LSI iBBU01 User Manual page 13

Chapter 1
Introduction
®
LSI provides a family of MegaRAID intelligent battery backup units
(iBBUs) for use with its high-performance MegaRAID 1078-based SAS
RAID controllers. The battery backup units protect the integrity of the
cached data on MegaRAID RAID controllers by providing backup power
if there is a complete AC power failure or a brief power outage. The LSI
MegaRAID iBBUs provide an alternative to using an uninterruptible
power supply (UPS), and a second level of fault tolerance when used in
conjunction with a UPS.
The cache memory available on MegaRAID controllers can improve
overall system performance. Writing data to the controllers cache
memory is much faster than writing it to a storage device. Write
operations appear to complete very quickly at the software application
level. The MegaRAID RAID controller then writes the cached data to the
storage device when system activity is low or when the cache is getting
full. The risk of using write-back cache is that the cached data can be
lost if the AC power fails before it has been written to the storage device.
This risk factor is eliminated when the MegaRAID RAID controller has an
onboard iBBU.
The MegaRAID iBBUs monitor the voltage level of the DRAM modules
on the MegaRAID RAID controllers. If the voltage drops below a
predefined level, the battery backup module switches the memory power
source from the RAID controller to the battery pack attached to the iBBU.
As long as the voltage level stays below the predefined value, the
MegaRAID iBBU provides power for memory. If the voltage level returns
to an acceptable level, the iBBU switches the power source back to the
MegaRAID RAID controller, and all pending writes to storage devices are
completed with no data loss.
LSI offers several battery designs in the MegaRAID 1078-based SAS
RAID controller family, as listed in Table 1.1. This table shows which
Intelligent Battery Backup Units for 1078-based MegaRAID® User Guide 1-1
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