Massive arrays of idle disks (MAID) are high-capacity, lower cost disk arrays for storing less active enterprise data and saving energy. The primary value proposition of MAID is lower operational costs, which stems from its capability to power down a portion of drives within the array, thereby lowering power consumption. Because spinning disk drives typically account for 80% of a storage array’s power consumption, MAID is, in concept, an effective technology for greening storage. Moreover, 90% of organizational data that is more than three months old, 70%-80% of all data in the organization overall, is inactive or never accessed. The basic concept of MAID platforms is to group and store data based on access frequency, placing rarely accessed data on devices that are turned off. The concept of MAID was pioneered by Copan Systems, and early criticisms of MAID included concerns about shutting down enterprise disk drives, which unlike laptop devices are engineered to be always on. Mainstream manufacturers have begun to introduce MAID-like features into disk arrays, lending credibility to the concept, although there appears to be some debate about the exact definition of MAID -- a key distinction being an architecture designed specifically to accommodate shutting down a proportion of drives completely. Two main issues drive MAID and spin-down adoption:
- Cost. Increasing amounts of data in the enterprise are ‘tier inactive’ candidates for placement on SATA devices. Historically, this information would be stored on tape, which consumes little or no energy, but the prevalence of SATA devices for tier 3 and 4 applications make it increasingly cost-effective to move data to an idle storage tier;
- Technology innovations.
Which technology innovations are noteworthy in MAID?
There are two main sources of technology innovation around MAID and spin-down: 1) hard drive manufacturers and 2) array vendors. Device manufacturers including Hitachi, Seagate, Western Digital, Samsung, and Fujitsu all have announced varying degrees of settings that are invoked through software commands. In concept, users should consider five main points on the power management spectrum when evaluating MAID and spin-down solutions:
- Normal – for online active data – no power savings;
- Park – which parks the disk drive’s heads while the platters continue to spin – 20-24% power savings;
- Slow Down – a mode that decelerates the spin speed of the disk platters from 7200 RPMs to 3600 - ~50% power savings;
- Sleep – a standby mode – which can deliver up to 85% power savings;
- Off – shuts down the disk device – effectively 100% savings.
[Note: These figures are savings at the device, not the array level].
Recently, a number of array suppliers including EMC, Nexsan, Hitachi, and DataDirect Networks have introduced features within disk arrays that, through software, allow administrators to take advantage of some of these different power settings and apply parameters such as time-of-day on a drive-by-drive basis. Nexsan in particular offers a wide range of MAID levels. Copan has introduced intelligent software that proactively manages spin-up of idle disks (i.e. disk aerobics) to ensure devices are exercised and that data are migrated off devices that are high probability candidates for failure.
What are the main constraints of MAID/Spin-down?
The Wikibon community sees two main barriers to adoption with MAID and spin-down:
- The lack of good data classification practices. Candidates for MAID placement generally should be isolated from, for example, thinly provisioned volumes spread across many devices, which limit the effectiveness of MAID;
- The lack of MAID-aware file systems. Long delays mean application time outs. Today, there are few MAID-aware operating systems and file systems, except for specialized archive software and virtual tape library (VTL) solutions with specific extensions. Less aggressive power management feature levels, while not offering as great a power savings as MAID, avoid error handling/timeout problems and allow for wider adoption.
Advice for users
Spin-down generally and MAID specifically are maturing and increasingly becoming logical fits for VTL applications including backup and recovery, archiving applications, the inactive tier of a tiered storage management system, large sequential applications (e.g. scientific and entertainment), and even general purpose file systems such as NFS and CIFS (especially with power modes that are more spin-up friendly - e.g. Nexsan. Organizations should think of tiered storage in multiple dimensions, including: 1) data value and 2) access frequency. Begin grouping data by I/O activity and ensure the existence of a ‘no activity’ group of data. If aggressive exploitation of sleep or off mode is a goal to reduce power consumption, users should choose a MAID-aware application or solution — but in many cases, be aware that not all devices can be powered up simultaneously. Organizations looking to manage less frequently accessed storage intelligently could in theory roll their own solution using robust archive software and low cost disk and green tape to improve efficiency. MAID is, however, a packaged solution that if deployed correctly can simplify the objective and lower integration risks, albeit at a price premium.
Action Item: The bottom line on energy consumption is that getting rid of stuff is always the best approach to lowering the energy bill. Start by classifying data and setting policies to delete information that is not needed. Include an inactive tier that can exploit MAID/spin-down and consider storing more data on that tier, but be sure to remove older hardware in the process.