During a scheduled maintenance window, a technician replaces a failed 1 TB 10 000-RPM SAS drive in a hardware RAID 5 array that originally contained three identical drives. Because no matching SAS disks are on-hand, the technician installs a 1 TB 5400-RPM consumer-grade SATA drive instead. The array rebuilds successfully, yet users immediately notice that file writes and large transfers are much slower than before the swap. Which of the following is the MOST likely root cause of the degraded performance?
The RAID controller's battery-backed cache has failed, forcing all writes to bypass the cache.
The array now contains mismatched drives, so the controller must operate at the speed of the slowest disk.
Controller firmware is several versions behind, resulting in inefficient write algorithms.
The logical volume is nearly full and heavy fragmentation is reducing disk throughput.
RAID controllers complete each stripe write only after all member disks finish their individual I/O. When one member is slower or uses a different interface, the entire array is throttled to that drive's speed, so overall throughput drops even though the rebuild succeeded. In this scenario the low-RPM SATA disk is dramatically slower than the remaining 10 000-RPM SAS disks, so the set performs at the slower drive's rate.
A failed cache battery (distractor) can force write-through mode and hurt performance, but there is no evidence of cache warnings and the slowdown coincides with the mixed-drive replacement. Fragmentation or low free space would affect only heavily used logical volumes, not begin immediately after a single-disk swap. Out-of-date firmware might reduce performance in some cases, yet the abrupt change right after inserting a slower drive points to the mismatch rather than firmware age.