A multi-tier RAID storage system with RAID1 and RAID5

Redundant Arrays of Inexpensive Disks (RAID) is a popular technique used to improve the reliability and performance of secondary storage. Of various levels of RAID discussed, RAID1 and RAID5 have become more popular. Mirroring or RAID1 maintains multiple copies of the data, generally provides best performance and is easier to configure. Rotating parity scheme or RAID5 is the least expensive RAID scheme with good large update performance. It suffers from poor small update performance and performance drops sharply when a diskfails and the array enters degraded mode. Configuring RAID5 is more involved. This paper presents the design and implementation of a host-based driver for a multi-tier RAID storage system, currently with 2 tiers: a small RAID1 tier and a larger RAID5 tier. Based on access patterns, the driver automatically migrates frequently accessed data to RAID1 while demoting not so frequently accessed data to RAID5. The prototype provides reliable persistence semantics for data migration between the tiers using ordered updates. Mechanisms are separated from policies through an API so that any desired policy can be implemented in trusted user processes. Finally, we present comparison of the performance of our system with comparable systems using striping and RAID5