Storage that makes you think

Anandtech has a nice article up right now that compares SAS, SATA, and SSD drives in a database environment. Go read it. I'll wait.

While the bulk of the article is about how much the SSD drives blow the pants off of rotational magnetic media, the charts show how SAS performs versus SATA. As they said at the end of the article:
Our testing also shows that choosing the "cheaper but more SATA spindles" strategy only makes sense for applications that perform mostly sequential accesses. Once random access comes into play, you need two to three times more SATA drives - and there are limits to how far you can improve performance by adding spindles.
Which matches my experience. SATA is great for sequential loads, but is bottom of the pack when it comes to random I/O. In a real world example, take this MSA1500CS we have. It has SATA drives in it

If you have a single disk group with 14 1TB drives in it, this gives a theoretical maximum capacity of 12.7TB (that storage industry TB vs OS TB problem again). Since you can only have LUNs as large as 2TB due to the 32-bit signed integer problem, this would mean this disk group would have to be carved into 7 LUNs. So how do you go about getting maximum performance from this set up?

You'll have to configure a logical volume on your server such that each LUN appends to the logical volume in order, and then make sure your I/O writes (or reads) sequentially across the logical volume. Since all 7 LUNs are on the same physical disks, any out-of-order arrangement of LUN on that spanned logical volume would result in semi-random I/O and throughput would drop. Striping the logical volume just ensures that every other access requires a significant drive-arm move, and would seriously drop throughput. It is for this reason that HP doesn't recommend using SATA drives in 'online' applications.

Another thing in the article that piqued my interest is there on page 11. This is where they did a test of various data/log volume combinations between SAS and SSD. The conclusion they draw is interesting, but I want to talk about it:
Transactional logs are written in a sequential and synchronous manner. Since SAS disks are capable of delivering very respectable sequential data rates, it is not surprising that replacing the SAS "log disks" with SSDs does not boost performance at all.
This is true, to a point. If you have only one transaction log, this is very true. If you put multiple transaction logs on the same disk, though, SSD becomes the much better choice. They did not try this configuration. I would have liked to have seen a test like this one:
  • Three Data volumes running on SAS drives
  • One Log volume running on an SSD with all three database logs on it
I'm willing to bet that the performance of the above would match, if not exceed, running three separate log volumes running on SAS.

The most transactional database in my area is probably Exchange. If we were able to move the Logs to SSD's, we very possibly could improve performance of those databases significantly. I can't prove it, but I suspect we may have some performance issues in that database.

And finally, it does raise the question of file-system journals. If I were to go out and buy a high quality 16GB SSD for my work-rig, I could use that as an external journal for my SATA-based filesystems. As it is an SSD, running multiple journals on it should be no biggie. Plus, offloading the journal-writes should make the I/O on the SATA drives just a bit more sequential and should improve speeds. But would it even be perceptible? I just don't know.


"I would have liked to have seen a test like"..... goes for any commercial review

I have two thoughts on this.First, yes, I wonder if the gains would be perceptible. Often, as professional geeks, we strive for theoretical improvements that, frankly, no one can actually perceive without special monitoring tools.Second, how long will it be before you hit the theoretical limit on writes to these new SSD drives? I haven't really read much about that and, in fact, don't know if there is even any data on that, especially when it comes to database usage. Maybe I'm too conservative, but, I've wondered about that as so many places start adopting these SSD drives in production environments. When will the inevitable problems start to show up?