Conserving Disk Energy in Network Servers

ACM 17th annual international conference on Supercomputing

Presented by Hsu Hao Chen

Outline Introduction Conserving disk energy

Idle Replace Combined Multi-speed

Evaluation Combined Multi-speed

Conclusions

Introduction(1/2) The Google search engine

15K servers These large clusters consume a significant

amount of energy Energy costs can reach 60% of the

operational cost of data center

Evaluating four approaches to solving it Idle Replace Combined Multi-speed

Introduction(2/2)Multi-speed

Combined

Idle Most of them are based on powering disks down du

ring periods of idleness Break-even threshold

cost of powering the disk down and up (on the next access) Testing

Assuming that load peaks reach only 50% of this maximum throughput

A large memory cache Memory cache miss rate that is lower than 0.03%

Result Average idle time of 15.2 seconds

In summary, not appropriate for network servers

Replace(1/2) Replace each high-performance disk

with one or more lower power disks

1-to-1 ratio for laptop disk problem Storage capacity Performance: access latency

1-to-2 ratio for low power SCSI disk Energy consuming

Two low power disks > high-performance disk

Replace(2/2) 1-to-n ratio for laptop disk

reliability problem We would need at least four (RAID) laptop disks

for each HP

Combined(1/4) The idea is to associate each high-performa

nce disk with a lower power disk, called a secondary disk.

The disks should have the same size and mirror each other

Coherence actions updates while the set of disks coming up was powered of

f

Combined(2/4) Implementation

Combined(3/4) Linux module

Allows the creation of multiple virtual devices Each virtual device is mapped to a pair of disks Module is inserted at a low level, all disk traffic (i

ncluding metadata accesses) is visible to it Module intercepts all calls to the ll_rw_block()

kernel routine

Combined(4/4) The module has three key components

A translation table per virtual device specifies which physical disk drive to use on each access

ll_rw_block() Monitors the load on the disks and measure of the load offered

EWMA α=0.875

Selects which disk to use depending on the load on the disk subsystem

A bitmap per disk specifying all the blocks that have been written since the disks

of the corresponding virtual device were last made coherent. A bit is set in a bitmap when an intercepted ll_rw_block() call p

roduces a disk write

Multi-speed This approach does not require mutiple disks, coh

erence, bitmap etc..

Switching threshold Decides to changes speeds

Emulation Because multi-speed disks are not available in the market The emulation keeps our two SCSI disks powered on all t

he time All write accesses are immediately directed to both disks The emulation also assigns performance and energy cost

s to the speed transitions

Evaluation(1/7) Network server hardware

P4-1.9GHz 512MB RAM OS: Linux 2.4.18 Storage disks

the SCSI Ultrastar 36Z15 disk the SCSI Ultrastar 73LZX disk (when evaluating Multi-speed) the laptop Travelstar 40GNX disk (when evaluating Combined)

a Gigabit Ethernet network interface

Web server is able to service a maximum of 2520 requests/second for the Clarknet trace

ClarkNet:These two traces contain two week's worth of all HTTP requests to the ClarkNet WWW server

Proxy server can service up to 335 requests/second for the Hummingbird trace

Evaluation(2/7) Combined

Power-saving 1%

Evaluation(3/7) Combined

In summary, we do not consider very realistic for network servers.

Power-saving 41%

Evaluation(4/7) Multi-speed for web-server

Power-saving 16%Power-saving 22%

Evaluation(5/7) Multi-speed for proxy-server

Power-saving 15%Power-saving 17%

Evaluation(6/7) Multi-speed

web-server proxy-server

Throughput Degradation 1%Throughput Degradation 3%

Evaluation(7/7) Multi-speed

In summary, the two-speed disk should perform well in a wide range of scenarios.

Conclusions Two-speed disk techniques

energy savings without performance degradation in network servers

Our results suggest that this technique should be carefully considered by disk manufacturers

The other techniques we studied cannot provide any disk energy savings