Practical SQL Server

SQL Server Database Corruption, Part III: Preventing Corruption

In previous posts we touched upon an overview of what causes corruption and looked at ways to simulate corruption. In this post we’ll talk about more about corruption – in terms of addressing how you can prevent it. (Though, if you’ve been paying attention in previous posts, you’ll probably guess what’s coming up).

How to Prevent SQL Server Database Corruption

In answer to the question: “How can I prevent corruption?” the simple, and honest, answer is: “You really can’t”. As covered in previous posts, corruption is ‘more or less’ a random problem that occurs at the disk subsystem level and really can’t be prevented. (Or as I like to tell all of my consulting clients: “There’s a reason that EVERY major Operating System out there ships with something like CHKDSK.”)

So, while disk/storage errors should be rare within any production system, the fact is that these kinds of errors aren’t so much a question of IF they’ll happen, but WHEN they’ll happen – simply because the reality is that magnetic storage is insanely complex and subject to the occasional ‘hiccup’ that can result in corruption.

However, given that corruption SHOULD be a relatively rare problem that you will encounter in your environment, it IS safe to say that if you keep bumping into seemingly-regular or semi-regular problems with corruption, it’s fair to say that you may need to start becoming concerned about whether there’s a problem with your hardware or possibly even drivers and so on. (The problem, however, is that troubleshooting these kinds of problems is insanely hard – so if you do start to recognize regular problems with corruption it may be time to start thinking about migrating critical systems to new hardware.)

However, even though you can’t prevent corruption, that doesn’t mean that you are powerless against its effects. In fact, not only does SQL Server readily accept or anticipate that disk subsystem errors can and WILL occur, but it also provides a number of great tools and functionality that can be used to address and compensate for corruption when it happens. Consequently, by taking advantage of these features, it’s possible to become very protected against the kinds of damage that corruption could otherwise cause. All you really need, then, to protect ‘against’ corruption is a decent amount of pro-active effort that can help protect you against the EFFECTS of corruption.

Caveat: Incurable Corruption

Of course, while the underlying purpose behind this series of posts is to provide a solid overview of ways to detect, correct, and protect against corruption, there ARE some kinds of corruption from which you can’t recover – such as corruption that happens in the ‘boot page’ (or page 9) as mentioned in my previous post. So, in cases where you bump into these kinds of corruption problems, the only thing you’re really going to be able to do is resort to using backups. That said, backups happen to PRETTY MUCH be the KEY to responding to most OTHER forms of corruption as well. So, in other words: one huge component to being able to properly deal with corruption when it happens (not if it happens) is to have viable, regularly tested, backups on hand – so that you’ll have every option possible at your disposal for dealing with corruption.

The Key to Dealing With Corruption

So, given that corruption can’t really be prevented and given that some forms of it are actually so nasty that you’ll need backups (though the truth is that many/most forms of it can be nasty enough that you’ll need PART of your backups in many cases), the next question becomes: “What is the key to being able to successfully deal with corruption when it occurs?”

And, in my experience, Without a doubt, the best and easiest way to address with database corruption within SQL Server environments is to detect it early and address it BEFORE options to address it run out – because, as we saw in the previous post, it IS possible for corruption to happen and then, effectively, go unnoticed for a potentially LONG time if nothing ends up querying the locations or pages that were trashed by corruption.

In other words, imagine a mission-critical database that happens to encounter corruption during when writing data that IS important but which, for the sake of argument, maybe doesn’t get queried except at the end of every month. If this corruption goes unnoticed for, say, a few weeks its entirely possible in many organizations that the use of rolling backups will severely hamper the ability to go in and address this problem once it’s discovered – simply because the backups that could have best been used to recover from this issue will have been purged, lost, etc. Whereas, on the other hand, if this problem were found within just a few days (for example) then there would likely be more options available in terms of addressing this problem.

Consequently, the biggest key to being able to deal with corruption is to have as many options and possibilities at your disposal as possible – and that, in most cases, can only happen when corruption is discovered relatively early on. Otherwise, corruption almost always becomes more expensive and less easy to deal with.

Up Next

So, with those ideas in mind, my next post will take a look at how and why the use of CHECKSUM page verification is such a great thing. Then, in a subsequent post we’ll take a look at ways to pro-actively be alerted to whenever SQL Server detects any kind of disk problem related to data purity, and then we’ll keep ploughing our way through some tips, techniques, approaches, and examples of how to deal with corruption once it’s been discovered.

Part IV: CHECKSUM Page Verification

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