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 near-line data storage. 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 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 comes at the cost of much increased latency, significantly lower throughput, and no or lower redundancy. Most large hard drives are designed for near-continuous spinning; their reliability will suffer if spun up repeatedly to save power. 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. MAID can supplement or replace tape libraries in hierarchical storage management.

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 Architecture allow dense packaging of drives and are designed to have only 25% of disks spinning at any one time. This allows for high throughput to get data to this platform quickly. Since persistent data is accessed very little, any data can be accessed at any time, and stay within the power budget of 25% of drives spinning.

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