Managed sequences in Postgres Pro Shardman are implemented on top of regular PostgreSQL sequences with some additional cluster-wide metadata, which among other things holds the interval of cluster-wide unused sequence elements.

When CREATE SEQUENCE .. WITH (global) is issued, an ordinary PostgreSQL sequence with the same name is created on every cluster node. The range of this unmanaged sequence is a bounded sub-interval of the managed sequence (as defined by MINVALUE and MAXVALUE parameters), and it contains at most block_size elements. The nextval function returns values from the unmanaged sequence until it runs out, then a new sub-interval with block_size elements is allocated from the managed sequence using a broadcast query involving all cluster nodes. So, smaller block size values make the generated numbers more monotonic across the cluster, but incur a performance penalty since the broadcast query may be rather expensive. Another way to describe the block size parameter is to say that it controls the size of the second cache level, similarly to how the CACHE parameter works, except at the level of an entire Postgres Pro Shardman cluster.

Also note, that every time a new sub-interval is allocated the underlying unmanaged sequence is modified (as in ALTER SEQUENCE), which will lock it for the transaction duration, preventing any other local concurrent transactions from obtaining next sequence values.