public long usedBytes() {
      return usedCount * itemSize;
    }
How many bytes are currently taking up space in this bucket size's buckets Note: If your
items are less than the bucket size of this bucket, the actual used bytes by items will be
lower than this value. But since a bucket size can only allocate items of a single size, this
value is the true number of used bytes. The difference will be counted in
{@link #fragmentationBytes()}.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketAllocator.java