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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