What is the problem with replacing a 10000rpm SAS drive in a RAID 5 array with a low-latency SSD?

Replacing a 10,000 RPM SAS drive with a low-latency SSD in a RAID 5 array presents challenges primarily due to the mixed environment created by using different types of storage media. When RAID arrays are configured, they typically rely on uniform characteristics across the drives to ensure optimal performance and reliability.

In a RAID 5 configuration, data is striped across multiple disks with parity information for fault tolerance. If you introduce an SSD into the mix alongside SAS drives, the differing performance characteristics, such as latency, throughput, and data access patterns, can significantly degrade the RAID's overall performance. The SSD may operate at a much faster speed compared to the spinning SAS drives, leading to inefficiencies in data handling within the array. This can create bottlenecks because the SSD would be capable of processing requests faster than the SAS drives, resulting in uneven performance that could hinder data retrieval and writing processes.

In this case, it would be advisable to replace all SAS drives in the RAID array with SSDs to maintain consistent performance and reliability across the entire RAID setup. This ensures that all drives operate at matching speeds and latencies, thus optimizing the performance of the RAID array.