A Battery Backup Unit (BBU) in RAID controllers serves two critical functions during power failures:

// Pseudo-code of BBU operation
void handlePowerFailure() {
    if (powerLost && bsuPresent) {
        activateBatteryPower();
        flushCacheToNonVolatileStorage(); // Step 1: Save cached data
        if (batteryCapacity > threshold) {
            maintainDiskPower(1000ms);     // Step 2: Optional disk power
            completePendingWrites();
            gracefulDiskShutdown();
        }
    }
}

Most enterprise RAID implementations follow this behavior pattern:

• 72-hour cache preservation: Protects unwritten data in cache memory until power returns
• 1-3 second disk power window: Some controllers provide brief power to complete writes

Major RAID controller manufacturers handle this differently:

# LSI MegaRAID BBU configuration example
MegaCli -AdpBbuCmd -GetBbuProperties -aALL
# Shows retention time and cache protection status

# HP Smart Array BBU settings
hpssacli ctrl all show config detail | grep -i battery
# Displays battery-backed cache configuration

Technical constraints prevent universal disk power maintenance:

Battery Backup Units (BBUs) in RAID controllers serve two critical functions during power outages:

1. Preserving cached data in non-volatile memory
2. Enabling controlled write operations to disks

While disks can physically write data quickly, the complete I/O path involves multiple latency-sensitive operations:

// Simplified write path latency breakdown (in microseconds)
1. RAID controller processing: 50-100μs
2. Cache commit: 10-50μs
3. Disk seek time: 2000-9000μs (for HDDs)
4. Physical write: 200-400μs

Controller vendors specify extended BBU durations because:

• Enterprise systems may need time for proper shutdown procedures
• Some environments require data retention during extended outages
• Allows for orderly recovery when power returns

Here's how you might implement BBU status checks in a storage application:

# Python example using MegaCLI for BBU status
import subprocess

def check_bbu_status():
    try:
        output = subprocess.check_output(
            ["MegaCli", "-AdpBbuCmd", "-GetBbuStatus", "-aALL"],
            stderr=subprocess.STDOUT
        )
        return "BBU status: Optimal" in str(output)
    except subprocess.CalledProcessError as e:
        print(f"BBU check failed: {e.output}")
        return False

def set_write_policy(has_bbu):
    if has_bbu:
        # Use WriteBack caching for performance
        subprocess.run(["MegaCli", "-LDSetProp", "WB", "-LALL", "-aALL"])
    else:
        # Fall back to WriteThrough for safety
        subprocess.run(["MegaCli", "-LDSetProp", "WT", "-LALL", "-aALL"])

Modern systems often implement hybrid approaches:

For optimal data safety:

# Recommended MegaCLI settings for BBU-protected arrays
MegaCli -AdpBbuCmd -SetBbuProperties -learnCycle 7 days -aALL
MegaCli -AdpBbuCmd -SetBbuProperties -autoLearnPeriod 28 days -aALL
MegaCli -AdpBbuCmd -SetBbuProperties -enableBbu -aALL