The Power Aware RAID: PARAID

NSF grant CNS-0410896

Charles Weddle, Mathew Oldham, Florida State University
RuGang Xu, UCLA, The Laboratory for Advanced Systems Research
An-I Andy Wang , Florida State University
Geoffrey H. Kuenning, Harvey Mudd College
Peter Reiher , UCLA, The Laboratory for Advanced Systems Research


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.

By introducing a skewed striping pattern, PARAID 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.


PARAID Architecture


We propose Power-Aware RAID (PARAID), which implements a skewed striping strategy to achieve power savings without performance degradation. By assigning data blocks in a skewed pattern, the number of powered-on disks can vary according to the demands placed on the system.

The figure shows one simple strategy for striping data blocks across two RAID gears, each with two disks. In this case, 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.



PARAID Benefit


Conventional RAIDs are designed to maximize peak performance. The uniform striping design is elegant and avoids hot spots. However, in the context of energy savings, conventional RAIDs tend to over-provision the resources necessary to meet user demands.

Uniform striping
Conventional RAIDs exploit the parallelism of disk drives to achieve high performance. The homogeneous design of RAID makes data management simple. Uniform striping allows all drives to be accessed in the same way. The load balancing ensures that no disk becomes a bottleneck.

However, when a system is not near the peak load, load balancing leads to over-provision resources. A uniform striping RAID requires all disks to be active to serve any requests, even when high performance is not required. The same request could perhaps be served with a fraction of the disks without violating performance constraints, while concurrently saving power. Ideally, we want a minimum number of disks kept active to meet system demands, but a maximum kept idle when not needed, so that we can power down the idle disks.

Load characteristics
Server loads exhibit daily and weekly variations. For each day, average Web servers exhibit peak-to-trough load ratios around 10:1. Files also have different characteristics (i.e., large, small) and are accessed differently (i.e., real-time, infrequently used). The one-size-fits-all approach of RAID makes simple caching and disk spin downs suboptimal, since a 90% cache hit rate does not equate a 90% disk idle time. Additionally, uniform striping and current disk layouts do not allow for the expression of these load characteristics, which is required for energy-saving mechanisms to work effectively.



Publications


Charles Weddle
PARAID: A Gear-Shifting Power-Aware RAID USENIX 5th Conference on File and Storage Technologies (FAST'07), San Jose, February 2007.
 
Charles Weddle
PARAID: The Gear-Shifting Power-Aware RAID Technical Report 060323, Florida State University, January 2006.
 
Charles Weddle
PARAID: The Gear-Shifting Power-Aware RAID Masters Thesis, Florida State University, June 2005.
 
Mathew Oldham
A Power and Performance Measurement Framework for Server-Class Storage Honors Thesis, Florida State University, April 2005.


Presentations


Charles Weddle
The Power Aware RAID: PARAID. Presented at the USENIX 5th Conference on File and Storage Technologies (FAST'07), San Jose, February 2007.
 
Charles Weddle
The Power Aware RAID: PARAID. Presented at the Florida State University Annual Technical Conference, Tallahassee, March 2006.
 
An-I Andy Wang
Conquest-2: Improving Energy Efficiency and Performance Through a Disk/RAM Hybrid File System Presented at the Computer Science Colloquium, University of California, Riverside, May 2005.
 
Charles Weddle
The Power Aware RAID: PARAID Work In Progress report. Presented at the 2005 USENIX Annual Technical Conference, Anaheim, April 2005.
 
Charles Weddle
The Power Aware RAID: PARAID Poster. Presented at the 2005 USENIX Annual Technical Conference, Anaheim, April 2005.
 
RuGang Xu
Conquest: Combining Battery-Backed RAM and Threshold-Based Storage Scheme to Conserve Power. Presented at the 19th Symposium on Operating Systems Principles (SOSP),New York, October 2003.


weddle@cs.fsu.edu

This material is based upon work supported by the National Science
Foundation under Grant No. 0410896.

Any opinions, findings, and conclusions or recommendations expressed in
this material are those of the author(s) and do not necessarily reflect
the views of the National Science Foundation.


Last modified:  February 26, 2007