Dell PowerEdge RAID Controller 5i - Manual (Page 21)

PERC 5 Features
19
Battery Management
The Transportable Battery Backup Unit (TBBU) is a cache memory module with an integrated battery
pack that enables you to transport the cache module with the battery into a new controller. The TBBU
protects the integrity of the cached data on the PERC 5/E Adapter by providing backup power during
a power outage.
The Battery Backup Unit (BBU) is a battery pack that protects the integrity of the cached data on the
PERC 5/i Adapter and PERC 5/i Integrated controllers by providing backup power during a power
outage. Unlike the TBBU, the BBU is not directly attached to the cache memory module and is therefore
not transportable with the controller.
The TBBU and BBU offer an inexpensive way to protect the data on the memory module. The lithium
battery provides a way to store more power in a smaller form factor than previous batteries.
See "Transferring a TBBU Between Controllers" on page 30 for detailed procedures about handling
controller cache in case of a controller failure.
Introduction to Write Cache Policy
The cache controller writes a block of data to cache memory, which is much faster than writing to
the physical disk. The cache controller sends an acknowledgement of data transfer completion to the
host system.
Write-Back versus Write-Through
In write-through caching, the controller sends a data transfer completion signal to the host system when
the disk subsystem has received all the data in a transaction. The controller then writes the cached data
to the storage device when system activity is low or when the write buffer approaches capacity.
In write-back caching, the controller sends a data transfer completion signal to the host when the
controller cache has received all the data in a transaction. The cached data is not written to the
storage device.
The risk of using write-back cache is that the cached data can be lost if there is a power failure before it is
written to the storage device. This risk is eliminated by using a battery backup unit on selected PERC 5
controllers. Refer to Table 2-1 for information on which controllers support a battery backup unit.
Write-back caching has a performance advantage over write-through caching.
NOTE:
The default cache setting is write-back caching.
NOTE:
Certain data patterns and configurations perform better in a write-through cache policy.
How Firmware Manages Cache
Firmware manages cache based on the condition of the battery. Learn Cycle is a battery calibration
operation performed by the controller periodically (approximately every 3 months) to determine the
condition of the battery.
Dell_PERC5_UG.book Page 19 Tuesday, February 13, 2007 6:02 PM