Undocumented functions of NTDLL

NTSYSAPI 
NTSTATUS
NTAPI
RtlGetCompressionWorkSpaceSize(

  IN ULONG                CompressionFormat,
  OUT PULONG              pNeededBufferSize,
  OUT PULONG              pUnknown );

CompressionFormat
See RtlCompressBuffer for valid CompressionFormat flags.



pNeededBufferSize
You must allocate temporary compression buffer for system internal use in compression process. 
Buffer must have pNeededBufferSize bytes length.


pUnknown
-?, propably PageSize (0x4000).