Motivation

1. disk 1 disk 2 disk 3 disk 4 load load Soft-State Block Replication RAID-5 Layout drives drives time time gear 1 gear 2 gear 3 Challenges • Tension between energy and performance • Performance • Maximize parallelism • RAIDs with uniform striping • Energy • Minimize powered components load utilization threshold time gear 2 gear 1 PARAID: The Gear-Shifting Power-Aware RAID Charles Weddle - Mathew Oldham - An-I A. Wang - Florida State University RuGang Xu - Peter Reiher – University of California, Los Angeles Geoffrey H. Kuenning – Harvey Mudd College Abstract:The energy consumption of disks is a growing concern. It now contributes 27% to the operating costs for data centers. Unfortunately, existing RAIDs are not designed to conserve energy. This is due to uniform striping in which even light loads generate activities on all drives. Power-Aware RAID (PARAID) introduces a skewed striping pattern that can be gear shifted to a minimum striping level so the performance matches the load. Unused disks are powered down. Based on our “2-gear” prototype, PARAID consumes 15% less power than a traditional RAID, while not compromising performance Motivation • Energy costs are rising • An increasing concern for servers • No longer limited to laptops • Energy consumption of disk drives • 24% of the power usage in web servers • 77% of the power usage in proxy servers • 27% of operating costs for data centers PARAID Design Skewed Striping: By assigning data blocks in a skewed pattern, the number of powered-on disks can vary according to the demands placed on the system. To be able to create the gears, PARAID uses spare storage on the RAID device for soft state replication. Preserving Peak Performance: A RAID-5 layout is used for the primary copy of the data blocks which allows for maximum parallelism between the drives when the load on the system demands it Observations • Cyclic fluctuation of loads • RAIDs are configured for peak performance • Uniform striping over-provisions resources for light loads • Over-provision of storage capacity • Avoid frequent upgrades • Limited power cycles for server-class drives • Cannot power-cycle drives too frequently PARAID creates tiers of disks within the RAID that can be gear-shifted so that the least number of disks are used to meet system load while not degrading performance. Power: The same file can be delivered with either the lower two-drive gear or the higher four-drive gear settings. Under high loads, all four drives will be powered to meet the performance demand. Under light loads, two drives are sufficient to deliver the data. Reliability: Since a RAID-5 layout is used for the primary copy of the data blocks, the distributed parity of RAID-5 will tolerate a single disk failure. PARAID creates a bimodal usage pattern so that busy drives can stay powered on most of the time and idle drives can stay powered down most of the time, leaving only middle range disks to be powered cycled more often, reducing the total number of times a drive is powered cycled. PARAID Benefits Initial Findings Performance: Based on the replay of a web server trace, our 2-Gear PARAID-0 prototype consumes 15% less power on average than conventional RAID-0 as shown in the Total Power Consumption graph. The peak PARAID-0 performance is comparable to RAID-0 under the PostMark benchmark. Conventional RAID: The “One-size-fits-all” approach of convention raid creates a suboptimal strategy for spinning down drives. PARAID: The number of disks can be adjusted to meet load with out degrading performance.