Writing to disk or server cache?

Hello, all. IDS 9.21 on HP-UX 11, raw devices. Another thoughts about checkpoint time. During checkpoint IDS does not write data to server cache memory, but tries to write directly to disk array. Yet, writing to memory can be done more quickly. Is it possible to make Informix to use cached disk write during checkpoint? For example by moving tablespaces from raw devices to file systems storage or is there other way to force this? If cached write is possible how it can impact Informix server reliability, any ideas about that? Thanks! Andris