Provided by: fusefile_1.0-1_amd64 
NAME
fusefile, fusedisk - FUSE file mount for combining file fragments
SYNOPSIS
fusefile [fuse-opts] mountpoint [overlay] filename/from-to ...
fusefile -dump [fuse-opts] mountpoint [overlay] filename/from-to ...
fusedisk [fuse-opts] mountpoint [overlay] filename/from-to ...
DESCRIPTION
fusefile is a FUSE file mount that presents a series of fragments of other files as a
contiguous concatenation. Technically it bind mounts a driver on top of the filename
mountpoint to provide access to the given file fragments as if in a single, contiguous
file.
fusefile accepts over-writing on the fused file (i.e. the mountpoint) which gets
distributed accordingly to the fragments. But neither the fused file nor the fragments can
change size; any writing thus merely over-writes content without truncating fragments. All
fragment files are held open while fusefile is active.
By using the optional -overlay:filename argument between the mount point and the
fragments, an overlay file may be set up. The overlay file will then be used by fusefile
for capturing writes to the fused file (i.e. the mountpoint). The overlay file will
contain any new written fused file regions followed by meta data to distinguish between
new, written content and old content that comes from the fragments.
The option -dump as first argument together with a fusefile setup will print the setup to
standard output rather than establishing a fusefile mount. This is of most use with a
prior overlay setup, where then the printout includes the portions of updates that have
been captured in the overlay. The printout is the series of fusefile fragment argments to
give in order to intersperse the captured overlay portions according to the overlay table.
fusedisk is a helper script to set up a fusefile as a block device (via fuseblk) by using
the device mapper (dmsetup) to manage an empty block device mapping where content is
handled at the mountpoint via fusefile. (Note that the same thing may be done with the
device manager directly, but then all fragments need to be in sectors of N*512 bytes
whereas with fusedisk, only the fused file as a whole is "clipped" at nearest N*512 bytes
below actual size)
FRAGMENT ARGUMENTS
The fragment arguments include the filename of a source file, and optionally start and end
byte positions. All in all there five variations:
filename or filename/
include all of the file. A pathname that includes "/" must be ended with an extra
"/" since that last "/" separates the filename from the range detail.
filename/start:end
include the range from the given start to end. Either "start" or "end" or both may
be omitted, to mean the beginning and end of the file respectively. If "start" or
"end" are less than 0 then it means relative to the end of the file.
filename/start+length
include "length" bytes from the given start. A negative "start" means relative to
the end of the file. If "length" is negative or omitted it means that position
relative to the end.
filename/start include bytes from the given start. This is the
same as "/start+"
Note that a negative start position is clipped to 0 and a too large end position is
clipped to the end of the file.
Character devices are treated as being of any given finite size, but have size 0 by
default. For example, "/dev/zero/:100" means a fragment of 100 NUL bytes.
OPTIONS
This section enumerates the most interesting options to use with fuesfile. See "man fuse"
and "man mount" for more options.
-dump
The -dump "option" tells fusefile to print out the applicable fragment sequence for
the current setup, including the overlay table, if any. The printout is done
instead of setting up a mount point.
-oallow_other
The fuse option -oallow_other is needed for sharing the fused file with other users
who otherwise will not have access to it (including "root"). Note however that this
must first be enabled in /etc/fuse.conf.
-ononempty
The fuse option -ononempty may need to be used when reusing an existing file as
mountpoint.
-ouid=... and -ogid=...,
These mount options, where ... is a user or group id respectively, are useful for
root when using fusedisk and thereby give user or group ownership for the mount to
the nominated user or group.
EXAMPLES
This section illustrates uses of fusefile.
Exanple 1.
Insert a file "y" into a file "x" at position 1200.
$ fusefile -ononempty x x/:1200 y x/1200:
This also shadows the original file "x" and presents the fused file instead.
Example 2.
Make fused file y be a swap of the beginning and end of file "x", at position 2442.
$ fusefile y x/2442: x/:2442
Example 3.
Replace partition 2 of an image file, A, with a different file, X. For this example the
partition table looks as follows.
$ partx -oNR,START,SECTORS A
NR START SECTORS
1 2048 2097152
2 2099200 409600
3 2508800 14268383
As each sector is 512 bytes the clipping points around partition 2 are 1074790400 and
1284505600 and the insertion size is 209715200 bytes. The fusefile comman will therefore
be as follows.
$ fusefile -ononempty A A/:1074790400 X/:209715200 A/1284505600
Note that the fused file shadows the original file A.
Example 4.
Protect raw disk image file with an overlay:
$ fusefile -ononempty disk.raw -overlay:today disk.raw
By that set up, the overlay file, "today", will protect the disk image file, "disk.raw"
from changes, and also override the pathname "disk.raw" to be the fused file.
Example 5.
A fusefile mount with an overlay file is writable regardless of the fused fragments, but
all updates are written to the overlay file instead of to the fragments.
$ fusefile -ononempty A -overlay:B A
The overlay file, B in the example, contains all changes to the shadowed original file, A.
The overlay file contains only the newly written regions and content is otherwise obtained
from the original file.
To that end, the overlay file also contains a "marker table" at the end as if appended to
the fused file. That part of the file is outside of the fused file; and it's simply an
element count followed by pairs of byte addresses that tell which regions of the fused
file have been captured into the overlay file. (The marker table is of course maintained
so that adjoining regions are collapsed)
Thus, an overlay file may be reused to later re-establish the same fused file with overlay
as previously, to continue capturing more changes.
Example 6.
As final example, we set up a fused block device y as a swap of the beginning and end of
file "x", at position 2442:
$ sudo fusedisk -ouid=1000 y x/2442: x/:2442
Note the use of sudo for becoming root, which is required for block device handling, and
also the -ouid=1000 option so as to make the block device y be owned by the user with id
1000.
NOTES
fusefile opens the nominated source files before any bind mounting. With the fuse option
-ononempty it will bind over an non-empty file, which may be useful. The source files
remain open, but the source fragments are not recomputed. If a source file changes the
fused file will present the new content. If a source is reduced in size, access will be
inconsistent.
If the mountpoint file doesn't exist, then fusefile creates it.
Unmounting is done with "fusermount -u mountpoint" as usual. A fusedisk mount is unmounted
by root using umount.
SEE ALSO
fuse, fusermount, mount, dmsetup
AUTHOR
Ralph Rönnquist <ralph.ronnquist@gmail.com>.
fusefile(8)