Thursday, 12 January 2012

Why Compellent is going to be more efficient than Equallogic … for me…

I tweeted about EQL and Compellent efficiency the other day, and thought I would elaborate a little, to clarify my point.

First off, our Equallogic units are great, especially our SUMO’s. We are still going to be using them for the next few years and I do think that we made the correct choice in buying them just over a year ago.

Going forward we will be using Compellent and these are a few of the efficiency reasons why:

Block size
Equallogic uses a 15MB block size compared to Compellent’s 2MB or 512KB. For structured data that means that the EQL is an order of magnitude less efficient when it comes to snapshots. I have seen a log of a few hundred MB generate a snapshot of hundreds of GB on the EQL which becomes expensive on their SSD units. On the flip side, if your app is sequentially adding data to a file, the block size will be completely irrelevant so it does come down to your application.
I do not believe this is something they can change on a whim and is something you buy into when you buy EQL.

Thin provisioning
On EQL right now there is no space reclamation on thin provisioned volumes. As a result, once you have written to a LUN, it doesn’t know when you have deleted data (under windows anyway) and hence eventually becomes fat provisioned. Compellent has an agent that communicates with the array and tells it which blocks can be freed up, allowing thin provisioning to reclaim space. In addition Compellent only writes thin – there are no fat provisioned LUNs as well as the fact that if it sees the OS zeroing out a large amount of disk space, it doesn’t commit them to disk. No more accidental full formats on LUNS…

On the EQL you have to pre-allocate LUN space and snapshot space among other things. This is great for knowing that you won’t run out of capacity, but does reduce the efficiency of your space utilisation. Space allocated to snapshots on one LUN cannot be temporarily used by another LUN. On the Compellent arrays you don’t need to pre-allocate snapshot space per LUN but do run a greater risk of running out of capacity on badly managed systems.

Tiering / Data Progression
EQL implementation of auto tiering is great for consistent workloads, and fantastic at balancing loads on similar arrays. Where it’s not as good is inconsistent workloads or giving you granular control on how to tier data to lower performance disks. I will do a post at some stage about Compellent’s tiering, but for now lets just say that it’s far more robust and has some efficiency advantages like restriping snapshot data from R10 to R5. This robust tiering will allow us to more effectively use our SSD tier on the Compellent, than we are able to right now on the EQL.

For me these enhancements are theoretical as our arrays have arrived, but have not gone into production yet. Over the next few months I’m pretty sure I’m going to be giving the Compellents some stick when I get frustrated by its quirks, but for now I’m just really excited to see if it can live up to my expectations!


  1. Hi Brett,

    A few months down the track, are you happy you made the decision of Compellent over EQ? Are there more features that have impressed you with Compellent?


    1. Hi Andrew,

      Been itching to do a followup post on this, but a change in role has killed my free time.

      In a nutshell - yes, very happy with the decision to go Compellent. I think the most surprising feature was the improved management workflow - adding luns to large groups of servers is trivial, and works very intuitively etc. Considering the additional complexity of the Compellent, management is also extremely easy - all of my team have commented on this.

      In terms of efficiency - the Compellent's have lived up to expectations so far. I'm getting significant savings in terms of snapshot space, and the tiering has not had any noticeable negative side effects. Biggest advantage being that next time we grow our capacity, it will be at our cheapest tier, rather than our most expensive.

      Only thing I don't like about Compellent is the mandatory engagement process to get a quote from them. This is great for people who are first time buyers, but if you already know what you want, it can be an irritation.

      Would I buy Compellent again? Definately - I'm about to buy another array for our DR site.