In computing, a massive array of idle disks (more commonly known as a MAID) is a system using hundreds to thousands of hard drives for nearline storage of data. MAID is designed for 'Write Once, Read Occasionally' (WORO) applications. In a MAID each drive is only spun up on demand as needed to access the data stored on that drive. This is not unlike a very large 'Just a Bunch Of Disks' (JBOD) but with power management.
Compared to RAID technology a MAID has increased storage density, and decreased cost, electrical power, and cooling requirements. However, these advantages are at the cost of much increased latency, significantly lower throughput, and decreased redundancy. Low drive utilization rates may actually reduce reliability in consumer-oriented large PATA and SATA drives.[1] Drives designed for multiple spin-up/down cycles (e.g. laptop drives) are significantly more expensive. Latency may be as high as tens of seconds.[2] MAID can supplement or replace tape libraries in hierarchical storage management.
To allow a more gradual tradeoff between access time and power savings, some MAIDs such as Nexsan's AutoMAID[3] incorporate disks capable of spinning down to a lower speed, such as 4000 RPM; this achieves 60% of the power reduction achieved by a full spin-down and sleep, and requires less power and time to spin back up to operating speed.
With the advent of SATA disk drives that are designed to be powered on and off, MAID architecture has evolved into a new storage platform for long term, online storage of persistent data. Large scale disk storage systems based on MAID architectures allow dense packaging of drives and are designed to have only 25% of disks spinning at any one time.[2]