Wednesday, 25 January 2012

inexpensive iSCSI solutions

We are in the unenviable position of upgrading many servers to meet the growing requirements of an ever increasing sales force. Basically we need a new Exchange server, whilst we are at it, the additional data means more backup space is needed... oh and could we have another 4 full development environments, P.S. the SAP server could use another 500GB.

Servers are easy in today's new order of virtualising everything. And indeed given the way that we retire production web kit to development servers, CPU and memory resource is cheap. Hard disks however are a bit more difficult. Yes hard disks are cheap, but hard disk enclosures fill up fast, then your only choice is to get *all* new disks rather than just adding a couple more. Once you do proper capacity planning you suddenly have some quite big bills on your hand and actually you are no better off in a years time :(

Yes, the solution is obvious, get a  nice big SAN:
  • It makes vitalisation simpler
  • Improves your redundancy by allowing you to easily move vm's between hypervisors
  • Its fast
  • Very flexible (you can repartition on the fly)
  • High disk redundancy
A quick lesson on what a SAN is. Essentially SAN falls under the category of external storage. Its a big dedicated disk array, highly optimised to deliver data over the network, sometimes over its own dedicated network to reduce interference from other devices. Here comes the hard part... SAN stands for Storage Area Network as apposed to NAS or Network Attached Storage. The difference is that a NAS looks like a shared drive on the network that anybody can attach to (provided they have permission) it looks for all the world like a server in its own right.

A SAN is somewhat more esoteric, normal network users cannot even see the device. It presents itself like a raw hard drive usually through an iSCSI interface that you attach to and use literally like a real internal hard drive. The big thing is that normally only one server uses that iSCSI partition, most often it is the main drive for a virtual machine.

The principle difference between a SAN and a NAS is in the way that data is transferred between the network storage decice and the data consumer. On a NAS data is transferred as files in a structured manner, irrespective of how the data is represented on the disk. On a SAN the data is transferred at a block level mimicking the way data is stored in blocks on the disk. In theory a SAN is more efficient and faster than a NAS as it is optimised for data transfer rather than file structure and so the data can transfer without having to be assembled up into files. Very good for large unstructured data, i.e. databases. In practice, there actually isn't a lot of difference until you get to quite large devices.

The big selling point with a SAN nowadays is it's compatibility with VM-Ware and other Hypervisors, giving you the ability to leave the hard disk in one place and run the actual server on any old CPU that you have spare without having to transfer terabytes of data around!

So, SAN == Good. Unfortunately it also usually == $$$ (about £20K in 2012) especially for a reasonably large (>10TB) or redundant (RAID 50 + redundant PSU) often with about 1 months wait for assembly and commission.

OK, we need to put something in place as an interim solution. It doesn't have to be that high availability, it's only for the development environments until the full SAN arrives. Step up to the mark Netgear with the ReadyNAS Pro. These great little NAS devices also publish themselves as an iSCSI endpoint. Put four 2TB disks in it and you have 6TB of RAID which can easily service 4 Apache + Glassfish stacks for about £1000 (2012 prices) hell have 2!

Little Netgear NAS devices are actually pretty good at this, they have dual GbE networks, built in backup and replication as well, in fact we have a company that has often helped us who would like an offsite backup solution and we will probably re-purpose this for him after we have learnt it's capabilities! Alan Schofield has a huge repository of photographs that he has always backed up manually. Two NAS devices (one at his office and one in ours 5 miles away) will give him a maintenance free solution and let him concentrate on wonderful photography rather than IT!

Sounds like win-win to me :)

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