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Originally, lsa allocated each segment independently what could result in high memory fragmentation. As a result many compaction and eviction passes may be needed to release a sufficiently big contiguous memory block. These problems are solved by introduction of segment zones, contiguous groups of segments. All segments are allocated from zones and the algorithm tries to keep the number of zones to a minimum. Moreover, segments can be migrated between zones or inside a zone in order to deal with fragmentation inside zone. Segment zones can be shrunk but cannot grow. Segment pool keeps a tree containing all zones ordered by their base addresses. This tree is used only by the memory reclamer. There is also a list of zones that have at least one free segments that is used during allocation. Segment allocation doesn't have any preferences which segment (and zone) to choose. Each zone contains a free list of unused segments. If there are no zones with free segments a new one is created. Segment reclamation migrates segments from the zones higher in memory to the ones at lower addresses. The remaining zones are shrunk until the requested number of segments is reclaimed. Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>