Oracle 12c (12.1.0.1) SecureFile LOB segment size growing larger than expected and exhausts tablespa

Hi Oracle Experts,

In Oracle 12c (12.1.0.1 + RHEL6 environment), observe that rotating log table (max 10k rows) with SecureFile LOB column has a LOB segment growing larger than expected: LOB segment size is much greater (+GB larger) than the sum of LOB column length using dbms_lob.getlength.

LOB ddl:

LOB ("LOGOBJ") STORE AS SECUREFILE (

  TABLESPACE "LOG" ENABLE STORAGE IN ROW CHUNK 8192

  NOCACHE LOGGING  NOCOMPRESS  KEEP_DUPLICATES

  STORAGE(INITIAL 106496 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645

  PCTINCREASE 0

  BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT))

. 

LOB retention_type is default

LOB retention_type is null

segment_size_blocks:1944600

segment_size_bytes :15930163200

used_blocks        :1995