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RAID's Days May Be Numbered
storagedude sends in an article claiming that RAID is nearing the end of the line because of soaring rebuild times and the growing risk of data loss. "The concept of parity-based RAID (levels 3, 5 and 6) is now pretty old in technological terms, and the technology's limitations will become pretty clear in the not-too-distant future — and are probably obvious to some users already. In my opinion, RAID-6 is a reliability Band Aid for RAID-5, and going from one parity drive to two is simply delaying the inevitable. The bottom line is this: Disk density has increased far more than performance and hard error rates haven't changed much, creating much greater RAID rebuild times and a much higher risk of data loss. In short, it's a scenario that will eventually require a solution, if not a whole new way of storing and protecting data."
Honestly, there really aren't that many unsolved problems in computing if you are sufficiently aware enough to include mainframes and mainframe operating disciplines in your consideration. The basic way the mainframe community solved this particular problem long ago was to, first, take a holistic view about mitigating data loss. Double concurrent spindle failures are just one possible risk element. What about, for example, an entire data center exploding in a spectacular fireball? (Or whatever.) IBM, for example, came up with several different flavors of GDPS and continues to refine them, and they include multiple approaches to data storage tiering across geographies, depending on what you're trying to achieve. Data loss, whether physical or otherwise (such as security breaches), is not a particular problem with this class of technology and associated IT discipline, nor does there seem to be any signs of a growing problem in this particular technology class.
Enterprise arrays copy all the good data off the drive to a spare drive, use RAID to recover the failed sector(s), then fail the broken disk.
This is something the ZFS creators have been talking about for some time, and been actively trying to solve.
ZFS now has triple parity, as well as actively checksumming every disk block.
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That's what any raid controller worth their salt does. I've seen 3ware and areca controllers do this, and those aren't the most expensive controllers on the market by far.
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You don't rely on RAID to avoid data loss; you rely on it as a first line in providing continuity. We run backups of large systems here, but we tend to do other things too: synchronous live mirroring between sites of the critical data. And beter system design. There are some systems where, whilst we _could_ go back to tape (or VTL) at a pinch, having to do so would be a disaster in itself.
We're designing systems that permit rapid service recovery (the most live critical data) and a second tier of online recovery to get the rest back. We just can't afford the downtime.
Double-spindle failures on RAID systems are just one of those things that you _will_ see. Deciding whether a system deserves some other measure of redundancy is mostly an actuarial, rather than a technical, decision.
And when RAID 6 has a high enough risk that it's worth expanding the scheme everyone will start switching from double parity schemes to triple parity schemes since their much less expensive in terms of spindle count than RAID 6+1.
I don't think you've quite understood the problem described. You can have an infinite number of parity disks, but it does you no good if recovering one data disk causes another data disk to fail.
Imagine a disk fails on every 100TB of reads (10^14). You have ten 1TB data disks. Imagine you keep them in perfect rotation so they've spent 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100% of their lifetime. The last disk dies and you replace it with a new drive (0%). To rebuild the drive you read 1TB from each data disk and use whatever parity you need. They've now spent 11, 21, 31, 41, 51, 61, 71, 81, 91 and 1% (your new disk) of their lifetime and you can read another 9TB before you need a new disk.
Now we try doing the same with ten 10TB disks and the same reliability. The last disk dies and you replace it, only now you must read 10TB from each disk. Instead of adding 1% to the lifetime it adds 10% so that they've spent 20, 30, 40, 50, 60, 70, 80, 90, 100 and 10% (your new disk) of their lifetime. But now another disk fails, you can recover that but then another will fail and another and another and another.
Basically, parity does not solve that issue. If you had a mirror, you would instead copy the mirrored disk with significantly less wear on the disks. RAID is very nice as a high-level check that the data isn't corrupted but it's a very inefficient way of rebuilding a whole disk.
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The problem is IT guys and PHB's that think RAID=Backup.
It's not and it never has been a backup solution. RAID is high availability and nothing more.
RAID does it's job perfectly for high availability and will continue to do so for decades. Sorry but I have yet to see any other technology deliver the capacity I use for my small 30TB Database we have at work. Our Raid 50 array works great. We also realtime mirror that to the Backup SQL server (not for backup of data but backup of the entire server so that when SQL1 goes offline SQL2 picks up the work.)
SQL2 is backed up to a SDAT tape magazine nightly.
RAID does what it's supposed to do perfectly, it's days are not numbered because no other technology other than RAID can provide high availability.
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I'll assume you aren't trolling, and point out that disks work BECAUSE OF the air inside. The heads gain lift.
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Air is necessary for the read/write head to operate. The piece that comes into close proximity of the platter is essentially a tiny hovercraft. It's about the size of a pepper flake, and has a microscopic pattern called an "air bearing" carved into the side facing the platter. Designing this air bearing is an exercise in fluid dynamics -- it is the shape of the bearing and how air flows over it that allows the read/write head to skim over the surface of the platter at a distance measured in microns without actually contacting the surface of the platter.
If the read/write head does contact the surface of the platter, that is called a head crash, and is bad.
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