Many sysadmins wonder why RAID controllers need battery-backed cache (BBU) when servers are connected to UPS systems. While UPS protects against power failures, the battery on RAID controllers serves a different purpose: it ensures data consistency during unexpected crashes, not just power loss.

Consider this scenario: Your server experiences a kernel panic or hardware failure while write-back cache contains unwritten data. The UPS remains powered, but the system isn't functioning to flush cache to disk. This is where BBU becomes critical.

// Example of a kernel panic scenario
void faulty_driver() {
    int* ptr = NULL;
    *ptr = 42;  // Kernel panic occurs here
    raid_controller_flush_cache();  // Never reached
}

Forcing write-back mode without BBU introduces several risks:

• Data corruption during system crashes
• Incomplete transactions in database systems
• Filesystem metadata inconsistencies

Testing shows significant performance differences:

For systems with UPS, consider this balanced approach:

# MegaCLI configuration example
./MegaCli -LDSetProp WB -LAll -aAll  # Enable write-back
./MegaCli -LDSetProp -DisableBatteryWarning -LAll -aAll  # Keep warnings active
./MegaCli -AdpSetProp -EnableJBOD -aAll  # Additional protection

Implement these checks in your monitoring system:

#!/bin/bash
# Check BBU status
bbu_status=$(storcli /c0 show all | grep "BBU Status")
if [[ $bbu_status != *"Optimal"* ]]; then
    echo "CRITICAL: RAID controller BBU status $bbu_status"
    exit 2
fi

Large enterprises often combine both solutions:

• UPS for clean shutdowns during prolonged outages
• BBU for microsecond-level protection during crashes
• Replicated storage for true high availability

While your reasoning about UPS protection is technically correct, enterprise storage systems implement battery-backed cache (BBU) as an independent failsafe mechanism rather than redundant protection. Consider these scenarios where BBU remains critical even with UPS:

// Pseudocode example of storage controller behavior
if (power_failure_detected) {
    if (UPS_available && shutdown_triggered) {
        flush_cache_to_disk(); // Normal UPS shutdown procedure
    } else if (BBU_present) {
        maintain_cache_in_volatile_memory(); // BBU takes over
        write_back_later();
    } else {
        force_write_through_mode(); // Performance impact
    }
}

In my experience with Dell PERC and HP Smart Array controllers, these situations justify separate BBU protection:

• UPS communication failures: iLO/iDRAC may fail to receive UPS shutdown signals
• Graceful shutdown timeout: Large caches may need more time than UPS runtime provides
• Host OS crashes: Kernel panics bypass normal shutdown procedures

The write-back mode performance benefit is substantial (2-4x throughput in benchmarks), but consider these metrics before forcing it:

For Linux systems using megacli, here's how to verify BBU status:

# Check battery health
megacli -AdpBbuCmd -GetBbuStatus -aALL

# Force write-back if BBU is healthy
megacli -LDSetProp WB -LAll -aAll

# Fallback to write-through if BBU fails
megacli -LDSetProp WT -LAll -aAll

Windows Server administrators should monitor BBU status through PowerShell:

Get-PhysicalDisk | Where-Object {$_.MediaType -eq "SSD"} | 
Get-StorageReliabilityCounter | Select-Object *

Most overlooked is the BBU aging factor. Lithium batteries typically last 3-5 years, and their capacity degrades over time. A "healthy" BBU might not provide sufficient runtime if:

• Cache size has increased since initial deployment
• Ambient temperature exceeds 35°C (reduces battery efficiency)
• Controller firmware hasn't performed recent battery calibration