Tuesday, May 10, 2011

ASM – It's not just for RAC anymore


I'm super critical of Oracle when they screw stuff up or try to push technology in a direction that's bad for DBAs. You'll be hearing some rants about it in upcoming posts. But I also think that Oracle is a company that is actually good for the direction that technology is heading, unlike some companies whose names begin with "Micro" and end with "soft". Yes, they're a vast, stone-hearted corporation that would sell their grandmothers to raise their stock price. So is every other technology company – get used to it. But when they do something right, I'll be fair and sing their praises. Once every version or so, Oracle does something that really changes the game for DBAs. In version 8 it was RMAN. In 9i it was locally managed tablespaces. In 10g, it's definitely ASM - Automatic Storage Management. Yeah, I know this is kinda old news - ASM has been out for a good long while. What surprises me, though, is how many DBAs think that ASM is only useful for RAC architectures. "I don't run RAC, why would I need ASM?"

When ASM came out, it both intrigued and terrified me. The claim that it could produce I/O performance almost on par with raw devices without all the grief that comes with using them was exciting. But the idea of putting your production data on a completely new way of structuring files was pretty scary. I trust filesystems like UFS and ext2/3 (maybe even NTFS a little, but don't quote me) because they've stood the test of time. If there's one thing a DBA shouldn't screw around with, it's the way that the bits that represent your company's data are written to disk. I'm skeptical of any new way to store Oracle data on disk, since I'm the loser that has to recover the data if everything goes south. So I entered into my new relationship with ASM the way you should – with a whole lot of testing.

I originally moved to ASM out of sheer necessity. I was running RAC and using a woeful product called OCFS – Oracle Clustered Filesystem – to store the data. Performance was bad, weird crashes happened when there was heavy I/O contention, it wasn't pretty. Nice try, Oracle. It's cool that it was an open source project, but eventually it became clear that Oracle was pushing toward ASM as their clustered filesystem of choice. To make a long story short, we tested the crap out of it and ASM came through with flying colors. Performance was outstanding and the servers used a lot less CPU, since ASM bypasses that pesky little filesystem cache thing. In the end, we moved our single instance databases to ASM as well and saw similar results. It's true that, since you give Oracle control of how reads and writes are done, ASM is a very effective global filesystem for RAC. But the real strength of ASM is in the fact that its a filesystem built specifically for Oracle databases. You don't use it to store all your stolen mp3 files (unless you're storing them as blobs in the database, wink), you use it for Oracle datafiles. You give Oracle control of some raw partitions and let it go. And it does a good job. Once you go ASM, you never go back.

I'm not going to do a sell job on the features of ASM, since I don't work for the sales department at Oracle. Really, the positives for ASM boil down to three key features. 1) It bypasses the filesystem cache, thus going through fewer layers in the read/write process. This increases performance in essentially the same way that raw devices do. 2) It works constantly to eliminate hot spots in your Oracle data. This is something that your typical filesystem doesn't do, since it takes an intimate knowledge of how the particular application (in this case Oracle) is going to use the blocks on disk. Typical filesystems are designed to work equally well with all sorts of applications, while ASM is specialized for Oracle. 3) It works (with Oracle) as a global filesystem. In clustered systems, your filesystem is crucial. It has to be "globally aware" that two processes from different machines might try to modify the same block of data at the same time. That means that global filesystems need to have a "traffic cop" layer of abstraction that prevents data integrity violations. Normally this layer would impact performance to a certain degree. But ASM gives control to Oracle, which has a streamlined set of rules about what process can access a certain block and prevents this performance loss.

So consider using ASM. Even if you don't run RAC, benefits #1 and #2 make it worth your while. Our DBA team has been using it religiously on both RAC and non-RAC systems for years without any problems.

Of course, we're talking about Oracle here, so leave it to them to take the wonderful thing that is ASM and screw it up. Next time I'll tell you how they did just that in version 11g.

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