Dynamic share migration to maintain file health

[mmore]

Dynamic share repair to maintain file health. based on the following features already exist in Allmydata-Tahoe1.3 we can improve automatic repair:

1. Foolscap provides the knowledge of the alive nodes.

2. Verification of file availability can be delegated to other node through read-cap or a verify-cap without security risk.

The proposed auto repair process:

1. Using memory-based algorithm, because client know where the file shares exist so we can keep tack of alive file shares, for simplicity we consider that share availability from its node availability.

2. repair process triggered automatically from the repairer, repair responsibility has many technique based repair cost ; network bandwidth and fault tolerant.

3. time out , we can use lazy repair technique to avoid node temporary node failure, i.e waiting for a certain time before repair process starts.

4. reintegration, using memory-based repair technique remembering failed storage servers, who come back to life, will help in reducing Tahoe grid resources such as network bandwidth and storage space.

5. repairer, selection of repair responsibly takes many issues into consideration: security , repairer location , repairer resources.

[zooko]

I reformatted the original description so that trac will represent the numbered items as a list.