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I Don't Need Backups, I Use Raid1 (momjian.us)
58 points by keyist on June 1, 2012 | hide | past | favorite | 51 comments

I once started work somewhere where they did software releases by RAID.

Their systems involved shipping a server (effectively an appliance) to the customer with all of the working components on it. However, there was no build or deployment process for these components - so the only way to create a new server was to take an existing one and create a copy.

This was done by opening up a working server running with RAID 1, removing one of the disks and installing the disk into a new server. Let the RAID recover the data onto the other blank disk then remove it and put the other blank disk in and let it rebuild.... result, a copied server!

Same story; different company: http://www.erica.biz/2009/common-mistake-in-business/

It is amazing how even fairly technically-savvy people get sucked into the "RAID=backup" mentality. This story (in the above link) ended up costing the business owner tens of thousands of dollars.

Dud, Flood, & Bud.

Duds are hardware that goes bad, like a disk drive, network adapter, NAS, or server. There are an infinite number of ways and combinations things can break in a moderate sized IT shop. How much money / effort are you willing to spend to make sure your weekend isn't ruined by a failed drive?

Floods are catastrophic events, not limited to acts of God. Your datacenter goes bankrupt and drops offline, not letting you access your servers. Fire sprinklers go off in your server room. Do you have a recent copy of your data somewhere else?

Bud is an accident-prone user. He accidentally deleted some files... the accounting files... three weeks ago. Or he downloaded a virus which has slowly been corrupting files on the fileserver. Or Bud's a sysadmin who ran a script meant for the dev server on the production database. How can we get that data back in place quickly before the yelling and firing begins?

There are more possible scenarios (hackers, thieves, auditors, the FBI), but if you're thinking about Dud, Flood, & Bud, you're in better shape than most people are.

We live in a sad world where most companies don't have a real disaster recovery plan. Many times in my career I've had customers ask me to save them because they "thought" they were backing up but when they went to restore from the {tape|floppy|backup disk} media they found it to be corrupt.

Backup and Disaster recovery strategies seem really easy until you think through all the failure modes and realize the old axiom "You don't know what you don't know" is there to make your life full of pain and suffering.

Years ago my customers would literally restore their entire environments onto new metal to verify they had a working disaster recovery plan. Today most clients think having a "cloud backup" is awesome.. Until they realize in the moment of disaster that they are missing little things like license keys for software, network settings, passwords to local admin on windows boxes etc.

RAID is not a backup strategy. RAID is an availability strategy. Unfortunately, it appears that many people don't understand the distinction.

The community has discussed the idea of adding a feature to specify a minimum streaming replication delay

This is a feature of Oracle, the redo logs are replicated to the standbys as normal, so you have an up to date copy of them on the standby, but only applied after an x hour delay. You can roll the standby forward to any intervening point in time and open it read-only to copy data out.

Less need of it these days with Flashback, of course, but it saved a lot of bacon.

Using mk-slave-delay you can do this with Mysql as well. We always have a slave running behind a day. You can fast forward the slave using the 'START SLAVE UNTIL' command.

Don't MySQL slaves pull from the master, rather than the master pushing? In the Oracle way, the transactions are on the standby, just not applied yet. In the event of a catastropic failure of the primary, you're still covered.

Most companies I've worked for have had some kind of annual fire drill / alarm testing. They announce it the prior week, and then, say, Tuesday at 10am the alarm goes off, everyone files out of the building into the parking lot for 5 minutes, then back inside. In 15+ years (at several different companies), only once has there been an actual fire department call where the evacuation was "real" (even then, there was no actual fire).

In those same 15+ years, mostly working for startups, there have been numerous drive failures. Unfortunately, failure (a) to verify backups before there's a failure, and (b) to practice restoring from backups has often meant that a drive failure means loss of several days' worth of work. In one instance, the VCS admin corrupted the entire repo, there were no backups, that admin was shown the door, and we had to restart from "commit 0" with code pieced together from engineers' individual workstations. That was when I got religious about making & testing backups for my work and the systems I was responsible for...

