The QNX 4 filesystem achieves high throughput without sacrificing reliability. This has been accomplished in several ways.
While most data is held in the buffer cache and written after only a short delay, critical filesystem data is written immediately. Updates to directories, inodes, extent blocks, and the bitmap are forced to disk to ensure that the filesystem structure on disk is never corrupt (i.e., the data on disk should never be internally inconsistent).
Sometimes all of the above structures must be updated. For example, if you move a file to a directory and the last extent of that directory is full, the directory must grow. In such cases, the order of operations has been carefully chosen such that if a catastrophic failure (e.g., a power failure) occurs when the operation is only partially completed, the filesystem, upon rebooting, would still be "intact." At worst, some blocks may have been allocated, but not used. You can recover these for later use by running the chkfsys utility. For more information, see the Backing Up and Recovering Data chapter.