Shared object that supports the Power-Safe filesystem (QNX Neutrino)


driver … qnx6 qnx6_options… &

Runs on:

QNX Neutrino


The driver is any of the devb-* drivers, and qnx6_options is one or more of the following, separated by commas:

Attempt to align all reads and writes in sizes and offsets of the filesystem block size.
Set the low free space watermark, in blocks.
Enable or disable encryption support (disabled by default). In order to use encryption, you must have formatted the filesystem with the -E option for mkqnx6fs. Use fsencrypt to manage the encryption.
Control which users (if any) can suspend the taking of snapshots (via a flag in the DCMD_FSYS_FILE_FLAGS devctl() command). The default is root.
Enable a block overallocation heuristic for small file writes.
Set the frequency of automatic snapshots; the default is 10 seconds. A filesystem snapshot is explicitly made when you call sync() or fsync(), or from this periodic timer.
Specify the required disk synchronization capability. The mode mode must be one of the following:
  • mandatory (the default) — the drive must support synchronization to allow a filesystem to be mounted read/write. If it doesn't, the mount fails and returns EROFS. A read-only mount (mount -r) can always be performed on any device.
  • optional — attempt synchronization, but ignore any error if the drive doesn't support such an operation. The driver might be incorrectly advertising the capabilities, or the physical media might not require explicit synchronization (write-through).
  • none — never issue a synchronization command to the disk, and don't drain dirty blocks from the filesystem cache (until an explicit umount). This mode is suitable only for use with a UPS.
If the drive doesn't support synchronizing, fs-qnx6.so can't guarantee that the filesystem is power-safe. You can use the sync option to override this requirement at your own risk. Before using this filesystem on devices — such as USB/Flash devices — other than traditional rotating hard disk drive media, check to make sure that your device meets the filesystem's requirements. For more information, see "Required properties of the device," below.
Disable or enable support for TRIM, or use discard instead.

A managed NAND block device can't overwrite in-place and has no idea of whether content in a block is even valid or meaningful to a mounted filesystem. So the management layers have no choice but to preserve all written content, which can be a lot of wear-leveling overhead if in fact those blocks belonged to say a deleted file, or if the partition was freshly formatted.

The TRIM command is thus a hint to the managed NAND device from the filesystem that certain sectors are no longer live and can be discarded (i.e., the content doesn't have to be preserved or copied by wear-leveling, and/or logical blocks can be erased rather than be reclaimed from elsewhere).

Using the discard option gives better performance than enabling trim. When the filesystem tells the driver to discard a set of blocks, the driver simply marks them as discarded and returns, queuing them up for garbage collection later. If the filesystem requests the driver to trim a set of blocks, they're cleaned immediately, which may result in heavy disk I/O, depending on the current state of the system. In the end they do the same thing, just with different timing.

Note: SSDs don't support the discard option.


The fs-qnx6.so shared object provides support for Power-Safe (copy-on-write/snapshot) filesystems. It's automatically loaded by the devb-* drivers when mounting a Power-Safe filesystem.

Note: This filesystem uses UTF-8 encoding for presentation of its filenames; attempts to specify a filename not using UTF-8 encoding will fail (with an error of EILSEQ).

Required properties of the device

The Power-Safe filesystem was designed for and is intended for traditional rotating hard disk drive media. It operates by moving the on-disk filesystem state from one stable view to another stable view using copy-on-write (COW) to relocate modified blocks. To finalize this transition, all dirty blocks involved in the new view must be committed to persistent storage, and then a new filesystem superblock/root referencing the relocated blocks is committed.

This provides power-safe robustness, because at any point in time either the old version is completely accessible or the new version is completely accessible (with no live data being overwritten in between). Thus to mount as read-write on a given device, that device must have the following properties:

Summary of filesystem commands

The following table shows the shared objects and related commands for the filesystems:

Partition type Filesystem Shared object Initialize with: Check with:
1, 4, or 6 DOS fs-dos.so mkdosfs chkdosfs
7 Windows NTa fs-nt.so N/A N/A
11, 12, or 14 FAT32 fs-dos.so mkdosfs chkdosfs
77, 78, or 79 QNX 4 fs-qnx4.so dinit chkfsys
131 Linux (Ext2) fs-ext2.so N/A N/A
175 Apple Macintosh HFS or HFS Plusa fs-mac.so N/A N/A
177, 178, or 179 Power-Safe fs-qnx6.so mkqnx6fs chkqnx6fsb
  Read-only compressed (RCFS) fs-rcfs.so mkrcfs N/A

a Read-only.

b Not usually necessary.

For more information, see the Filesystems chapter of the System Architecture guide.