You must test your backups. I used a commercial backup service that sent daily status emails. It seemed great for months until I realized it had a bug and there was nothing in the archive.

Yep. It helps to think of it as: Backups aren't the end product. Successful restores are the end product.

Cloud backup services have taken away any possible excuse for not remotely backing up any non-ginormous collection of data. It's push-button easy and a lot cheaper and easier than dealing with taking tape backups and moving them offsite.

Not to say that it's the best solution for everyone, but simply that it leaves people no excuse for doing nothing.

The underlying cognitive bias in "I don't need backups, I use raid1" seems to be the quite common one of "I don't do anything stupid, so I don't need anti-stupidity devices" (feel free to substitute "careless" or similar for "stupid"), maybe with a side-order of "if I set up systems that protect me from my stupidity then only stupid people will want to work with me". The fact is, most of us do many stupid things every day--some stupid at the time, some stupid in retrospect--and systems that don't let us recover from them are poor systems.

Never underestimate your RAID controller's ability to fail (silently!) and start writing corrupted garbage to your disks.

I once did a RAID data recovery on a system with a high-end Intel raid controller. The controller failed and they sent a new one - only the new one couldn't assemble the raid properly. It turns out that there was a flaw in the logic for where parity was stored. Normally parity is spread evenly across all the drives - not on this version. I had to reverse-engineer the crazy raid pattern and write a program to deraid it. It had gone undetected - as long as it was running.

and don't underestimate the manufacturers ability to screw it up. Like when people 'upgrade' the firmware in their Buffalo NAS box and after that they can't see the data anymore. Luckily the data was still there and undamaged but it took data recovery to get it back.

EDIT: people didn't like my humor. Well, look, the whole thing that you're buying with a raid controller is...redundancy. So if it's not redundant, failing silently, while telling you it's being redundant, how is this different from, say, paying for a house inspection that doesn't get done? If a raid controller is allowed to silently fail, it becomes a post-experience good.


Meaning that even while you're using it, you have no idea if it works.

My contention is that it's not a raid array if it can silently stop being redundant without telling you.

At best it's an Possibly Redundant Array of Inexpensive Disks.

(The below is how my comment first read.)

(sarcastic) Yeah, it's only prudent to grab a drive out from time to time and make a surprise inspection of whether it's actually filled up a full 4/5th of the way (or whatever) with the actual data the volume is supposed to contain! And the remaining fifth had better look a damn sight like parity information!

Seriously though, a controller that fails like this isn't a RAID controller, since what separates it from a paper plate and a cardboard box. On the paper plate you write "RAID controller" and tape it to an already attached hard drive, and you put the remaining members of the redundant array into the cardboard box. No setup or even connection required!

seriously seriously though, what you're suggesting is unacceptable. that's not a raid controller, that's a scam.

Of course a RAID controller isn't supposed to fail silently, but it can and it does. I can't think of many complex pieces of technology that work 100% all of the time.

You don't think that something that only exists to create disk redundancy is in a different category from complex pieces of technology that don't have this in their name?

I simply disagree that you should "never underestimate" your raid controller's ability to fail silently (which is the comment I was replying to). If this is even on your radar you don't have a RAID controller.

This is literally like saying. "Never underestimate your digest algorithm's ability to hash the same file to different values, making the checksum seem to fail." That's not a digest algorithm, that's a randomized print statement.

A RAID controller you should 'never underestimate' the ability of to fail silently is literally sometimes the same as a paper plate with "raid controller" written on it. Call it "sometimes raid". or "maybe raid" or "more raid". You don't have a raid controller.

In general, then, we shouldn't call RAM memory since it might misremember, we shouldn't call them computers since they might miscompute, we shouldn't call it encryption since it might not encrypt, and we shouldn't call them bicycles since one of the wheels might fall off. Is that about it? I think I can see your point, but as we're nowhere near the point at which machines become as reliable as humans, let alone utterly reliable, I'm not quite sure of the use of the fine distinction you're trying to draw.

No, I don't at all mean in general.

(See my cousin reply here).

That is not at all "about it". I mean, specifically, for the layer that a RAID produces. It's simple. When you add RAID, you add a layer on top of physical hard-drives to make them redundant.

