A short note on RAID levels

Posted by Martin P in Infrastructure      2 comments

Sep
21

So you have you shiny new NAS, or even JBOD (Just a Bunch of Disk) and you’re wondering how to make it safe a secure from disk failure.  For a long time, the standard answer would have been RAID 5 (with hot spare).  As a compromise between security and performance, RAID 5 always worked quite well.   Lose a disk to head crash or some other malady, and the dead disk is moved out, the hot spare moved in and rebuilt and everyone’s happy again.

But then disk capacities started to increase.  It is common now to have multi-TB disks in an array. The problem now becomes the amount of time it takes to rebuild the array.  It is now common for a disk rebuild to take many hours, which increases the chance that a second disk will fail during the process.  RAID 5 can’t handle that. Bang! You’re dead.

So in came RAID 6, which allows for 2 parity disks which means two disks can be lost out of the array and a rebuild still complete.  That rebalances the odds.  Or it did until disk capacities just kept on increasing.

So now Sun – increasingly repositioning itself as a storage company adds support for 3 parity disks into its storage line, underpinned by the rather cool (in a geeky kind of way) ZFS. While not the first product of it’s kind, the Sun storage line is strong and I’d love for the European competition regulators to get on and make a decision on the Oracle acquisition of Sun so customers know where they stand.  ZFS has been making good on its potential for a while now, and with the news over the summer than deduplication will be supported, anyone remotely interested in storage really must take a look.

22/09/09 EDIT: Larry Ellison seems to agree.

16/07/10 EDIT: I’ve been asked just how realistic is it that two disks would fail in quick succession?  If you took two disks at random – the answer would be that OK, perhaps its a bit of a stretch.  Disk failures do happen but not that often.  However, the disks in a RAID array are not disks at random.  There is a very high probability that all the disks in an array were manufactured at the same time, and of course, they were all spun up for the first time together.  This puts them on the same point in the bathtub curve.  That means that there is a higher probability of such drives failing in quick succession than the randomised scenario.  By how much?  That depends on a lot of factors – the quality of the hardware, the nature of the rebuild process, the precise position on the bathtub curve – and may not be a very large increase.  For the cost of an additional disk – would you take the risk?

Add High Availability to your PACS on a shoestring: Part I

Posted by Martin P in DCM4CHEE, High Availability, Infrastructure, PACS General      3 comments

Sep
21

Introduction and Architecture

In most clinical organisations, PACS is a critical part of the operational workflow.  RIS is important but PACS must stand firm even when all around it have failed.  In environments which include Emergency, Intensive Care and Theatre (to name a few), the non-availability of PACS can be a danger to patient safety.

Most vendors, however, cost HA not just as an extra, but as a luxurious extra, adding a significant percentage to the total cost of a system.  It needn’t be that way.  HA is a well understood practice and the tools and materials needed can be easily worked into a standalone package that can provide HA for almost any PACS (see section on limitations, below).

I’m going to describe a solution for HA based on free, open source software (note: NOT ‘freeware’.  Know the difference).  All of the software in this solution is mature, proven software used by major organisations the world over.  Where there are choices I have selected software components that have full support options available, just in case you need a little more assurance.

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