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RAID systems

"RAID was one of the unsung heroes of the digital economy in the past 20 years. Without RAID - we wouldn't have the big data (and reliability) that we have gotten used to today. But don't get too hung up on learning the internal details of traditional RAID. With today's fast links and processors there are many other new ways to stripe data across drives and memory chips too - with lower latency and better efficiency." - Zsolt Kerekes, editor
RAID is an acronym for

Redundant Array of Inexpensive Disks.
In the mid 1980's when this term first entered public awareness, you could buy 2 types of disk drives, either low cost drives such as used in the average PC, or high speed high performance mainframe drives as used by Quantel in its digital video effects systems.

The huge market for PC disks soon became the leading edge technology drivers for disk storage and overtook the larger minicomputer and mainframe form factor disks in speed, reliability and capacity.

By the late 1990s RAID systems using PC form factor disks had become the most common form of bulk storage in enterprise servers and even some (Unix) mainframes.

Today the original RAID concept remains valid even though hard disks have changed form factors many times in the past 20 years (8", 5.25", 3.5", 2.5", 1.8" and below 1") and the concept may be useful in the near future when the "disks" in the array could actually be flash solid state disks and not traditional hard disks.

You can create a virtual disk array which looks electronically just like a bigger ordinary disk, by attaching a bunch of disks working in parallel and connected to a RAID controller interface.

The combined system can be programmed to provide desirable characteristics such as faster data throughput (for example a 4 disk wide system could have a data throughput capability 4 times faster than a single disk).

RAID can also provide fault tolerance, because redundant disks can be added into the array and the data split up in such a way with redundant error bits that there is no loss of data if any single disk fails (or if 2 disks fail in some RAID configurations) - provided the dead disk(s) is replaced and the data rebuilt before the next failure occurs.

RAID doesn't always result in an application speedup. It can slow down the access time in some types of application in which the data sets are small and randomly located - because the latency of the RAID controller is additional to the disk's own access time. ...from Megabyte's Storage Dictionary
An SSD ASAP is something which you install between 2 storage systems which have vastly different latency and capacity.

Historically the slow end was legacy RAID built from arrays of rotating rust platters.

But in more modern systems both ends can be SSDs.
Need SSD Acceleration ASAP?
Say "hello" to SSD ASAPs

Nibbles Re: RAID History
IBM received the first patent for a disk array subsystem in 1978, and co-sponsored the research by the University of California at Berkeley that led to the initial definition of RAID levels in 1987.

IBM launched the first modern style RAID systems in 1990

It wasn't until the late 1990's that RAID technology became a "must-have" building block in commercial Unix servers.

In the future RAID technology will be already integrated in most home PC's and entertainment systems, because home users don't do backups, but they will have large digital entertainment libraries which won't fit neatly onto a single disk.

Nibble:- RAID and Me

first came across the concept of RAID in 1986 when I joined a company called Databasix as their hardware engineering manager.

Databasix, based in Newbury in the UK, was founded by John Golding from the nucleus of an earlier company he had been in called Microconsultants.

Microconsultants had spawned rich sister company Quantel. Peter Michael, who had been the business and technical brains in both these earlier companies, gave Databasix a very generous start in life from what I could see by the pool of talented people and money which had mostly been already spent when I joined the 70 people or so in the new "start-up".

Among the many things on my to-do list was to build a working RAID controller and RAID array demonstration system.

"I don't know much about hard drives" - I said. I could afford to be honest - because these guys had already seen the worst when they first met me. My VC backed networked data acquisition company was going bust and they had been a potential buyer.

"There's not much too it." My boss said. "Just read the manuals that come with the disk drives. We want to see if RAID will give us fast real-time disks at a cost that's significantly less than the video disks from Japan used by our sister company Quantel."

"OK" I said. "I don't know much about RAID either."

"Nobody does. Here's a bunch of articles. They tell you all you need to know. We'd like the demonstration ready in 3 weeks."

"That sounds like a very short time to me."