This type of layer has a completely different expectation from all of your other examples. The example in my cousin reply is apt: it would be like expecting a checksummong algorithm (which you're ONLY using to add verification that a file is genuine) to sometimes fail and produce a random checksum in the space of possible checksums the algorithm can produce, instead of the checksum that the algorithm actually produces for that particular file. Or if it has a comparison function, to sometimes fail and say that the file checksums to the provided checksum, regardless of whether it does so.

This is ridiculous: such a layer wouldn't be a checksum, it would be completely different. The idea that I have to physically roll a layer on top of my checksum, to check whether it's currently acting like a randomized print statement or a comparison function whose truth value is randomly negated, is ridiculous.

I don't know how else to put this. Maybe instead of your RAM, bicycle, examples, I can give you these examples: -> Imagine if you are adding a fuse to a circuit to protect it, but the fuse sometimes actually just saves up electricity so it can release it one quick burst and override the circuit. That's not a fuse.

-> Imagine if you hire an auditor to make sure your employees aren't misappropriating funds, since the business involves a lot of cash, but your auditor sometimes just pockets cash. That's not an auditor. You only thought you hired an auditor. The solution isn't to make sure the auditor has an auditor, it's to hire an actual auditor instead of someone you mistakenly think is one.

-> Imagine if you buy insurance, but actually the company sometimes will just spend lawyers on defending having to pay out, even when the event clearly happened and you were clearly covered. That's not insurance - that's a scam. You shouldn't have to insure the layer of insurance with an insurance against the insurance company out-lawyering you. You should get an actual insurance policy.

-> Imagine if you buy a seatbelt, but after buckling it, there is a realistic chance that you really haven't, and it's just a clothing item draped across your body and not attached in any way at any point.

Well if that's possible, that's just not a seatbelt. It's a defective item that was supposed to be a seatbelt but isn't.

The point is, all these examples are optional layers on TOP of a process. If they have a realistic chance of failing as in the above descriptions, they simply are not what they're claiming to be. Their chance of failure should be so low you can't even think about it; if it isn't, you should just hire or buy a different on, since you made a mistake.

I don't know what you're getting at, but I think you are underestimating the chance that your RAID controller will fail.

Actually, I don't have a RAID controller. But at least I don't think I have one!

If I did get one, I wouldn't get one that tended to silently fail, since that would pretty well defeat the purpose of thinking my disks were redundant, wouldn't it?

"Tend to fail silently" is different from "could fail silently at some point, though it is unlikely." You are correct that RAID controllers shouldn't be the former; however, there is absolutely no way to prevent RAID controllers from being the latter.

Could you tell me your reasoning about the last sentence?

If you were talking about anything else, like a normal hard-drive, fine, of course it can fail silently. But the whole thing that a RAID drive is, is another layer on top of hard-drives, to make them redundant and chirp loudly when one dies or starts having wrong data and has to be removed, so that you can replace it and rebuild the RAID.

I mean, all the RAID controller does is write data that is always redundant (even when it thinks all drives are working fine). How is it not possible for it to check for this consistency as well? Especially in Raid-6 etc configurations, which are even more consistent?

Of course, on a probabilistic level random bit rot means "nothing is certain", but on a practical level, how can you not expect a raid controller not to fail silently, when all it does is corral redundant data around, create checksums, verify what's written, etc. It's the whole reason it exists.

To me this is like saying that a checksumming algorithm should be expected to sometimes fail and just return a checksum chosen randomly from the space of all possible checksums, instead of the checksum actually produced by the algorithm for that data.

That's ridiculous. I shouldn't have to even think about putting another layer on top of the checksum, so that I can checksum it. The very idea of having to do that means you don't have a checksumming algorithm.

This thing should be right up there with bitrot causing bash to execute an rm -rf whenever you drop down to root. Sure that's possible, but that's not even in the scope of anything you have to think about.

To me, a RAID is a layer on top of hard-drives that makes them redundant. Any controller that has a realistic chance of failing silently simply does not fit that definition.

