ability to connect two SAS cables between a controller and an enclosure for path
redundancy. The controller is able to tolerate the failure of a cable or Enclosure Management
Module (EMM) by using the remaining path. When redundant paths exist, the controller automatically
balances I/O load through both paths to each disk drive. This load balancing feature increases
throughput to each drive and is automatically turned on when redundant paths are detected. To set
up your hardware to support redundant paths, see the Setting up Redundant Path Support on the
PERC H800 Adapter section in the PERC H700 and PERC H800 User’s Guide
(support.dell.com/manuals).
4.5.4 Failed Physical Disk Detection The controller automatically detects and rebuilds failed physical disks when you place a new drive in
the slot where the failed drive resided or when an applicable hot spare is present. Automatic
rebuilds can be performed transparently with hot spares. If you have configured hot spares, the
controllers automatically try to use them to rebuild failed physical disks.
4.5.5 Using Replace Member and Revertible Hot Spares The Replace Member functionality allows a previously commissioned hot spare to be reverted back to
a usable hot spare. When a drive failure occurs within a virtual disk, an assigned hot spare
(dedicated or global) is commissioned and begins rebuilding until the virtual disk is optimal. After the
failed drive is replaced (in the same slot) and the rebuild to the hot spare is complete, the controller
automatically starts to copy data from the commissioned hot spare to the newly inserted drive. After
the data is copied, the new drive is part of the virtual disk and the hot spare is reverted back to