Enabling TRIM Support in SSD RAIDs

Deploying solid state drives (SSDs) in RAID configurations can leverage the performance of persistent storage systems into a new dimension. However, the Achilles’ heel of SSDs and, therefore, also SSD RAIDs is their write performance, especially for small random requests. To prevent a degradation of the write throughput, it is important to maintain an adequate amount of free flash capacity. This can be ensured by over-provisioning and additionally encouraged by notifying the utilized SSDs of storage space no longer required (e.g., because the occupying file was deleted) using the TRIM command. However, many current hardware and software RAID implementations do not propagate TRIM commands to their member drives at all and especially not in RAID setups using parity information to increase the reliability. This leads to a severe limitation of the intention behind the TRIM command because parity-based RAIDs are part of many persistent storage systems. In this report, we discuss the issues of enabling TRIM support in SSD RAIDs and explain how TRIM can be incorporated into RAID implementations for commonly used RAID levels.