"Missed by _that_ much": Disk Form Factor vs Rack Units

Apologies to 1965 TV series "Get Smart" and the catch-phrase "Missed by that much" (with a visual indication of a near-miss).

This is a lament, not a call to action or grumble. Standards are necessary and good.
We have two standards that we just have to live with now: too many devices depend on them for a change. Unlike the "imperial" to metric conversion, there would be few discernible benefits.

There's a fundamental mismatch between the Rack Unit (1.75 inches) or the vertical space allowed for equipment in 19 inch Racks (standard EIA-310) and the Disk form factors of  5.25, 3.5 and 2.5 inches defined by the Small Form Factor Committee.

There is no way to mount a standard disk drive (3.5 or 2.5 inch) exactly in a Rack. There are various amounts of wasted space.
Originally, "full-height" 5.25 inch drives could be mounted horizontally exactly in 2 Rack Units (3.5 inches), three abreast.

The "headline" size of the form-factor is the notional size of the platters or removable media.
The envelope allows for the enclosure.

So whilst "3.5 inch" looks like a perfect multiple of the 1.75 inch Rack Unit, a "3.5 inch" drive is  around 0.5 inch larger.
Manufacturers of vertical-mount "hot-swap" drives allow around 1mm on the thinnest dimension, 9 mm on the "height" and 1.5 inches (42-43 mm) on the longest dimension (depth).

A guess at the dimensions of hot-swap housings:
1/32 in (0.8mm) or 1mm sheet metal could be used between drives (upright)
and 1.5-2mm sheet metal would be needed to support the load (with an upturned edge?).

In total, around 0.5 inch (12.5mm) might need be allowed vertically for supporting structures.
An ideal Rack Unit size for the "3.5 inch" drive form-factor would be 4.5 inches.

Or, "3.5 inch" drives could be 3.00 - 3.25 inches wide to fit exactly in 2 Rack Units.

Different manufacturers approach this problem differently:

• Copan/SGI and Backblaze mount 3.5 inch drives vertically in 4 Rack Units (7 inches).
  Both of these solutions aim for high-density packing, 28 and 11.25 drives per Rack Unit .
• Copan, via US Patent # 7145770, uses 4U hot-swap "canisters" that store 14 drives in 2 rows, with 8 canisters per "shelf" (112 drives/shelf). In a 42 U rack, they can house 8 shelves, for 896 drives per Rack. Their RAID system is 3+1, with max 5 spares per shelf, yielding 79 data drives per shelf, and 632 drives per Rack.
  These systems are designed specifically to hold archival data, with up to 25% or 50% of drives active at any one time, as "MAID": Massive Array of Idle Disks.
• Backblaze are not a storage vendor, but have made their design public with a hardware vendor able to supply cases and pre-built (but not populated) systems.
  Their solution, fixed-disks not hot-swap, is 3 rows of 15 disks mounted vertically, sitting on their connectors. The Backblaze systems include a CPU and network card and are targeted at providing affordable and reliable on-line Cloud Backup services [and are specifically "low performance"]. Individual "storage pods" do not supply "High Availability", there is little per-unit redundancy. Like Google, Backblaze rely on whole-system replication and software to achieve redundancy and resilience.
• Most server and storage appliance vendors use "shelves" of 3 Rack Units (5.25 inches), but fit 13-16 drives across the rack (~17.75 inches or 450mm) depending on their hot-swap carriers.
• "2.5 inch" drives fitted vertically (2.75 inch) need 2 Rack Units (3.5 inches). Most vendors fit 24 drives across a shelf. "Enterprise class" 2.5 inch drives are typically 12.5 or 15 mm thick.

Another possibility,  not been widely pursued, is to build disk housings or shelves that don't exactly fit the EIA-310 standard Rack Units. Unfortunately, the available internal width of 450mm cannot be varied.

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The form factors:

"5.25 inch": (5.75 in x 8 in x 1.63 in =  146.1 mm x 203 mm x 41.4 mm)
"3.5 inch"  : (4 in x 5.75 in x 1 in =  101.6 mm x 146.05 mm x 25.4 mm)
"2.5" inch  : (2.75 in x 3.945 in x 0.25-0.75 in = 69.85 mm x 100.2 mm x [7, 9.5, 12.5, 15, 19] mm)

Old disk height form factors, originating in 5.25 inch disks circa mid-1980's.

low-profile = 1 inch.
Half-height = 1.63 inch.
Full-height = 3.25 inch. [Fitting well into 2 Rack Units]