For non-transaction WRITETHRU files, all updates are written through the server’s cache to the file system cache. The server flushes file system buffers on each ISAM update operation for WRITETHRU files. (Low-level updates on WRITETHRU files are written through the server’s cache to the file system cache but are not flushed to disk.)
For PREIMG files opened or created with the WRITETHRU attribute, the server behaves as follows:
WRITETHRU minimizes the possibility of lost updates in the event of a catastrophic event because it writes updated cache pages to disk at the time of the update operation. However, WRITETHRU does not provide the guarantee of recoverability that TRNLOG provides. When using WRITETHRU, it is possible for some updates to be lost or for data and index file inconsistencies to exist following a catastrophe.