@Override
  public ExtendedCell forceCopyOfBigCellInto(ExtendedCell cell) {
    MemStoreLAB mslab = this.mslabs.get(0);
    return mslab.forceCopyOfBigCellInto(cell);
  }
The process of merging assumes all cells are allocated on mslab. There is a rare case in which
the first immutable segment, participating in a merge, is a CSLM. Since the CSLM hasn't been
flattened yet, and there is no point in flattening it (since it is going to be merged), its big
cells (for whom size > maxAlloc) must be copied into mslab. This method copies the passed cell
into the first mslab in the mslabs list, returning either a new cell instance over the copied
data, or null when this cell cannt be copied.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/ImmutableMemStoreLAB.java