I have table with LCS in Cassandra cluster. I am observing too may tombstones in my cluster so I have decided to reduce GC grace seconds and performing major compaction. Ran nodetoolcompact keyspace table but compaction job ran within a second. it seems major compaction not worked. Can you please help me to understand.
If you're actually using the antique Cassandra 2.0, as the label on your question said, then indeed it didn't support major compaction on LCS, and "nodetool compact" only triggered a minor compaction which would have happened anyway (unless explicitly disabled). This was fixed in Cassandra 2.2 - see the issue https://issues.apache.org/jira/browse/CASSANDRA-7272.
There are still ongoing plans to improve major compaction on LCS further - see https://issues.apache.org/jira/browse/CASSANDRA-11817 - but it should already work reasonably well and shouldn't finish "within a second" as you reported - unless you really are running Cassandra 2.0.