SMR Disks for Mass Storage Systems

Disk drives have seen a dramatic increase in storage density over the last five decades, but to continue the growth seems difficult because of physical limitations. One promising approach to overcome the impending limit is shingled magnetic recording (SMR). It can be adopted while utilizing essentially the same physical recording mechanisms currently in use. Because of its manner of writing, a shingled write disk is unable to update a written track without overwriting neighboring tracks, potentially requiring the rewrite of all the tracks to the end of a "band" (where the end of a band is an area left unwritten to allow for a non-overlapped final track). Random reads are still possible on such devices, but the handling of writes becomes critical. We describe our recent experiences evaluating the behavior of shingled disks when used in an array configuration. We propose a novel use of SMR disks in an array with regular disks which we call RAID 4S that has the potential to improve the performance of storage systems. Our evaluation shows that RAID 4S clearly demonstrates the feasibility of using SMR disks in a RAID 4 array as it can out perform the use of SMR disks in standard RAID 4 with update in place by up to 89%.