During peak usage, write performance on a virtualization host drops from about 400 MB/s to under 20 MB/s. The 8-disk RAID-6 set attached to a hardware RAID controller shows every physical drive and virtual disk as Optimal, and no rebuilds are occurring. The controller's event log records the entry:
BBU disabled; changing WB virtual disks to WT
Which hardware component must be replaced to restore normal write performance while still protecting cached data if power is lost?
The coin-cell CMOS battery on the system board
The battery-backup unit (super-capacitor) that powers the RAID controller's cache
The redundant AC power supply installed in the chassis
The message indicates the controller has turned off write-back (WB) caching and forced write-through (WT) because it can no longer rely on its battery-backup unit (BBU) to preserve cached data. When the cache battery or super-capacitor fails, RAID firmware automatically disables write-back to prevent possible data loss, and throughput drops sharply. Replacing the RAID controller's cache battery/super-cap restores protected write-back caching and normal I/O performance.
Incorrect answers:
Replacing a SAS disk would not re-enable controller cache or improve overall write speed, because no disk failures or rebuilds are present.
The motherboard's CMOS battery only maintains BIOS settings and does not affect RAID cache policies.
The server's redundant AC power supply feeds the entire chassis; swapping it will not change the controller's battery status or cache mode.
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What is the function of a RAID controller's BBU and why is it important?
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What is the difference between Write-Back (WB) and Write-Through (WT) caching in RAID?
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