"We've already ordered the disks to save time. You order whatever chips you need, and use some of the software guys to help on this."

From memory, I think I got the demo deadline pushed out to about 4 to 5 weeks.

We also had an Artificial Intelligence demo being worked on at the same time by about 50 software engineers, and a parallel computing demo, but the RAID functionality was the "must have" thing which could not be easily dropped from the sales plan.

We did build a working 4 drive RAID system. One of our biggest problems had been the high rate of Dead On Arrival disk drives. That caused a lot of problems which we initially blamed on the software. Hard drives were a lot more sensitive in those days and could be killed just by putting them down on your desk.

But by then I knew a lot more about disks and realised that the Inexpensive Disks we used in our demonstrator weren't anywhere near as fast as they could have been, because they were the wrong standard. Then, as now, there were many interface standards for disk drives. If you're going to build a fast system then you might as well use fast building blocks. Dataquest was telling me that we should probably be using SCSI instead.

As my imposed wish lists started to pile up and commercial reality started hitting my new employer, I decided that it would be a heck of lot easier to partner with a disk controller company which was already down this part of the curve, and later we became a beta site for dozens of manufacturers of processor cards, array processors, hardware interfaces, memory and disk drives as we tried to make a business out of selling the technology curve to military buyers almost before it was really there. That was great fun, but a different story.

It was about 10 years after that before RAID systems next appeared in my life - when RAID companies like DEC (acquired by Compaq and now part of HP) and Data General (acquired by EMC) started promoting their RAID systems to readers of my Sun foused SPARC Directory.

Nearly 12 years after my first acquaintance with RAID it became one of the first 4 product categories here on And although the interface patterns have changed over the years, from DAS SCSI, then Fibre-channel SAN, then Ethernet NAS, and then iSCSI, the ideas inside the box have remained the same.

When you get to be an old guy like me, it's a lot easier if some of the new stuff which hits your brain, is actually a rehash of old stuff.

...more about Databasix.

I didn't want to interrupt the narrative flow above - but the core Directors at Databasix when I joined were:- John Golding, Andrew Bruce, Ray Potter and Dan Boxshall.


"New SSD enhanced hardware and software fabrics will have the same effect on how you come to view a single server - as RAID did on the limitations of a single hard drive."

the Top SSD Companies - 30th quarterly edition

AccelStor NeoSapphire  all-flash array
1U enterprise flash arrays
InfiniBand or 10GbE iSCSI
NeoSapphire series - from AccelStor
RAID, RAIC, RAISE etc - RAID's Many Aliases
The long established word "RAID" has been stretched by marketers into many other disguises to make their products sound better.

The RAID system concept itself is simple, being a box of disk drives with self healing properties which can be run in parallel for faster data throughput. But that's not sexy enough for most storage product marketers. So we now have the following refinements.
  • RAID connected by SCSI, Firewire or IDE can be called a DAS (Directly Attached Storage). The RAID has to be connected by something, but "DAS" sounds more modern, and indicates that you chose this method of connection in preference to all the others.
  • RAID connected by Fibre-channel can be called a SAN (Storage Area Network). That sounds better already.
  • RAID connected by Ethernet can be called a NAS (Network Attached Storage). Latterly the term IP-SAN has been used to add new freshness to this word-washing.
  • RAID connected by both Fibre-channel and Ethernet can be called a NUS (Network Unified Storage).
  • Even better than plain old vanilla NUS,apparently, is SUS, or Scalable Unified Storage, coined by the short lived startup Broadband Storage
  • More likely to endure than either NUS or SUS, is market research company Gartner's term FAS for Fabric Attached Storage which also lumps NAS and SAN together.
  • RAIN (Redundant Array of Independent Nodes) is an Adaptec creation
  • MAID (Massive Array of Idle Disks) is a whimsical term from COPAN Systems used for disk to disk backup systems.
  • RAIGE - (RAID Across Independent Gigabit Ethernet) is a creation of Pivot3 although some of concepts sound similar to how Google implements its internal storage infrastructure.
  • RAISE - (Redundant Array of Independent Silicon Elements) is a term invented by SandForce. Their SSD controller uses this protection scheme inside 1.8" and 2.5" SSDs.
  • RAIC - redundant array of independent chips - was coined in the 1990s by Solid Data Systems
  • DVRAID - is a proprietary RAID technology from ATTO Technology that is "optimized for digital content creation environments that require protection in the event of a disk failure without the performance penalty traditionally seen with parity RAID."
  • SAID - Self-maintaining Array of Identical Disks - a possibly overambitious term from Atrato
  • RAIDn - is an algorithm launched in 2003 by InoStor. It never achieved wide currency.
  • Finally, a RAID not connected to anything at all can be called a LUS (Lonely Unloved Storage)...

