In the past I used to perform a lot of "cross-platform" dumps. If the database is huge, it can take many hours to complete. The last part of the "cross-platform" process can be very long.
According to the doc, the WaitEventID 311 corresponds to a process waiting for another process to finish before writing datapages MASS (Memory Address Space Segment) to the disk:
http://infocenter.sybase.com/help/index.jsp?topic=/com.sybase.infocenter.dc00848.1502/html/monitor_tables/CHDIHEHJ.htm
Is that taking more time than usual? Does the 'dump database' process generates IO?
Is the number of physical IO increasing when you run:
select
convert(char(9),master..sysprocesses.cpu) cpu,
convert(char(9),master..sysprocesses.physical_io) io,
convert(char(9),master..sysprocesses.memusage) mem
from master..sysprocesses
where spid=<your_spid>