Why raid 5 2009

If this was even remotely accurate, RAID 5 rebuild failures would be so common as to make this a truism. So, show me. So far, I've yet to receive a single reference. Oh, I'm not talking about a mathematical analysis of UREs. Yes, I've read them. And, yes, their wonderfully interesting. I'm talking about a real, live, power-on field demonstration. The faithful say, "That's just anecdotal evidence. Just because you've never seen it happen doesn't make it not real. But that is not a persuasive argument.

Nor is simply calling it an anecdote proof of anything. I'm willing to be convinced. But I want to see the proof from a reliable source. Here are some pearls I've distilled down from the discussion. I'm reasonably confident that these are correct, but will be grateful for clarification if someone thinks I've erred. The error we're taking about is a "soft" URE. That is, a mis-read of magnetic data that, in a normally functioning array, would be re-written by the controller and "erased.

UREs can and do happen at any time, not only during a rebuild of a array. The majority of the time, they are corrected [edit - by a mechanism at a higher level than the drive itself] and rewritten transparently. The predicted occurrence of a URE is different for different quality drives. So, if you said to me, "I want to build a drive array that has A. You get A, B, and C. Should you go back and rip out all your old RAID5 setups that are 4x73 or 4x or 4x and based on HP enterprise drives because they are suddenly dangerous?

Raid 5 2 HDD Failure. Recovering data from RAID 5 with 2 drives failed! Both of these cases are 2-drive failures. Not failures during a rebuild. A horrible thing to happen, but not what I'm talking about.

And, would you agree, just as fatal when both disks that are mirrored in your RAID 10 die simultaneously? This is the article I quoted in my original post. Again, a great theoretical analysis. Also, data on the disk is addressed by sectors, so if one fails, this means you typically have at least bytes lost. It's true that even that might not completely break some kind of large media file, but you have to remember that RAID5 is a layer below your file system data, so if an error occurs when its trying to rebuild itself, it will not be able to give you your data back.

You might be able to recover a lot of your data from an error of this kind, but don't count on the RAID implementation to do it for you.

Yea, because we all backup 12TB of home data to an offsite location. Mine is my private evil island, and I've bioengineered flying death monkeys to carry the tapes for me. They make 11 trips a day. I'm hoping for 12 trips with the next generation of monkeys, but they're starting to want coffee breaks.

I'm sorry, but I'm getting seriously tired of people looking down from the pedestal of how it "ought" to be done, how you do it at work, how you would do it if you had 20k to blow on a backup solution, and trying to apply that to the home user.

Even the tape comment in the summary is horseshit, because even exceptionally savvy home users are not going to pay for a tape drive and enough tapes to archive serious data, more less handle shipping the backups offsite professionally.

This is serious news. As it stands, the home user that actually sets up a RAID 5 raid is in the top percentile for actually giving a crap about home data. This, at the same time as more and more people are actually needing a decent solution. Now, while 1TB onto DVDs seems like quite a chore and I'll admit it's not trivial , some level of data staging can help out immensely, as well as incrementally backing up files, not trying to actually get a full drive snapshot.

Say you backup like this: my pictures as of 21oct my documents except pictures and videos as of 22 oct etc. With digital cameras I would argue that home movies and pictures are the two biggest data consumers that people couldn't backup to a single dvd and that they would be genuinely distressed to lose. Yea, but DVD is transient crap. How long will those last? A few years? You cannot rely on home-burned optical media for long term storage, and while burning 12 terabytes of information on to one set of dvds double layer may not seem like a big deal, having to do it every three years for the rest of your life is bound to get old.

For any serious storage you need magnetic media, and though we all hate tape, 5 year old tape is about a million times more reliable than a hard drive that hasn't been plugged in in 5 years. So either you need tape in the sort of quantity that the private user cannot justify, or you're going to have to spring for a hefty RAID and arrange for another one like it as a backup.

Now, what do you do if you can't rely on RAID? No other storage is as reliable and cheap as the hard drive. You can still have failures, and as hard disk sizes increase, the amount of data jeopardized by a single failure increases as well. Sure, right now. The first hard drive I ever bought was 8 megabytes and cost dollars.

