Motivation

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 consum
es 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 sk
ewed 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 us
es spare storage on the RAID device for soft
state replication.
Preserving Peak Performance A RAID-5 layout is u
sed 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 th
e primary copy of the data blocks, the
distributed parity of RAID-5 will tolerate a
single disk failure. PARAID creates a bimodal u
sage 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.