Yes, the purpose of RAID is not to fail silently; but it's hardware, and hardware can be flawed. That's just a fact. There are different levels of RAID precisely because there are different levels of redundancy - that is, different extents to which the possibility of failure is minimized. No hardware is flawless, though.

Please note that I have not said "it is likely to fail" or "you should expect that it will probably fail." I agree that it shouldn't be something that keeps a person up at night. But the simple fact is that, when data is important, you should prepare for that possibility (and others) by backing up. RAID does not solve all problems, and it is not guaranteed, as unlikely as failure might be.

Moreover - in saying that it simply isn't RAID if it ever fails silently, you're attempting to define away a nonsemantic problem. The point of a starter motor on a car is to start the engine. If the starter motor fails to start the engine, I guess I could make an Aristotelian argument that it has ceased to be a starter motor, or even perhaps that it was never a starter motor in the first place. But what practical good does that do anybody?

All hardware has the potential to fail. Yes, people should buy hardware that is less likely to fail. I'm pretty sure they already do that, though.


You might read this first:


you can reply to that as well here if you want.

I think we're in very general agreement. Although you yourself did not say "it is likely to fail" or "you should expect that it will fail", this is exactly the sentiment I was replying to was.

Regarding your "all hardware possibly failing" and the example of a starter motor to imply that I am trying to disappear a technical problem with a semantic argument, I think I am (especially in that cousin reply) being quite a bit more specific.

Basically, when it comes to safety mechanisms that exist as a layer on top of a process and aren't necessary at all, I simply shouldn't have to even think about reinventing another safety mechanism on top of the safety mechanism. Get one that isn't defective.

A hard-drive isn't defective just because it fails: it's expected to. A RAID controller is also expected to fail...JUST NOT SILENTLY.

In the seatbelt example: should you even think about having to tie your seatbelt to the buckle with sturdy rope, for real safety in case the seatbelt just doesn't buckle when it seems to, or comes undone like a ripped shirt button at the slightest firm tug?

No. You should get an actual seatbelt.

Basically, the standard you hold a control layer to is different from the standard you hold an underlying process to.

It would be like the difference between your brake failing and your (for added safety) handbreak failing, which you only engage on top of the motor's brake anyway. If the motor brake fails you would start rolling (if you're on a bit of an incline). But you shouldn't even have to think about a hand-brake 'just failing' in the same condition.

Sure it can fail if you are being towed without being lifted, or whatever, in an extreme situation. But in a normal situation?

Basically, it is a difference of both category/kind AND of degree.

I am certainly not saying that a parking brake can never fail. I am not saying a raid controller can never fail.

I am saying that both of these, when they are layers on top of a normal process, should be out of sight, below your threshold of having to control for it. If they're not, you need to get a different one.

You don't get six insurance policies against the same earthquake possibility, hoping that they won't ALL decide to out-lawyer you or go bankrupt. You get real insurance that's properly reinsured. Check up on them. Find a real one.

Raid failure is fine. Silent raid failure is not fine.

(checksum failure with an exception is fine; checksum failure with no exception, warning, or error, just a random checksum produced - or a check randomly passing when the checksum doesn't match the one you provided, is not okay. fix your checksum, get a real one - don't build another layer on top, for the cases that your checksum is a randomized print statement or your insurance policy a monthly donation from you to a non-charitable organization that puts aside a portion to out-lawyer you with if you try to make a claim, with the rest spent on advertising or being their profit. That's not an insurance policy, that's a scam.)

Yeah, I think we're in general agreement. With RAID that is the way RAID is supposed and expected to be, the chance of silent failure very, very small.

In case anyone is confused, what happens when the server catches fire or is stolen?

Or even more likely, the RAID configuration is lost.

Or more commonly according to the article: a user accidentally removes (and perhaps even shreds) the wrong file.

I've worked with tapes offsite before hard drives became cheap enough to use for backup (of the appropriate amount of data of course).

My current setup goes as follows:

Servers in colocation get backup daily to a server in the office. That server in the office then gets backed up daily to a iosafe.com fire and water proof hard drive in the office which when I get a chance will be bolted to the desk for further security. Clones are then made of that server biweekly (which are bootable) and one is kept in the office and one is taken offsite.

