gpadmin=# \d gp_global_sequence
Table "pg_catalog.gp_global_sequence"
Column | Type | Modifiers
--------------+--------+-----------
sequence_num | bigint | not null

Tablespace: "pg_global"

The gp_global_sequence table contains the log sequence number position in the transaction log, which is used by the file replication process to determine the file blocks to replicate from a primary to a mirror segment.