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Thu, 28 Jun 2007

Device mapper sucks or recent changes in distributed storage development.


And thus will not be used.
It has serious limitation on table format - each line in table must form contiguous region, and it is not allowed to change limits (start and size) in constructor, which effectively breaks dynamic configuration of the storage.
So, no device mapper - I will use raw block device and create proper binary protocol to configure device from userspace instead of string based one for device mapper.

So far I only added couple of bits to dynamically add/remove storages and encoding algorithms, which actually can be revisited later if needed.
Each algorithm is an entity which will send/receive (and read/write via usual block IO path if node is local, which is supported too) data from nodes, it will determine if several nodes needs to be read/written (for example to recover if array is degraded or to update checksum) and perform all actions.
Initial trivial algorithm will use round-robin mechanism to write sequential data to several nodes.

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