So the office server is the offsite for the colo server and the clone of that is the backup for the office.

The clones allow you to test the backup (hook it up and it boots basically).

Added: Geographically the office is about 3 miles from where the backup of the office is kept. But the office is about 40 miles from where the colo servers are kept.

Fun anecdote: years ago, I worked for a department that had its server on a RAID setup, and when I asked about backups they said, "Don't worry". One day, a drive failed. They replaced it and started restoring from the other drive - which failed mid-sync. The two drives were from the same production lot and died literally within 12 hours of each other.

So: back up your data.

It happened to me. It happens the drives had a bug where there death time was hardcoded in the drives. Past a predetermined time of usage, they would fail. Of course, I never believed RAID was a backup strategy so I was able to recover.

I've had a raid controller fail by restoring as a raid 5 array where previously there was raid 10 array.

This is one of the problems I have with SQL Azure. They have yet to implement a satisfactory backup option: http://www.mygreatwindowsazureidea.com/forums/34685-sql-azur...

It's amazing to me that anyone is actually arguing that RAID negates the need for backup. That is just dumb.

If I ever heard an SA working for me advocate that position, I would probably get them off of my team ASAP.

Maybe I'm an idiot, but the vast majority of times I've needed to recover something from a backup are due to user error, not hardware failure. RAID sure doesn't help there.

A backup has not much value when stored in the same physical location with the original data. Any fire/flood/robbery will destroy all the data.

Of course it has value; fast recovery of data after hardware failure.

You still want off-site backups as well of course, in case of something more extreme, but they're usually going to be slower to recover from than nearby backups.

I was burnt once when both hard disks in my Raid1 fail at the same time, unlikely but it happened. And Raid is not a backup strategy.

Not as unlikely as many people would like to think! If the two drives are from the same production lot, they may suffer from a common manufacturing defect. And because they are in the same chassis, if a server fan fails, both drives may subsequently fail due to thermal damage within a very short interval.

Even if they don't fail simultaneously, the mirror drive may fail (or even more likely) have read errors or flipped bits that will corrupt the restore or render it impossible.

Personally, I don't place much trust in any RAID configuration other than RAIDZ2 (ZFS; you can lose two drives and still recover all your data; every block is checksummed to avoid reading or restoring corrupted data).

But even ZFS can't protect you against accidental deletion, fire, theft, or earthquake.

There's RAIDZ3 now!

I do RAID data recovery for a living. Customers always think that '2 drives failed at the same time'. Thats not what usually happens. What really happens is that 1 drive fails and the raid does what it supposed to do - it keeps on functioning. Sometimes for months. Then the second drive fails and it stops. They call in the IT guys and it appears that '2 drives failed at the same time'. Not.. Most raid data loss is caused by IT guys trying to fix it - because its a raid and its redundant and it cant be hurt so let me start swapping stuff and rebuilding and destroying...

redundancy != backup

Actually, backups are redundancy.

You just have to structure your redundancy to survive multiple threat models.

In which case, the redundancy offered by RAID alone is grossly insufficient.

There is a very important difference. Redundancy doesn't protect you against bad changes to your data, backups do. Backups should ideally be immutable, and append only. What happens when a disgruntled employee runs 'sudo rm -rf /'? With redundancy the effects of that decision are dutifully cloned on all media. With backups one has the ability to rollback to an older state.

Backups are redundancy out of firing range of problems like, say, hard drive meltdown, operator error, etc.

I've had gruntled employees, occasionally myself, run some variant of 'rm -rf' unintentionally far more often than I've had to deal with the other sort.

If you feel my grandparent post was advocating against backups, I'd strongly suggest you re-parse it. It's distinguishing between varieties of redundancy.

Using RAID1 as backup is OK as long as you occasionally run 'sudo rm -rf /' for maintenance.

for most of my stuff dropbox pro (with packrat addon for unlimited file histories) + github handle all my backup needs. Of course this wouldnt work for all scenarios but i dont work with/have loads of huge files.

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