At the heart of PeerStorage is a system that reliably stores data on devices that continually disconnect. Here is an illustration of a way to create a virtual block device from disparate devices, each providing a portion of its own memory.
Each square is a “data store”. It is composed of :
- Gray cells, they contain bits of data.
- Colored cells, they contain parity bits.
Try for yourself to understand how it works:
- Click on the gray cells to change data bits. Observe how the parity bits also change.
- Click on colored cells to destroy a “store”. Observe how the data is reconstructed from nothing.
- Try to change values while “store” is rebuilding. Note that your changes are taken into account.
- Try to destroy more than one “store” at a time. If bits are lost they are replaced by ‘-‘.
Experiment with different sizes. Observe how the redundancy ratio varies (total cells / gray cells).
|Rebuild all "stores"|
|Change array size|
|Change parity distribution|
A simple calculation of probabilities based on the following assumptions:
- we move to 3 dimensions instead of the 2 of the demonstration,
- the side of the “cube” is 11 “data stores”, so there is a total of 1331,
- the redundancy ratio is 1.33, ie 1331/1000,
- every minute, 10% of the “data stores” are disconnected,
- it takes 5 seconds to rebuild a “data store”.