Tuesday, April 22, 2014


Current RAID schemes, and going back to the 1987/8 Patterson, Gibson, Katz RAID paper, make no distinction between transient and permanent failures: errors or dropouts versus failure.

Monday, April 21, 2014

Storage: Spares and Parity in large disk collections

What approaches are available to deal with spare drives and RAID parity for 300-1,000 drives in a single box?
Will existing models scale well?
Do other technologies fill any gaps?

Storage: First look at Hardware block diagram

Stuffing 500-1000 2.5" drives in an enclosure is just the start of a design adventure.

The simplest being choosing fixed or hot-plug drive mounting. There's a neat slide-out tray system for 3.5" drives that allows hot-plug access for densely vertically packed drives that could be adapted to 2.5" drives.

Sunday, April 20, 2014

Storage: Challenges of high-count disk enclosures

Stuffing 500-1,000 2.5" drives in a single enclosure may be technically possible, but how do you make those drives do anything useful?

Increasing drives per enclosure from 15-45 for 3.5" drives to 1,000 requires a deep rethink of target market, goals and design.

Not the least is dealing drive failures. With an Annualised Failure Rate (AFR) of 0.4%-0.75% now quoted by Drive Vendors, dealing with 5-15 drive failures per unit, per year is a given. In practice, failure rates are at least twice the Vendor quoted AFR not the least because in systems, conditions can be harsh and other components/connectors also fail, not just drives. Drives have a design life of 5 years, with an expected duty-cycle. Consumer-grade drives aren't expected to run 24/7 like the more expensive enterprise drives. Fail Rates over time, when measured on large fleets in service, increase over time and considerably towards end of life.

It's isn't enough to say "we're trying to minimise per unit costs", all designs do that, but for different criteria.
What matters is the constraints you're working against or parameters being optimised.

Storage: How many drives can be stuffed in a Box?

How many 2.5" drives can be stuffed into a single enclosure, allowing space for power, cooling, wiring and a single motherboard? Short answer: ~500-1000.