On-demand snapshot and prune in a data storage system

A method of data progression in a data storage system having at least two tiers of storage space, a first tier comprising storage space in a single level cell (SLC) solid state device (SSD) and a second tier comprising storage space in a multi-level cell (MLC) SSD, the method comprising:

• setting a predetermined free space threshold for the first tier of storage space that is a threshold relating to the amount of free space available on the first tier of storage space;
• monitoring at least an amount of available free space in the first tier of storage space;
• when the amount of available free space in the first tier of storage space decreases to the predetermined free space threshold, generating an on-demand snapshot of at least a portion of the data of the first tier of storage space by designating that data as read-only;
• transferring the on-demand snapshot to the second tier of storage space, thereby freeing the corresponding at least a portion of data of the first tier of storage space for new writes;
• wherein a write operation received by the data storage system directed to data stored by the on-demand snapshot is directed instead to newly allocated storage space of the first tier of storage space, and the corresponding data in the on-demand snapshot is considered to be overwritten;
• and
• when a page of the on-demand snapshot contains overwritten data, freeing that page of the on-demand snapshot, such that the page is no longer associated with the on-demand snapshot;
• wherein the on-demand snapshot is utilized internally by the data storage system and is not generally accessible by a user of the data storage system.