Thursday, 4 September 2008

DMX Configuration Options

I've been looking at DMX configuration options this week. Essentially the question is how best to lay out a DMX-3 or DMX-4 array with a tiered configuration. For me there are two options and it's pretty clear which I prefer. First a little background. The following diagram shows the way DMX drives are deployed within a fully configured array. The array is divided into "quadrants", splitting each drive bay into two. Back-end directors (BED) provide connectivity to the drives as represented by the colour scheme. There are up to 4 BED pairs available for a full configuration.

Option 1 - Dedicated Quadrants

One option is to dedicate quadrants to a workload or tier. For example tier 1 storage gets given quadrant 1. Theoretically this should provide this tier with uncontended back-end bandwidth as all the tier 1 storage will reside in the same location. What it doesn't do is let tier 1 storage utilise unused bandwidth on the other BEDs, which as the array scales, may prove to be a problem.

There's also the question of physical space. As the drives are loaded into the array, if only a small number of them are tier 1, then potentially cabinet space is wasted. Either that or the configuration has to be build in an unbalanced fashion, perhaps placing more lower tier storage to the right of the array, using the expansion BEDs.

The second diagram shows how an unbalanced array could look - tier 2 devices on the left and right are loaded at different quantities and so lead to an unbalanced layout.

Option 2 - Mixed Workload

In this option, disks are spread across the whole array, perhaps placing tier 1 disks first followed by tier 2 devices. In this way, the I/O load is spread across the whole configuration. As new disks are added, they are distributed throughout the array, keeping performance even. The risk with this configuration lies in whether tier 2 storage will affect tier 1, as the array becomes busy. This can be mitigated with Cache partitioning and LUN prioritisation options.
I prefer the second option when designing arrays, unless there is a very good reason to segment workload. Distributing disks gives a better overall performance balance, reducing the risk of fragmenting (and consequently wasting) resources. I would also use the same methodology for other enterprise arrays too.

Bear in mind if you choose to use Enterprise Flash Drives (EFDs) that they can only be placed in the first storage bays either side of the controller bay and with a limit of 32 per quadrant. Mind you, if you can afford more than 32 drives then you've probably paid for your onsite EMC support already!!

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