O-AFA : Order Preserving All Flash Array

In this work, we present a new order-preserving mechanism for software RAID, called Order-Preserving All Flash Array, O-AFA. O-AFA is designed for barrier-compliant devices in a RAID system. Our contributions are three-fold. First, Barrier Stripe is employed to preserve the storage order in individual storage devices in the storage array. The cache barrier command stripe is dispatched to a flash array. Second, Interstripe epoch management is employed to follow the ordering constraint imposed by the filesystem. When the Linux MD dispatches the current stripe, it receives the information on whether there is a remaining write request among undispatched stripes. Based on this information, the Linux MD converts the current stripe to dispatch to the Barrier Stripe or not. Third, we use a shadow-page aware dispatch mechanism to replace the transfer overhead with the dispatch overhead. It brings the performance gain for the IO handling by 19%. The new order-preserving mechanism of O-AFA brings 75% performance gain for varmail and 77% for OLTP-Insert on MySQL.