    No, I just made this one up. But you can see the basic principle at work here. And no doubt there will be other terms later for RAID connected by the Internet or Infiniband.
See also:- Megabyte's Storage Glossary which includes definitions of the many other strange terms which appear from time to time in these pages.
RAID controller cards
SATA Raids the Datacenter
A Storage Architecture Guide
RAID Levels Outlive Their Usefulness
What do enterprise SSD users want?
Which RAID Manufacturers will Survive?
Where are we now with SSD software?
How fast can your SSD run backwards?
RAID storage basics guide for beginners
Market Trends in the Rackmount SSD Market
10 Ten Tips for a Successful RAID Implementation
RAID - editor mentions on
RAID - what it is, why we need it and how it works (pdf)
Using Solid State Disks to Boost Legacy RAID Performance (2004)

RAID news

Dell uses Avago's 12Gb/s SAS chips in new RAID systems

Editor:- September 10, 2014 - Avago Technologies today announced that Dell has selected Avago's 12Gb/s SAS technology (recently acquired from LSI) for use in RAID controllers in Dell's new PowerEdge Servers. See also:- SAS SSDs, storage glue chips

Intels oems LSI's RAID caching SSD technology

Editor:- April 8, 2013 - Intel - which already uses LSI's SandForce controllers in some SSDs - will oem LSI's dual-core RAID-on-Chip flash caching technology it was announced today.

LSI says their caching technology can double the number of VDI sessions supported in the same sever and flash environment.

"Intel's selection of LSI Nytro MegaRAID technology is another significant validation of our strategic focus and investments in flash-based server acceleration technology," said Gary Smerdon, senior VP and GM, Accelerated Solutions, LSI.

Old 3ware RAID 10 kills Intel SSD array performance

Editor:- February 11, 2013 - It's not really news that anything which wastes latency and raw CPU performance in the host interface and related controllers - also prevents and wastes the possible performance gain from SSDs. But sometimes users are the victim of circumstances outside their control.

This point is vividly demonstrated in the article - How slow can SSD be or why is testing a new server performance important? - which describes the problems someone had when using SSDs in an array with a slow controller designed for hard drives which was the default offered by their ISP company.

Amplidata gets $8 million C funding for - (don't call it RAID)

Editor:- February 29, 2012 - apparently you can still raise VC funding for HDD based storage array companies.

Amplidata today said it got $8 million in a Series C round of funding for its multi-petabyte scalable, efficiently data protecting and healing BitDynamics technology.

The idea is to implement the cheapest possible and easily expandable bulk storage capacity - what they call unbreakable object storage for petabyte-scale unstructured data (pdf) - so the storage nodes are stuffed with low power hard drives. But the controller racks can be internally accelerated by SSDs.

Pure Storage has amassed $55 million for bulk FC SAN SSD storage

Editor:- August 24, 2011 - Pure Storage yesterday unveiled its first SSD product line and announced it had received $30 million in series C funding bringing its total capital funding up to $55 million.

Pure Storage 's FlashArray provides bulk / utility SSD storage for FC SAN enviroments - which by using inline dedupe and compression - can in some applications (25TB and 50K IOPS per U) offer lower cost and yet still deliver higher performance than classic hard drive disk arrays.

