Since RAID was originally designed to parallelize disk drives for performance and resiliency, it seems pretty obvious that data protection techniques like RAID are constrained by the technology of the disk, and later solid state, drives they protect. It seems appropriate at this point to take a brief detour from explaining how we protect data from problems with disk drives into how the disk drives themselves have evolved.
While the prep school I went to had a Selectric Terminal connected to a mainframe running APL at IBM Poughkeepsie, the first computer I ever got to lay my dirty little hands on, the Chemistry department’s PCP-8i at university, didn’t have any disk storage. Grad students had access to the two 280KB DECtape drives, but as a lowly freshman, I had to save my BASIC programs with the 10cps paper tape punch on the ASR-33 teletypes we used as terminals.
By the time I was using hard drives regularly, they broke down into two types; 14” ‘high performance” drives that used the SMD interface and the 5 ¼” ST-506 drives that are the direct ancestors of all the disks we use today.
In the next few posts I’m going to discuss the changes that have affected how we as enterprise or storage architects use disks, and eventually, the SSDs that have emulated them.
The first big change was when disk media stopped being removable. The old SMD drives, as shown above, used removable disk packs. Organizations used disk packs to load data on demand for batch jobs and for all the reasons we make copies today from backups to use as a transport medium. By the 80s increasing track density made pack interchangeability impractical.
Then RAID was such a success that disk drive vendors stopped developing new SLEDs just a few years after Patterson Et Al published their seminal paper. While technologies, like voice coil positioners that first appeared in SLEDs would soon become common all of today’s drives owe their lineage to the ST-506.
In these posts, I’ll pay more attention to things like logical block addressing, which changed how we accessed data than to perpendicular magnetic recording or even Winchester heads, that increased bit density w/o requiring any changes in how the disks were handled.