In the last article we looked in depth at the heart of the PC build:
- The CPU (processor)
- The Memory (RAM)
- The Motherboard (main board)
Now we will look at the remainder of the PC:
- The Storage subsystem (hard disk or HDD)
- The Graphics Processor (GPU)
- The Case
- The Cooling (HSF or heatsink & fan)
Design…Select…Standardise…Optimise and Build… The Storage (hard disk drives)
When you save your work,
run an application or game, stream or encode to disk or in any other way require to access to or from the permanent storage your disk subsystem offers you are constrained by its performance. The storage system is almost the only mechanical system within the computer (although this is changing now with the advent of Solid State Drives, SSD’s). Data is stored on a hard drive magnetically and arranged in concentric circles around the disk from the centre to the edge. Therefore if your computer needs access to data at one end of the disk and then the other the drive has to physically move the head backwards and forwards, and wait for the disk to spin around to the right location to begin the read or write process. The time taken to do this is known as latency, and the average time to move the head to any given point and read data is given as Average Seek Time (usually in milliseconds, ms).
Manufacturers provide spin speed and average seek time data with their drives. In addition all drives have an onboard memory cache that the drive software uses to intelligently store previous (or read ahead) read requests so that it can almost intelligently pre-empt fetching of the data you want read. Data can then be quickly retrieved without requiring the disk to move the head. The other significant factor in drive performance is having as big a possible pipe (i.e. bandwidth) between the drive and the CPU to make sure that as soon as the drive has data available it can be transferred.
Our design imperatives for good performance storage are, in roughly this order:
- high bandwidth – achieved with;
- high areal density (more data packed into a smaller space can be read faster, target density around 300 gigabits per square inch)
- a high drive speed of at least 7500rpm or better still 10000 or more,
- high data transfer speeds of 3Gb/s,
- and supporting the latest interface standards of SATA-II or SAS (SCSI)
- a large onboard cache of at least 16MB and better 32MB or more
- best possible Average Seek time (should be under 10ms, under 5ms for extreme random access performance)
- RAID (Redundant Array of Inexpensive Disks) – using arrays of disks working in parallel to achieve higher speeds, higher transfer rates and redundancy for resilience in the unlikely event there is a drive failure
Most manufacturers of mainstream PC’s will only quote you disk capacity in GB and this is used as a selling point, they are often unable to give you the data above and are unlikely to have considered it in the design. There is a balance to be struck between performance and value, but also a delicate balance between bandwidth and seek times. If the work you do is predominantly sequentially accessing the disk it is probably more important to have high bandwidth than fast seek times (i.e. photography, multimedia). You also consider this in the design of RAID storage arrays as they impact on average seek times, write speeds and have their own critical configuration parameters that affect how effectively they will function. industrial air purifier