Feeds

Boffins say flash disk demands new RAID designs

Wear-levelling levels out

HP ProLiant Gen8: Integrated lifecycle automation

Solid state disks (SSDs) are wonderfully fast, but every time you write to them, the semiconductors involved degrade just a little, a property that means you swap them into established storage rigs at your peril.

That's the conclusion of a new paper, “Stochastic Analysis on RAID Reliability for Solid-State Drives” from a pair of boffins at The Chinese University of Hong Kong.

The paper explains its purpose as follows:

“Traditional storage systems mainly use parity-based RAID to provide reliability guarantees by striping redundancy across multiple devices, but the effectiveness of RAID in SSDs remains debatable as parity updates aggravate the wearing and bit error rates of SSDs.

In particular, an open problem is that how different parity distributions over multiple devices, such as the even distribution suggested by conventional wisdom, or uneven distributions proposed in recent RAID schemes for SSDs, may influence the reliability of an SSD RAID array.”

It's worth asking those questions of SSDs in RAID arrays because the all-silicon disks wear out over time. Disk-makers try to ensure their products have the longest possible working life with a trick called wear-levelling that spreads the work around inside an SSD so different bits of it don't get more work than others. The idea is that all parts of an SSD therefore age gracefully at about the same rate, instead of some regions being beaten to death at a young age.

Wear-levelling gets better with each passing year, but when data preservation really matters it is still disconcerting to know that bits of your disk might drop off the twig. Doubly so if you've gone to all the trouble of building a RAID-5 array to access the extra level of data protection it affords.

The pair therefore set up tests to see how much data survives, and is lost, on SSDs set up under the Diff-RAID scheme and conventional RAID 5. The study offers a very, very, complex test and measurement methodology involving a lot of reads, writes and erasures of data on SSDs, under various conditions, with subsequent measurement of how well the SSDs are at the end of the process.

Tests assume up to a terabyte of traffic passing through a disk each day, a not-unreasonable assumption given SSDs are often targeted for high I/O chores.

The results show that under some circumstances the all-SSD RAID arrays do well. On other occasions, things get nasty, especially as the drives age. And despite wear levelling, it seems the onset of ageing – and therefore bit loss - was possible to detect in the pair's tests.

The conclusion? “Performance and reliability analysis on RAID in the context of hard disk drives has been extensively studied. SSDs have a distinct property that their error rates increase as they wear down, so a new model is necessary to characterize the reliability of SSD RAID.” The paper offers that new model, for those willing to wade through some rather dense maths.

If that's not your bag, another out-take seems simple: you can't assume SSDs are always as reliable as spinning rust, especially when you use them in the same way. ®

Eight steps to building an HP BladeSystem

More from The Register

next story
THUD! WD plonks down SIX TERABYTE 'consumer NAS' fatboy
Now that's a LOT of porn or pirated movies. Or, you know, other consumer stuff
EU's top data cops to meet Google, Microsoft et al over 'right to be forgotten'
Plan to hammer out 'coherent' guidelines. Good luck chaps!
US judge: YES, cops or feds so can slurp an ENTIRE Gmail account
Crooks don't have folders labelled 'drug records', opines NY beak
Manic malware Mayhem spreads through Linux, FreeBSD web servers
And how Google could cripple infection rate in a second
FLAPE – the next BIG THING in storage
Find cold data with flash, transmit it from tape
Seagate chances ARM with NAS boxes for the SOHO crowd
There's an Atom-powered offering, too
prev story

Whitepapers

Seven Steps to Software Security
Seven practical steps you can begin to take today to secure your applications and prevent the damages a successful cyber-attack can cause.
Consolidation: The Foundation for IT Business Transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.
Designing a Defense for Mobile Applications
Learn about the various considerations for defending mobile applications - from the application architecture itself to the myriad testing technologies.
Build a business case: developing custom apps
Learn how to maximize the value of custom applications by accelerating and simplifying their development.
Consolidation: the foundation for IT and business transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.