Elsewhere, a summary of IDC's 2009 Worldwide HDD shipments and revenue:
- The transition from 3.5in. to 2.5in. performance-optimized form factor HDDs will be complete by 2012.
- Growing interest in new storage delivery models such as storage as a service, or storage in the cloud is likely to put greater storage capacity growth demands on Internet datacenters.
- The price per gigabyte of performance-optimized HDD storage will continue to decline at a rate of approximately 25% to 30% per year.
In their seminal paper, "A Case for Redundant Arrays of Inexpensive Disks (RAID)",  Patterson, Gibson and Katz discuss a number of disk performance metrics.
In their first figure they compare drive types across a number of metrics, looking for a top-line "Price/Performance" comparison, given that "reliability" can be maintained. Factors are:
- performance (bandwidth, latency, IO/sec)
- unit size or volume
... so we concentrate on the price-performance and reliability. Our reasoning is that (if) there are no advantages in price-performance or terrible disadvantages in reliability, then there is no need to explore further.and
(RAID) offers an attractive alternative to SLED (Single Large Expensive Disk), promising improvements of an order of magnitude in performance, reliability, power consumption, and scalability.Interestingly, weight has been omitted as a metric. Even the 8 in Fujitsu Eagle (6RU high and 600+mm deep) in 1981 stored 474.2MB, and at 65kg was heavy enough that planning for unpacking, installation and floor loading was necessary. Even now, the weight of "fully loaded" Disk Arrays with 3½ in HDD's has to be taken into account.
Since the paper was written, Disk Arrays have moved from using disk form-factors of 8 in, 5¼ in, 3½ in and are now flirting with 2½ in drives. In each of these transitions, there was no sudden change, rather a gradual phase-out as vendors introduced new models at premium prices, until the last old-format model was discontinued and customer-support gradually phased out. The whole process taking up to 10 years, but as with any adoption of new technology, the "majority" users (not early- or late-adopters) change in a 3-5 year window, depending on their "hardware refresh" cycles.
The driving forces for consumers have been Price and Size and the related Storage Density (Gb/volume) and Specific Power (W/Gb) which are Financial and Operational factors, not "technical" nor "performance".
For vendors, the technical driver is the on-going improvement in disk-drive capacity/recording densities, interface changes (SCSI to SAS) and improvements in CPU/RAM price/performance/capacity. Plus the inevitable commercial driver of "planned obsolescence", but in light of historical improvements in Silicon technologies (Moore's Law etc), this effect hasn't been significant historically.
Others have written serious technical comparisons of 3½ in vs 2½ in drives for Enterprise use, which are fully worth reading.
Here I've been attempting to describe a bigger arc:
the commercial and technical forces driving these on-going changes.
Understanding the forces may help inform us of:
- "limits to growth" - Will we end up with 1" drives or stop at 2½ in drives?
- "limits of technology" - The impact of hitting a recording density limit.
- The 8" Fujitsu Eagle put one drive in 6RU (10.5" platter). [DPD = 0.16/RU]
- 5¼ in drives fitted 3 abreast in 2RU (5.25" platter). [DPD = 1.5/RU]
Full Height Drive Bay: 3¼″ high by 5¾″ wide, and up to 8″ deep (83×146×200 mm)
- 3½ drives fit 14 drives per "shelf" of 3RU (3.5" platter) [DPD = 5/RU]
or 24 drives in 4RU [DPD = 6/RU]
- 2½ in drives allow around 2-3 times the density, 24 drives in 2RU [DPD = 12/RU]
2½ in drives are only 100mm deep, meaning they could be stacked 4-6 deep in a 2RU unit.
At 5W/drive, could those 100+ drives be kept sufficiently cool with standard front-to-back airflow (max 55°C operating)?
At 220-250gm per drive, plus fittings and support electronics/cabling, total weight would approach 50kg/shelf.
In producing "Design Rules" for the latest generation of disk drives, 2½ in, how can Drive Packing Density be maximised without compromising access and cooling? Tucking drives away inside a 19" rack works very well until one needs changing. Moving operating drives is not good practice, so slide-out trays aren't a good choice, especially as they will also block access ways and create a potential tipping hazard. E.g. early IBM 2311 drives were known for being unstable with drawers extended .
Packaging could be the Packaging Density limit for the 2½ in generation of drives.
An academic treatment of the topic would be inadequate if based solely on the raw drive volumes, 377 cm³ vs 105 cm³ [1 cubic-centimetre is also 1 millilitre]. Physical factors dominate choice of mounting geometry, meaning side-to-side cooling or an internal vertical chimney would be needed with simple front-and-back lineal packing.
Moving to the next generation of drives, 1.8 in (54 mm × 8 mm × 71 mm vs 70 mm × 7–15 mm × 100 mm for "2½ in"), will only exacerbate these wasted space/cooling problems. As yet, there is no commodity production/use of 1.8 in drives, it's unlikely that Enterprise HDD's will progress there anytime soon.
The prime candidates for 1.8 in form-factor are SDD's for installation in servers or laptops/netbooks.