Advanced Power Management

The Revolution series carefully manages the number of on/off disks to ensure that data access and cost are continually in balance. Power Management ensures that only a maximum of 25% of the drives in each Enclosure are powered on at any one time, in the event that data is required from a LUN on a RAID group that is powered down, Power Management will spin down an operational LUN when it is no longer in use and then power up the LUN that contains the data being requested.

It's therefore key that applications or "personalities" for which there may be a delay in getting data on or off LUNs depending on what drives are currently spinning at the time of the request.

Power Management or Power Budget is the key to delivering power reduction and thus delivering the associated benefits in a MAID array. To manage the application or Personality to a power budget there are three possibilities:

• Configure the personality or application so that they will never demand more than 25% of the drives to be actually spun up
• Use best practices to artificially limit the inbound workload
• Accept longer service times as IOs are queued

Power Managed RAID Software

Software combines power management and RAID processing to provide access to all 112 disk drives in a shelf. It provides full RAID protection for all data, with disks that spin only when necessary. The embedded shelf controller monitors power allocation and determines the next available set of disks (RAID groups). A maximum of 25% of the drives in a storage shelf are available to the user at any given point in time. Power Managed RAID software also manages the power budget, and determines if there is available power capacity to run Disk Aerobics™ in the background.

In a Power Managed RAID environment, maximizing power efficiency is critical. The use of large RAID groups would diminish the power budget available to applications, and increase the risk of data loss. This is because access to any data on a drive, would require the entire RAID group to be powered on.

Dual parity systems will involve lengthy rebuild times after a failed drive (especially as larger drive capacities become available).This is compounded by poorer performance as two writes are required for every parity vs. one write in a RAID 5 set. Calculations show that using small RAID groups for large drive decreases the chance of data loss, compared with larger multiple parity drives which have longer exposures to RAID recalculations.

The Revolution Series reserves five drives per Enclosure (in the latest release) as spares for each Enclosure. A further three drives are dedicated to the Always on Region™ (AOR). This means that there are actually 104 drives used for client data, arranged in 26 3+1 RAID sets.

Always on Region

The Always on Region (AOR) is an area on drives which are always spinning (up to 8 GB). This area is used by COPAN to store metadata, and can be configured to be used by an application or Personality that needs access to a limited amount of constantly spinning disk. The COPAN VTL for example will use this area for storing VTL metadata and High Availability shared metadata.