All modern RAID 4 and RAID 5 systems use a cache to improve write performance. This is true of both software RAID systems, like SoftRAID and hardware RAID controllers. The write cache allows the RAID systems to combine many small writes into a single larger write which the system can handle more efficiently.
The disadvantage of using a cache is that if you lose power between the time a file has been written to your RAID volume and when the file data gets written out from the cache, that file data will be lost. We recommend that you use a UPS (Uninterruptible Power Supply) on your Mac and disks whenever you are using a RAID 4 or RAID 5 volume. If you are not using a UPS, we recommend that you disable the write cache using the SoftRAID preferences.