For non-WRITETHRU files, the server stores data and index updates in its internal cache and does not write updates immediately to disk. For TRNLOG files, this is not a concern because committed updates to TRNLOG files are logged to the transaction logs. For non-TRNLOG files, however, in the event of an abnormal server termination the contents of the cache (and hence any unwritten updates to data and index files) will be lost.
In this situation, the server marks non-TRNLOG files as corrupt to indicate that the file was opened, updated, and not properly closed, so its state is unknown. Attempting to open such a file fails with error FCRP_ERR (14, file corrupt at open). Rebuilding the data file and its associated indices resets the update flag and allows the application to open the file, but all cached updates that had not yet been written to disk are lost.
The WRITETHRU file mode can be applied to c-tree data and index files to cause the server to write updates through the server’s cache to the file system cache or to disk, thereby minimizing the potential for loss of cached updates in the event of an abnormal server termination. While ensuring the updates are written to the file system or to disk, WRITETHRU preserves the updates in the server’s cache so that reads can be satisfied from the server’s cache.
A data or index file can be created as a WRITETHRU file (in which case WRITETHRU is a permanent attribute of the file), or it can be opened as a WRITETHRU file (in which case the file is treated as WRITETHRU until it is closed).