Shouting at a disk drive will cause it to stop. But what about the constant nagging they get in busy data centers? That’s a bigger problem.
Bad, bad, bad vibrations
The use of consumer-grade SATA drives in the enterprise raises the concern. A 2005 study, Performance Impact of External Vibration on Consumer-grade and Enterprise-class Disk Drives, by Thomas Ruwart and Yingping Lu found that consumer
. . . disk drives are more sensitive to the vibration from physically coupled adjacent disk drives. . . . [G]reater care needs to be taken in enclosure design, particularly for the 3.5-inch form factor disk drives due to their higher-energy seek operations when compared to seek operations on a 2.5-inch form factor disk drive.
Yet another reason to end-of-life 3.5″ drives.
Mechanical engineers know that drive vibration is a serious problem. How serious is the question. We do know that steel does a good job of transmitting vibrations as this graph shows:
Courtesy Rockwell Automation
An internal study of a vibration-damping rack by a major storage and server vendor obtained by StorageMojo – and not available on the web – found a
20% decrease in IO throughput performance (read) and 25% increase in total time to perform a task of typical 2U servers resulting in 25% increase in energy consumption . . . when vibration level went up from 0.1 GRMS to 1 GRMS. This is a conservative result, as newer HDDs are more sensitive to vibration, â€œwrite operationâ€ is 20% more sensitive than â€œread operationâ€ to vibration and effect of vibration on rotating components , HDDs & fans, are not accounted for.
[GRMS – properly Grms – is the root mean square of acceleration measured in g’s. Check out The Calculation of GRMS (pdf) for a technical discussion.]
Drives use accelerometers to compensate for shock and linear and rotational vibration but the info isn’t available externally so it’s difficult to directly quantify vibration impact. But data centers are noisy places quivering with the vibration of fans, air conditioners, disk drives and 60 cycle hum, all supported by steel racks.
A limited study
In a USENIX paper presented at the SustainIT ’10 conference Julian Turner reported on limited tests of a prototype anti-vibration rack. The AVR-1000 is an engineered carbon fiber composite rack designed to dissipate vibration across a wide frequency range.
His observations include:
Performance improvements for random reads ranged from 56% to 246% while improvements for random writes ranged from 34% to 88% for a defined set of industry benchmarks. Streaming sequential reads and writes had a much smaller performance improvement. . . .
The internal study found similar results:
IO throughput performance (write) increased by 285% and total time and energy to perform a task of a 4U storage server with 48 HDDs decreased by 64% when vibration level went down from 1 GRMS to 0.1 GRMS.
Evidently the combination of random head movements and vibration has a substantial effect on disk read performance.
For the enterprise and Internet-scale data centers the implications are substantial:
- Energy savings. Anti-vibration racks can save power by improving disk performance and reducing run times.
- SSD value. Flash SSDs have fast random read access. But disks can improve their performance by 50% through vibration damping, changing the SSD value proposition.
- Array sizing. Enterprise arrays are over-configured to improve performance. If disks were suddenly 50% faster that could be reduced or, alternatively, the utilization could be increased.
The StorageMojo take
The research is limited, but everything we know about disks and vibration today suggests this is real. Assuming further research finds similar results we could see an explosion of products using engineered materials to improve disk performance.
This is already the norm in chip fabs, where nanometer precision requires extensive vibration damping. Disk feature sizes are even smaller, so why not?
Drive vendors should make accelerometer data available to researchers. Vibration-damping racks could make disks much more competitive with SSDs – and mean billions to drive vendors – but only if we have the data.
For home users of multi-drive towers it may be that damping carbon fiber towers or disk mounts could improve performance. It may not be economic, but it sure would look awesome.
The impact for Internet-scale infrastructures is more nuanced. Google, for one, doesn’t use high-density storage – maybe 120 drives per rack – so the performance gains may not be economic for them. But users of many drive servers with, say, 300+ drives in a rack would find it easier to justify.
Courteous comments welcome, of course.