vacuum messges

What do these mean?

NOTICE:  FlushRelationBuffers(message, 2): block 2 is referenced
(private 0,
global 1)
FATAL 1:  VACUUM (vc_repair_frag): FlushRelationBuffers returned -2
pqReadData() -- backend closed the channel unexpectedly.
        This probably means the backend terminated abnormally
        before or while processing the request.
connection to server was lost
vacuumdb: vacuum failed


NOTICE:  FlushRelationBuffers(message, 2): block 2 is referenced
(private 0,
global 1)
FATAL 1:  VACUUM (vc_vacheap): FlushRelationBuffers returned -2
pqReadData() -- backend closed the channel unexpectedly.
        This probably means the backend terminated abnormally
        before or while processing the request.
connection to server was lost
vacuumdb: vacuum failed