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Ignite Durable Memory is basis for all data structures. There is no cache state saved on heap now. 

To save cache state to disk we can dump all its pages to disk. First prototypes used this simple approach: stop all updates and save all pages.

Page store is the storage fo for all pages related to particual cache and its partition.

Using page identifier it is possible to map from page ID to file and to position in particular file:

pageId = ... || partition ID || page index
and pageId can be easily converted to file + offset in this file
offset = idx * pageSize

Partitions of each cache have corresponding file in page store directory (particular node may own not all partitions). 

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