Why didn’t you call? …

May 9, 2012

Can’t remember exactly what that line is from (possibly a TV ad from my younger days), but it basically tells a story in the RAID industry. I’m continually getting customers (a) telling me what they are doing, then (b) ending up asking me how to put it all together because “they are not really experts at this” etc. Considering the fact that normally these people are doing something pretty bizarre or high-end, this is somewhat strange.

So …

Basic tips for looking at how to build a system. These are very, very generic, but there are some truisms to these, with one basic proviso – IF YOU ARE NOT 100% SURE OF WHAT YOU ARE DOING – CALL YOUR RAID VENDOR (in this case Adaptec of course).

Capacity – performance – redundancy (paranoia) – cost

These are the basic building blocks of storage – and you can’t have them all. Of course if you disregard cost you can get whatever you want in this world, but there are not many customers who actually mean it when they say “we are not worried about the cost, we just want the best” etc (they pretty quickly change their tune), so I’ll base everything below with a strong eye towards the “cost” of the system).

Capacity – it’s pretty easy to get big storage these days – 3tb drives are commonplace, but keep an eye on performance while you are calculating this variable. A general rule of thumb would be that the more spindles you have working for you, the quicker the box will be. Eg – 4 drives in a RAID 10 will be quicker than 2 drives in a mirror. So keeping cost down too much can often hurt your performance, though it generally doesn’t hurt your redundancy.

So think carefully about capacity, but don’t let it be the only consideration.

Performance – what exactly are you doing with your data, and what sort of data is it? Notice that we haven’t even bothered looking at hardware yet – the data type will determine most of the configuration. This can be broken down into basically streaming and random data – streaming can be in either direction (capturing or delivering content) and random is generally read (with a lesser degree of write – eg database).

So if you are doing streaming data then you will go for a parity raid (5, 6, 50, 60) as the parity calculations won’t hurt your performance and you’ll get a lot more capacity for your dollar. If on the other hand you are doing random data like a database, then you’ll be more interested in a non-parity raid such as 1 or 10.

Streaming data works fine on SATA drives (7200rpm), but random data (database) tends to work much better on SAS (10K or 15k drives). Of course SSD comes into play here but the cost generally wipes them out for anything except small specialised installations.

So if you are doing streaming data you are probably looking at SATA drives in a RAID 5, 6 or 50, while if you are doing database you are probably looking at SAS drives in a RAID 10, 1E or 1 (in that order please).

Of course, after all that, you need to work out which card can give you the raid levels and drive connections you are looking for, but that becomes the easy part.

So … back the heading … if you’re not sure, call :-)



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