Reading , wRighting and Recording – Measure how your applications hit your disks!
I’ve spent the last week thinking more about storage than I usually would, particularly in the light of some of the conversations I’ve been having over Tech Field Day with the other delegates & sponsors who have had varying levels of interest & expertise within the storage world. If, like me you have a basic appreciation of storage but want to get in that little bit deeper , a good primer would be Joe Onisick’s storage protocols guide at DefinetheCloud.net
Admins working in smaller shops probably have a little closer control over the storage they buy as they are likely to be the ones specifying , configuring and crying over it when it goes wrong ; It’s one of the con’s of working for a large enterprise is that the storage team tends to be separate – they guard their skills and disk shelves quite closely , sometimes a little too closely – I do wonder if their school reports used to say “does not play well with others” . The SAN is seen as a bit of a black box by the rest of the department and generally as long as the required capacity is available to someone when they ask for it , be it a lun or VMware datastore , then everyone is happy to let them get on with it.
As soon as there is a performance issue however , that happy boat starts to rock .The storage team starts to get defensive, casting forth whitepaper & best practice guide as if they were a World of Warcraft character holding a last stand. At some point you may well find that you hit the underlying best performance of the SAN , no matter how well tuned it is. You are then left in a bit of a quandary of what to do, in the worst case you have to bite that bullet and move that application which looked like the lowest of the low hanging fruit back onto a physical server with direct attached storage , where it’ll smugly idle at 5% utilisation for the rest of its life , ever causing reproachful looks when you walk past it in the datacenter.
How do you avoid the sorry tale above ? In a nutshell, “Know your Workload!” When you start to map what your applications are actually using you can start to size your environment accordingly. One of the bigger shocks that I’ve come across when doing such an exercise is a much heavier proportion of writes than the industry would have us expect. This causes a big problem for storage vendors who rely on flash based cache to be able to hit their headline performance figures. When reading from a cache , of course the performance will be great , but under a heavy write intensive load the performance of the underlying disk starts to be exposed and it seems to come down to number and speed of spindles. Running a system that uses intelligent tiering to write hot blocks in the fastest way then cascade them down the array as they get cooler could help in this instance. Depending on your preference for File or Block level storage , there are a number of vendors who could help you with this, for example Avere Systems or 3PAR or the next Generation of EMC’s FAST technology.