Editor's comments:- This looks like a spreadsheet based value proposition rather than a disruptive new product - and follows a market groove already established by WhipTail Technologies and Nimbus Data Systems. The market for this type of SSD market will be huge - but along the way to proving itself will have to fight off competition from auto-tieing SSDs and white box SSD RAID which will nibble away at the same customer SSD budgets.

Pushing data reliability up hard drive hill

Editor:- July 4, 2011 - Why didn't hard drives get more reliable? Enterprise users are still replacing hard drives according to cycles that have haven't changed much since RAID became common in the 1990s. So why didn't HDD makers do something to make their drives better?

Error correction code inventor Phil White - founder of ECC Technologies has recently published a rant / blog in which he describes the 25 years of rejections he's had from leading HDD makers - and the reaons they said they didn't want to use his patented algorithm - which he says could increase data integrity and the life of hard drives (and maybe SSDs too.) It makes interesting reading for any other wannabe inventors out there too. Phil White's article

But I think another reason for past rejections might simply have been market economics.

The capacity versus the cost of HDDs has improved so much throughout that period - and at the same time data capacity needs have grown - maybe the user value proposition didn't make sense.

If you (RAID user) find that all your 5 year old drives are still working (instead of being replaced) - how much is that really worth? By now those 5 year old drives might only represent 3% to 10% of the new storage capacity you need anyway. (The reliability value proposition is different outside service engineer frequented zone - but I don't want to get side-tracked into SSD market models here.)

Looking ahead at the future of the HDD market my own view is that whatever the industry does with respect to reliability won't tip the balance against SSDs in the enterprise.

The best bet for the future of hard drive makers is in consumer products where fashion ranks higher up the reason to buy list than longevity. Most people I know replace their notebook pcs, tvs and phones not because the old ones have stopped working - but because the new ones have lifestyle features which make them more desirable.

the missing link?

software which sits between HDD RAID and SSDs

Editor:- July 1, 2011 - earlier this week I spent an interesting hour talking to FlashSoft's CEO - Ted Sanford about the company's business plans and technology.

The company recently launched software which enables almost any SSD to act as a cache accelerator front end for hard disk storage arrays in enterprise servers. By automatically learning data hot spots as little as 15 minutes after being installed - the new software speeds up SQL queries for example 4x - and enables users to use less servers. more

SANBlaze ships PCIe to 1.8" SSD RAID adapter

June 13, 2011 - SANBlaze Technology is shipping a new rear transition module which connects upto 8x 1.8" SSDs to PCIe with RAID options.

optimizing SSD architecture to cope with flash plane errors

Editor:- May 24, 2011 - a new slant on SSD reliability architectures is revealed today by Texas Memory Systems who explained how their patented Variable Stripe RAID technology is used in their recently launched PCIe SSD card - the RamSan-70.

TMS does a 1 month burn-in of flash memory prior to shipment. (One of the reasons cited for its use of SLC rather than MLC BTW.) Through its QA processes the company has acquired real-world failure data for several generations of flash memory and used this to model and characterize the failure modes which occur in high IOPs SSDs.

Most enterprise SSDs use a simple type of classic RAID which groups flash media into "stripes" containing equal numbers of chips. RAID technology can reconstruct data from a failed Flash chip. Typically, when a chip or part of a chip fails, the RAID algorithm uses a spare chip as a virtual replacement for the broken chip. But once the SSD is out of spare chips, it needs to be replaced.

VSR technology allows the number of chips to vary among stripes, so bad chips can simply be bypassed using a smaller stripe size. Additionally, VSR provides greater stripe size granularity, so a stripe could exclude a small part of a chip rather than having to exclude an entire chip if only part of it failed - "plane error". With VSR technology, TMS says its SSD products will continue operating longer in the installed base.

Dan Scheel, President of Texas Memory Systems explained why their technology increases reliability.

