SQLIO is a fantastic utility for testing the abilities of the disks in your system. It has a very unfortunate name though, since it's not really a SQL Server testing utility at all. It really is a disk utility. They ought to call it DiskIO because they'd get more people using I think. Anyway, branding is not the point of this blog post. Writes are the point of this blog post.

SQLIO works by slamming your disk. It performs as mean reads as it can or it performs as many writes as it can depending on how you've configured your tests. There are much smarter people than me who will get into all the various types of tests you should run. I'd suggest reading a bit of what Jonathan Kehayias (blog|twitter) has to say or wade into Denny Cherry's (blog|twitter) work. They're going to do a better job than I can describing all the benefits and mechanisms around using this excellent piece of software. My concerns are very focused.

I needed to set up a series of tests to see how well our product SQL Storage Compress worked. I wanted to know the effects it would have on a system, the disk for sure, but also memory and CPU. How to stress the system? SQLIO of course. But when I set it up and ran it, following the documentation that comes with it, I was seeing better than 99% compression on the files. Don't get me wrong. Our product is magnificent, wonderful, all things great and beautiful, gets you coffee in the morning and is made mostly from bacon. But 99% compression. No, it's not that good. So what's up?

Well, it's the configuration. The default mechanism is to load up a file, something large that will overwhelm your disk cache. You're instructed to load the file with a character 0x0. I never got a computer science degree. I went to film school. Because of this, I didn't memorize ASCII tables so when I saw this, I thought it was zero's or something. Nope. It's NULL. That's right, you're making a very large file, but you're filling it with NULL values. That's actually ok when all you're testing is the disk sub-system. But, when you want to test a compression and decompression, that can be an issue.

I got around this fairly quickly. Instead of generating a file filled with NULL values, I just copied a database file for my tests. And to test it with SQL Storage Compress, I used a database file that had already been run through compression (about 40% compression on that file if you're interested). Now the reads were taken care of. I am seeing very realistic performance from decompressing the information for reads through SQLIO. But what about writes?

Well, the issue is, what does SQLIO write? I don't have access to the code. But I do have access to the results. I did two different tests, just to be sure of what I was seeing. First test, use the .DAT file as described in the documentation. I opened the .DAT file after I was done with SQLIO, using WordPad. Guess what? It's a giant file full of air. SQLIO writes NULL values. What does that do to compression? I did the test again on a copy of an uncompressed database file. Then I ran the original and the SQLIO modified copy through ZIP to see what happened. I got better than 99% compression out of the SQLIO modified file (original file of 624,896kb went to 275,871kb compressed, after SQLIO it went to 608kb compressed).

So, what does SQLIO write? It writes air. If you're trying to test it with compression or maybe some other type of file storage mechanism like dedupe, you need to know this because your tests really won't be valid.

Should I find some other mechanism for testing? Yeah, if all I'm interested in is establishing performance to my own satisfaction, yes. But, I want to be able to compare my results with other people's results and we all need to be using the same tool in order for that to happen. SQLIO is the common mechanism that most people I know use to establish disk performance behavior. It'd be better if we could get SQLIO to do writes in some other fashion.

Oh, and before I go, I get to brag a bit. Measuring IOPS, SQL Storage Compress outperforms my disk alone by about 30%.