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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."

above quote from - the Top SSD Companies - 2014 Q3

See also:- after AFAs - what's the next box? (2017)

And for a philosphical detour (covering wider ground than the narrow intersection of RAID with SSDs) - how about looking at the enterprise flash story... could it have been simplified?
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.
  • LI-RAID (Layer-Interleaved RAID) is a concept for 3D nand flash which exploits the different quality of blocks. Revealed in a research paper presented June 2018.
  • 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.
SATA Raids the Datacenter (2004)
A Storage Architecture Guide
0 to 60 - choosing the right RAID
What do enterprise SSD users want?
RAID Levels Outlive Their Usefulness (2001)
Where are we now with SSD software?
How fast can your SSD run backwards?
Which RAID Manufacturers will Survive? (2000)
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

new thinking in SSD controller techniques reveals "layer aware" properties exploitable in 3D nand flash

Editor:- August 28, 2018 - A new twist using RAID ideas in SSD controllers has surfaced recently in a research paper - Improving 3D NAND Flash Memory Lifetime by Tolerating Early Retention Loss and Process Variation (pdf) by Yixin Luo and Saugata Ghose (Carnegie Mellon University), Yu Cai (SK Hynix), Erich F. Haratsch (Seagate Technology) and Onur Mutlu (ETH Zürich) - which was presented at the SIGMETRICS conference in June 2018.

The authors say that in tall 3D nand (30 layers and upwards) the raw error rate in blocks in the middle layers are significantly worse (6x) compared to the top layer. Therefore to enable more reliable and faster SSDs using 3D nand for enterprise applications they propose a new type of RAID which pairs together the best predicted half of a RAID word with the worst predicted half from another chip in the same SSD.

This new RAID concept starts to be feasible in a very small population of chips - unlike traditional 2D nand schemes which need more chips to be installed in the SSD.

The new RAID is called Layer-Interleaved RAID (LI-RAID) - which the authors say "improves reliability by changing how pages are grouped under the RAID error recovery technique. LI-RAID uses information about layer-to-layer process variation to reduce the likelihood that the RAID recovery of a group could fail significantly earlier during the flash lifetime than the recovery of other groups." ... read the article (pdf)

Editor's comments:- the new RAID is just one of many gems in this research paper. Others being the discovery that remanence in 3D nand includes a significant short term charge loss (in the first few minutes after writes), and also that an endurance based characterization of a small part of each chip can be used to predict an optimized layer dependent threshold read voltage for all the layers in the chip. I've discussed the significance of adding the concept of "layers" to "number of raw chips" to the thinking in SSD controller design in my recent home page blog.

Seagate will acquire Dot Hill

Editor:- August 18, 2015 - Seagate today announced it will acquire Dot Hill Systems in an all-cash transaction valued at $9.75 per share, or a total of approximately $694 million.

SuperCloud rebuilds RAID 20x faster with CoreRise PCIe SSD

Editor:- July 3, 2015 - CoreRise today noted some record breaking performance results from one of its customers - SuperCloud (a well known Chinese cloud server manufacture) based on a configuration with CoreRise's PCIe SSDs in a 4U server with 2x 56Gbs InfiniBand ports.

Among other things SuperCloud said its lab results showed that RAID rebuilding was 20x faster than without the SSD - using a RAID5 configuration of 6D+1P. While RAID throughput was 10 to 14GB/s and 1 to 1.5 million 4KB IOPS.

Nimble video discusses 5 9's in 5,000 systems

Editor:- February 21, 2015 - Nimble Storage recently disclosed (in a sponsored video fronted by ESG) that its customer deployed rackmount storage systems are achieving better than 5 9's uptime - 99.999% availability.

news image Nimble reliabilityThis has been attained in a field population of 5,000 arrays representing 1,750 years of system run time thanks to a combination of factors including the crowd sourced intelligence of its InfoSight management system which can alert users to potential down time events so they can take evasive action before bad things happen.

Editor's comments:- While useful in telling us how many systems Nimble has sold it's less useful as an indicator of availability given that the average run time across the population is about 4 months.

It would be more impressive if they could repeat the disclosure in a few years time and selectively extract the up-time of systems over different run times, upto 1 year, 1 to 2 years etc.

