How to shrink LOB segment without downtime and how to perform it fast

HI ,

We have LOB segment of size over 160 GB, we would like to shrink it.

When I ran the query using dba_segments to find the size of the segment, I see that it is using about 165 GB.

SELECT SEGMENT_NAME, sum(BYTES)/1024/1024/1024 GB

FROM DBA_SEGMENTS

/

GB

--------

165.04

But when i ran the query to see actual size I found that it is using about 37 GB.

SELECT nvl((sum(dbms_lob.getlength(column))),0)/1024/1024/1024 AS GB FROM Table

/

     GB

----------

37.0427774

I would like to shrink it without downtime and do it fast. We have more than enough space for archives.