`devf-ram`

Flash filesystem RAM disk (QNX Neutrino)

You must be root to start this manager.

Syntax:

devf-ram
    [-a] [-b priority] [-c]
    [-E] [-f verifylevel] [-i arrayindex[,partindex]] 
    [-l] [-m mountover[,cmpover]]
    [-p backgroundpercent[,superlimit]] [-r] 
    [-s baseaddress[,windowsize[,arrayoffset[,arraysize[,unitsize][,buswidth[,interleave]]]]]
    [-t threads] [-u update] [-V] [-v]
    [-w buffersize]

Options:

-a

Don't automount filesystem partitions present on the media.

-b priority

Enable background reclaim at the specified priority. By default, background reclamation is disabled.

-c

Disable compressed mountpoints.

The -c option is for use with flash filesystem version 2 (ffs2) only.

-E

Do not daemonize. Exit on EBADFSYS with partition num + 1 of corrupt filesystem.

-f verifylevel

Enable flash verify. (default=0, 0=none, write=1, erase=2, all=3)

-i arrayindex[,partindex]

Starting socket index and first partition index; 0 >= index >=15. The default is 0,0. Use this to give multiple drivers unique IDs. The -i option is just a suggestion for the resource database manager; the selected indexes can be larger.

-l

List the available flash databases and exit.

-m mountover[,cmpover]

Override the mountpoints assigned to a file system that are formatted with an empty (i.e. flashctl -p/dev/fs0p0 -f -n "") mountpoint. The mountover argument can include two %X format specifiers (like those for printf()) that are replaced by the socket index and the partition index.

The cmpover argument can include two %X specifiers, or one %s format specifier that's replaced by the formatted mountover.

The cmpover argument is only available to use with flash filesystem version 2 (ffs2).

The default is /fs%Xp%X,%s/.cmp.

The -m option doesn't override a mountpoint specified with mkefs.

-p backgroundpercent[,superlimit]

Set the background-reclaim percentage trigger (stale space over free space) and, optionally, the superseded extent limit before reclaim. The default is 100,16.

-r

Enable fault recovery for dirty extents, dangling extents, and partial reclaims. By default, recovery isn't performed, which, when power faults are likely, can waste space on the media or make the media read-only.

-s baseaddress[,windowsize[,arrayoffset[,arraysize[,unitsize[,buswidth[,interleave]]]]]

Set socket options, normally the base physical address, window size, array offset, array size, unit size, bus width, and interleave. The format is left flexible for socket services with customized drivers.

A RAM disk has no fixed base physical address; it can sit anywhere in memory. Therefore, specify 0 for the base.

For SRAM, specify the physical address for the base.

You can specify the base physical address, sizes, and offset in octal (0777), hexadecimal (0x1ff), or decimal (511). The sizes must be a power of two, and you can specify them with any of the following suffixes:

(nothing) -- bytes
k -- kilobytes
m -- megabytes

-t threads

Number of threads; 1 >= threads >= 4 (default is 2). Extra threads increase performance when background reclaim is enabled (with the -b option) and when multiple chips and/or spare blocks are available.

-u update

Update level for timestamps; 0 for never update, 1 to update files only, and 2 to update files and directories. The default update is 0.

-V

Display filesystem and MTD version information.

-v

Display verbose information.

-w buffersize

Write (append) buffer size in bytes. The default buffersize is 512. Using a larger write-buffer prevents the creation of very small extents, reducing overhead. If buffersize is 0, appending is disabled.

Description:

The devf-ram manager provides Flash filesystem support for a RAM disk using the following default filenames (the ID, n, appended to /dev/fs can be changed via the -i option):

/dev/fsn: Default mountpoint for socket n.
/dev/fsnp0: Raw access for socket n, partition 0.
mountpoint: Flash filesystem mountpoint for socket n, partition 0 with transparent decompression.
mountpoint/.cmp: Flash filesystem mountpoint for socket n, partition 0 without transparent decompression (only previously compressed files are available here).

You can specify the mountpoint above with the mount attribute of the mkefs command, and override it with the -n option to flashctl. By default, it's /fsnp0.

The driver probes the hardware to determine its block size. If you need to know the block size, you can:

Look in the documentation for the hardware.
Or:
Start the driver in verbose mode by specifying the -v option. In the output, U: indicates the number of units (i.e. blocks), and S: indicates the block size. Both numbers are in hexadecimal.
Or:
Start the driver, and then run flashctl, specifying the -i option.

Examples:

Start devf-ram with compression mountpoint disabled and no append buffering.

devf-ram -c -w0 &

Start devf-ram and automatically mount the Flash filesystem partitions, with an initial fault recovery process, most POSIX semantics enabled and background reclaim at priority 5:

devf-ram -r -u2 -b5 &

If you specify a blocksize in your buildfile for DRAM-based flash filesystems, limit the size to the default, which is 64K.

Caveats:

Alhough the Flash filesystem supports most POSIX semantics, some functionality isn't implemented in order to keep the driver simple and efficient. The unsupported POSIX semantics include:

Hard links, and everything related to hard links (the . and .. directories don't exist, struct stat's nlink member is hard-coded, and unlink() of directories returns ENOTSUP).
Access times aren't updated on the media; they're set to the modification time.

QNX Neutrino flash filesystem version 3 no longer provides built-in decompression. The flash filesystem's decompression functionality has moved into the inflator resource manager. You should now use the deflate utility to compress files.

Performance might be slow when multiple writers are writing randomly to a shared file or to a shared directory (e.g. using unlink or rename). In these cases, the offset pointers have to be rewound for every access. There's no performance penalty when appending to a file, or when creating files with open(O_CREAT), mkdir, mknod, or link.