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QNX4FS is the filesystem used by the QNX4 and QNX6 operating systems.

It is extent-based.

The filesystem remains consistent even after a power failure, but this is NOT achieved by using journalling. Instead, the writes are carefully ordered and flushed to disk at appropriate intervals so that the on-disk structure always remains consistent, no matter if the operation is interrupted. (Note: However, that data may be lost, no filesystems can work around this in fact because the actual contents of the hard-disk's cache RAM will always get lost on a power failure.) This design has a considerable performance gain over journalling, by just bypassing that step.

An other notable property of this filesystem is that its actual metadata (like inode information and disk bitmaps) are accesible in the same way as any other file on the filesystem. (/.inodes and /.bitmap) This is consistent with QNX's (in fact, Plan9's, or historically UN*X's) philosophy that: "Everything is a file."

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