"...Consider a hypothetical SSD made up of 25 individual flash chips. If a plane failure occurs that disables 1/8 of one chip, a traditional RAID system would remove a full 4% of the raw Flash capacity. TMS VSR technology bypasses the failure and only reduces the raw flash capacity by 0.5%, an 8x improvement. TMS tests show that plane failures are the 2nd most common kind of flash device failures, so it is very important to be able to handle them without wasting working flash."

Editor's comments:- by wasting less capacity than simpler RAID solutions - more usable capacity remains available for traditional bad block management. This extra capacity comes from the over provisioning budget which figure varies according to each SSD design (as discussed in my recent flash iceberg syndrome article) but is 30% for TMS.

SSDs accelerate disaster recovery

Editor:- September 24, 2010 - an update to Intel's SSD Bookmarks - published today on - includes links to a case study in which RAID rebuild times for a real-time education server were reduced from 12 hours to 40 minutes, while response times became 25x faster. the articles
RAID seekers, click here to see more storage news
Megabyte found that tying lots of barrels
together to cross the data stream worked
well. And if one of them got punctured,
the raft didn't sink.
"In 1998's RAID page (this page you're seeing now) listed 51 manufacturers."
...from - storage history
SSD ad - click for more info
SSDs in RAID's past and future
SSDs have been used in RAID configurations since the 1990s. But there are problems with this approach when using conventional RAID controllers and 2.5" SSDs.

Although SSD IOPS (and throughput) does scale in arrays - latency is slugged by slow controllers which were originally designed for HDDs. This approach is also wasteful of flash capacity compared with other redundancy approaches designed into proprietary rackmount SSDs. And finally - the RAID approach - when applied to 2.5" SSD modules - doesn't address the problem of end to end data integrity - which has been mentioned in this article.

Some enterprise RAID companies, recognizing that hard disk based RAID in the enterprise will be a dead market in a few years, have moved into the emerging SSD ASAPs market which blends the advantages of HDD (cheap capacity) and SSD (fast IOPS) into Auto-tuning SSD Accelerated Pools of storage..
SSD ad - click for more info
"The performance impact from RAID rebuilds becomes compounded with long rebuild times incurred by mutli-terabyte drives. Since traditional RAID rebuilds entirely into a new spare drive, there is a massive bottleneck of the write speed of that single drive combined with the read bottleneck of the few other drives in the RAID set."
Dave Wright, CEO - SolidFire - in his blog - Say farewell to RAID storage (March 2013)
"When it comes to hard drives RAID was better than what it replaced in the 1980s but if you started again with the internet connectivity and processors we have today and started to design big arrays of disks from first principles then you wouldn't see just the RAID systems you see today. That's because RAID is small controller architecture..."
Editor:- in the article - Why size matters in SSD controller architecture (June 2011)
"What we need to do now is recognize that in very large systems, with lots of components failures are no longer a rare anomalous event - they are a frequent normal condition - and we need to quote performance and availability and reliability assuming that there is always some repair going on and we need to acknowledge that all these redundancy schemes reduce the probability of data loss they don't eliminate data loss..."
Garth Gibson, founder and chief scientist, Panasas Long Live RAID (video) (May 2012)
"...Cached RAID solutions are starting to run out of gas in high-performance, transaction-intensive applications: while the performance demands on e-business infrastructures are being driven to new levels by exploding demand, unpredictable peak loads, and an increasingly impatient population of on-line customers. It's time for another architectural innovation."
Michael Casey, Solid Data Systems (November 2000) - in his article - Solid State File-Caching for Performance and Scalability
RAID History
see this page back in

RAID - 1999
RAID - 2000
RAID - 2001
RAID - 2002
RAID - 2003
RAID - 2004
RAID - 2005
RAID - 2006
RAID - 2007
are you looking for an elusive shiny needle - in an over towering RAID haystack?

My lists of RAID systems vendors started with just a handful of companies in 1991 and grew to hundreds of companies.

When lists are that large they are boring. And RAID is now a commodity. So I've zapped the RAID oems list which used to be on this page.

Instead - you'll get better results using site search - with whatever is your favorite search-engine.

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