Compellent and SSDs

There's been a lot of talk this week about Compellent and their support for solid state drives. See the press release here. So now we have two vendors offering SSD devices in their arrays, Compellent join the club with EMC. Which is best?

At a meeting I had last week, we discussed SSD drives and EMC's implementation in particular. The consensus was that SSDs (or should I be calling them EFDs?) in existing DMX array were more of an "also supports" rather than a mainline feature. The reason for that thinking was that DMX was never engineered specifically to support EFDs, but rather they've been added on as a recent value-add option. What's not clear is whether this bolt-on approach really means you get the best from the drives themselves, something that's important with the price point they sit at. Consider that EFDs sit behind a shared architecture of director ports, memory, front-end ports and queues. Do EFDs get priority access (I know they have to be placed in specific slots in the DMX storage cabinet so presumably they are affected by their position on the back-end directors).

The other problem with the EMC approach is that entire EFD LUNs must be given up to a host. With large databases, how do you predict which parts of the database at any one time are the hot parts? How does a re-org or reload affect the layout of the data? Either you need to put all of your database on EFD or spend a lot more time with the DBAs and Sys Admins creating a design that segments out active areas (and possibly repeating this process often).

If Compellent's technology works as described, then LUNs will be analysed at the block level and the active blocks will remain on the fastest storage with the least active moved to lower tiers of disk (or to other parts of the disk) within the same array.

This should offer a more granular approach to using SSDs for active data. In addition, if data can dynamically move up/down the stack of storage tiers, then as data profiles change over time, no application re-mapping or layout should be necessary. Hopefully this means that SSDs are used as efficiently as possible, justifying their inflated cost.

Just to conclude, I'm not saying Compellent have the perfect solution for using SSDs but it is a step in the right direction for making storage usage as efficient as possible.

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!!