It has serious potential for long term storage. Yea, it's too expensive But in the long run it's the most promising thing out there. I agree that SSDs are inevitable I've got something in the area of GB of data on the machine which I'm currently using to type this, but very little of that data has any intrinsic or sentimental value to me.

Most of it is applications and games that could easily be reinstalled from the original media or re-downloaded. I have 11 old hard drives sitting in the closet should I ever need that data, and the likelihood of a hard drive failing in the first year after the first 30 days is phenomenally low. I just wish all the density improvements that hard disks get would propagate to tape. Tape used to be a decent backup mechanism, matching hard disk capacities, but in recent time, tape drives that have the ability to back up a modern hard disk are priced well out of reach for most home users.

Pretty much, you are looking at several thousand as your ticket of entry for the mechanism, not to mention the card and a dedicated computer because tape drives have to run at full speed, or they get "shoe-shining" errors, similar to buffer underruns in a CD burn, where the drive has to stop, back up, write the data again and continue on, shortening tape life.

Of course, disk engineering and tape engineering are solving different problems. Tape heads always touch the actual tape while the disk heads do not touch the platter unless bumped. I have CD-Rs dating back to or that are just fine -- and they're off-brand media too. I've got a mainframe circa that's been using the same type of drive since Last year we pulled all the year-end financial numbers off the yearly backups dating back to that point.

Zero failed tapes. It can degrade in anywhere between 2 to 5 years Longer if you keep it in a cool dark place, but not 20 years. Oh come on. Do you have 12TB of home data? And if you do, it's not that hard to have another another 12TB of external USB drives at some relatives place.

It's not that hard. Buying a computer system you cannot afford to properly use is crazy. Yes, some people are crazy, and those crazy people are going to lose data, but there's no sense in defending it. Well, i guess i'm crazy, i have 3TB of space on my home PC, and no way to back it all up offsite. I do have some important folders from one drive automatically copy to another drive periodically, so if one drive dies the other will be okay, but if i lose them both or the place burns down or i get a nasty virus, it's all going to hell.

Most of my space is taken up by pirated HD movies. And porn, lots of porn. Either way, i'm not too worried if i lose that, it's just the things i back up i really care about. This article has inspired me to look into Tape Backup but i worry that it's not cost effective i haven't looked yet. I should fill up some tapes with a few hundred gigs of porn, write "confidential" on them, and stash them in a bag, under some bush, across the street from HP near my apartment.

I'm sure some curious person would come looking, only to discover their contents and wonder why the hell someone went to all that trouble Prioritize your data.

I cannot believe that a home user has 12TB of important stuff. Back up your critical records both on site and off [1]. Back up the important stuff on site with whatever is convenient. Let the rest go hang. Few home users will have a critical need for that stuff beyond the life of the media. Any that do can copy it over every five years, and take the opportunity to delete the obsolete stuff.

Wow, how incite-ful. Doesn't matter what the discussion is, some geek is bound to weigh in with all the shortcomings of any idea. Newsflash: there is no perfect backup! No method is foolproof, especially when it's bound to be boring as hell, and you've got an inevitable human factor.

You get lazy moving the tapes offsite, you put off fixing a dead drive because there are 4 others, you wipe your main partition upgrading your distro and forget that your CRON rsync script uses the handy --delete flag, and BOOM wipes out your backup. My data backup scheme is to steganographically embed my entire filesystem into nude pictures of Sarah Palin, and then upload them to usenet. Pointing out what we all already know doesn't do anything helpful. In today's marketplace it is usually IT left holding the bag when things go south anyhow Jesus Christ, you must be one unlucky soul.

Do you live your entire life in a worst-case scenario? This system may not be foolproof what is? Not too shabby for what I would consider to be a fairly robust backup system for a home user.

I suppose the biggest challenge is deciding what goes into rsnapshot. But this is mostly video content, and really, if I lose my mythtv shows, it is not exactly as catastrophic as if I lost, say, my quickbooks data.