If indeed Nimble is still in a position to do so, and if it would still meaningful given the consolidation in hardware and software which lies ahead for the enterprise SSD market may mean that vendors will be using the same hardware.

InnoDisk spins off rackmount SSD business into AccelStor

Editor:- November 27, 2014 - InnoDisk today spun off the business unit associated with developing and marketing its FlexiArray systems (1U InfiniBand / iSCSI 10GbE enterprise flash arrays) into a new company called AccelStor.

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

rackmount SSDs - new reports from Evaluator Group

Editor:- September 24, 2014 - Evaluator Group today announced it's expanding its comparison report coverage (priced from around $2,750 for IT end-users) related to rackmount SSD and hybrid array vendors.

The latest addition to EV's research area are product analyses for 15 vendors, including: Cisco, EMC, HDS, HP, IBM, Kaminario, NetApp, Nimble, Nimbus, Pure Storage, SanDisk, SolidFire, Tegile, Tintri and Violin.

"Over the next 3 years Evaluator Group expects Solid State Storage Systems to be the architecture adopted for primary storage," said Camberley Bates, Managing Partner & Analyst at Evaluator Group. "Performance to reduce latency and improve consistency, along with reliability and efficiency functionality will drive this change. It is important IT end users understand the trade-offs of design and technical implementation to best suit their needs."

Using the Solid State Evaluation Guide to understand the critical technology characteristics EV says IT end users can clearly identify their requirements and priorities. The Solid State Comparison Matrix allows for side-by-side comparison of product specifications and capabilities. Evaluator Group guides IT end users through the process with product reviews and expertise on managing and conducting a Proof of Concept. Evaluator Group Solid State Storage Systems coverage includes products specifically designed to exploit the characteristics of all solid state deployment.

What will you be getting?

EV is offering a free evaluation copy of their report for the IBM FlashSystem to people who sign up for it.

Editor's comments:- with so many different architectural roles for enterprise SSDs and different user preferences - it's unrealistic to suppose that any simple side by side product comparisons will suit all permutations of user needs. But having said that - any reliable information which assists user education and comprehension into SSD arrays is a good thing.

Some flash array vendors - realizing the futility of expecting that users will understand what their products do and how they will interact with the bottlenecks and demands of user installations and workloads - have instead opted to side-step these delay laden hard user selection quandries - which are exaggerated by the concerns of getting it wrong - by instead offering new risk delineated pricing models - as described in my article - Exiting the Astrological Age of Enterprise SSD Pricing.

See also:- storage market research, 90% of enterprise SSD companies have no good reasons to survive

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.

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.

Later that grew to hundreds. Although most of them have gone away.

In 2009 I said that the modern era of SSDs had signed the death warrant for hardware RAID controllers.

The cutting edge of storage performance hardware controller wizardry moved from SAN to PCIe SSDs (and fabrics) then lingered a while in the memory channel and everthing will change again due to new memory defined software.

See also:- SSD controllers and IP
SSD ad - click for more info
SSDs in RAID's past and future
SSDs have been used in RAID configurations since the 1990s when all enterprise SSDs were RAM SSDs.

In the early phases of the enterprise flash market a seminal white paper (pdf) (2007) based on lab measurements showed that SSD IOPS (and throughput) could scale in arrays - using traditional RAID controllers designed for HDDs.

And that was an approach taken by many storage vendors to ease their way into the emerging rackmount SSD market.

Disadvantages of that technique, however, were that system latency was wasted by slow controllers and also the extra parity writes in fault tolerant RAID schemes worsened the reliability of the flash arrays by placing greater burdens on endurance.

In more modern products - vendors have adopted new approaches for arrays of flash which take better cognisance of flash's characteristics. For example:-
  • big controller architecture
  • lower latency RAID management
  • write amplification reduction (by software stacks, new types of RAID algorithm and use of big front end RAM caches)
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)
"The misapplication of consumer grade SSD technology to enterprise applications was once a real problem for the reputation of the SSD market. One organization had installed RAID systems using Intel SSDs in a high performance environment. About half the SSDs had "burned out" after a year. Worse than that - when the customer investigated more closely they found that some SSDs had failed in a way which had not been detected by the RAID controllers."
Fusion-io's CEO talks SSD - with (2010)
"...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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