Medley Data Availability and Computer Reach-ability

Medley pools the resources of several computers to provide a fast, reliable Medley drive.  Medley does this by making these computers interact according to well-defined rules.  These rules include expectations about the reach-ability of the computers in the pool.  If computers in the pool are temporarily unreachable, data in the Medley drive may be temporarily unavailable.

What Does "Reachable" Mean?
From Medley's perspective, computers can "reach" each other if their Medley software can communicate with each other.  The Medley software on two computers can communicate with one another only if the following are true:

• Both computers are turned on and not in a "suspended" state
• The computers are connected via the network and can "ping" each other via TCP/IP
• Medley is running on both computers

You can find out which computers are reachable from a specific computer via the "Membership" page of the Medley control panel on that specific computer.

What Happens When Computers Become Unreachable?
Different things happen when computers become unreachable, depending on the number of computers that become unreachable and the number of computers that are in the pool.

If at least half of the computers in the pool are unreachable from one computer, that computer will no longer be able to access the Medley drive.  Medley intentionally does this to prevent inconsistent changes from being made to the Medley drive if the LAN becomes partitioned.

For example, imagine a pool that contains five computers named A through E that has a LAN failure such that A through C are on one side of the failure and D and E are on the other.  Medley will prevent D and E from modifying the Medley drive, but it will allow A through C to modify it.  Just like people in a meeting, A through C are said to have quorum - they can reach more than half of the members in the pool and can therefore "do business"; D and E do not have quorum and therefore cannot do business.

If at least two of the computers in the pool are unreachable from one computer, that computer may be unable to access files.  Even though Medley makes two copies of data stored in the Medley drive, if both of those copies are stored on computers that are unreachable, that data will be unavailable.

If only one computer in a pool is unreachable, all the other members of the pool will still be able to access all of the data in the pool (unless it is a two node pool, in which case neither computer has quorum).

In addition, if only one computer in a pool is unreachable and there is space available, the remainder of the pool that has quorum will create one extra copy of all the data stored on the unreachable computer.  This reduces your risk of data loss in the event that the one computer became unreachable because, for example, it had a hard drive crash.