There are a lot of things that keep me awake at night, but loss of important data is not one of them. The vast majority of Egypts writings were stored on perishable papyrus, not carved or painted on stone. Of all that they ever wrote or stored, we have but the tiniest fraction remaining.

If we lost technology today, there would be nothing left but paper in 20 years. In a thousand, there wouldn't even be much paper. That's why I chisel all my data ones and zeros onto stone tablets. In a few years the pile of stones will be taller than Everest. And in a thousand years some bearded guy will discover couple of those stones, come down the mountain and will base a religion around it. These things are cyclical. The Egyptians found a way to preserve their message over thousands of years, surely we can come up with something.

And they would have saved future generations from vast amounts of confusion and effort, if they'd only been a little more diligent backing up their pyramid construction HOWTO files.

RAID has protected me from random disk failures. RAID 0, psudo-ironically, is not redundant at all. RAID 1, often called mirroring, are the arrays that are redundant. In RAID 0, which is actually a marketing term, there's striping, but no redundancy that can infer the contents of a missing member of the array. From the perspective of availability, it has none. As you cite, RAID 1 is a mirrored pair, usually the same type of drive, and it also is likely the fastest RAID-- and most expensive in terms of available net data after redundancy for availability.

Well, Windows does. NTFS is actually a pretty good file system. It's probably because it was originally designed by IBM. If I striped the JBOD into 6TB 7 drives and one drive failed all the near-line data would be virtually off-line and certainly read-only while the array re-built. With JBOD, should a. I've had TWO occasions where it has failed me. Once, a lightning strike that zotched both drives. The second time a rubber isolator failed in the case and the master drive fell onto the backup.

In both cases the bad spots in the two drives were different so I got back most of my data, but now I use Mozy as well as mirroring. You get your first RAID controller from a trusted friend. That looks fast. With a controller for swap, one for data and one for the system will Windows now be fast? Yeah, sorta. Those drives sure are quiet - from a click-click busy noise perspective, NOT from a "sounds liks a jet airplane when running" perspective.

Heat is an issue, too. I just use a laptop now and make several sets of backup DVDs or just copy to spare drives. I love RAID to death. But it's really only marginally worth the effort in the real world. But if you need fast, OMG. I mean, WTF? Many people regard RAID as something magical that will keep their data no matter what happens.

This pattern of data access is extremely common for all sorts of applications. And this raid 5 "problem" is simply the fact that modern sata disks have a certain error rate. But as the amount of data becomes huge, it becomes very likely that errors will occur when rebuilding a failed disk.

While using RAID 6 instead may seem like a solution, where RAID 6 is two drive failures are allowable instead of just one, the increased redundancy may not be cost effective. Also, as hard drive capacities continue to increase exponentially, year after year, even RAID 6 may soon become prone to the same problems.

When single disk drives become terrabytes in size, even a direct drive-to-drive copy may commonly encounter these read errors. The use of disk drives that have smaller capacities and improved unrecoverable read rates could be a solution to avoid these potential headaches.

The problem comes from the increasingly tight data density packed onto drive platters. Using traditional means, bit magnetic poles can often leak their polarity onto other adjacent bits, causing a switch in an otherwise normal bit. Manufacturers have switched to perpendicular recording methods to avoid such problems and increase density, but even this method has its physical limits. Manufacturers will have to find more creative solutions down the road if drives are going to exceed 2TB in size.

Topics Storage. That order of magnitude makes a big difference on the calculations of expected read errors. Let me set the stage by going back over the probability equation used by Robin Harris and Adam Leventhal. We can use sectors, bytes or bits for this calculation as long as we stay consistent.

He calculates that a GB disk array has million byte sectors. He determines the chance of array failure during a rebuild is only 0. This means that on average, 0. I wanted to flip this equation from probability of success to calculate the probability of a failure directly and then express it as a percentage so I just subtract the success rate from 1 and multiply times Personally I think it is easier to do all this by leaving all the numbers in bits.