SANs are different

Simple title...but to me, it makes sense, especially when we're talking about disk performance.

My understanding of disk performance has changed dramatically these last few days. I went into it believing that it was all about disk speed and max throughput, so an array of 6 15k SAS disks in RAID5 was fast to me, and there was no way anything SATA could equal it.

However, when you speak of IOPS (I/O per second) - a term thrown around like jellybeans by SAN sales people - it really comes down to spindles, as in, the number of disks you have.

A good metaphor someone told me was to think of a library. It's a small thing for one person (one disk) to get six books that are on the same shelf. It's even easier for six people (the six SAS disks) to look for 6 books, especially if they are all together. However, in an SQL environment, requests are asking for data all over the place.

So, if you then think of one person trying to get six books from all corners of the library, it makes sense for you to have one person for each book - in the time it takes one person to get one book, you have actually gotten six.

But...then we move from our RAID5/6 disk array to a 16 disk array...sure, the 'people' getting the books are a little slower, but you have an extra 10 people! With the EqualLogic option we're looking at, we'll be doubling the number of disks next year, meaning we would have gone from 6 spindles on our heaviest of duty SQL DB disk array to 32 spindles... Yeesh!

The SATA option we're looking at is probably 25% slower than the 15k SAS unit we're testing this week, but I'll have concrete numbers next week when we test out the 16x250GB disk SATA array. The 500GB option may even come into play, as the price difference is not that big.

Let's now look at throughput.

You can apply the analogy to the size of the door you're trying to fit the books through. So, you've retrieved 100 books, and you can just fit them through the door. But what if you need more than 100 at a time? Throughput allows you to carry a certain amount of books through the door at one time.

It's all well and good to boast 150MB/s max throughput for this RAID5 array, specifically for sequential writes - they monster the SAN speeds. However, it still comes down to what the SAN does best - allow fast access for everything that needs disk.

More to come in Part 2.


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