NAME

       nbdkit-cow-filter - nbdkit copy-on-write (COW) filter

SYNOPSIS

        nbdkit --filter=cow plugin [plugin-args...]

DESCRIPTION

       "nbdkit-cow-filter" is a filter that makes a temporary writable copy on top of a read-only
       plugin.  It can be used to enable writes for plugins which only implement read-only
       access.  Note that:

       •   Anything written is thrown away as soon as nbdkit exits.

       •   All connections to the nbdkit instance see the same view of the disk.

           This is different from nbd-server(1) where each connection sees its own copy-on-write
           overlay and simply disconnecting the client throws that away.  It also allows us to
           create diffs, see below.

       •   The plugin is opened read-only (as if the -r flag was passed), but you should not pass
           the -r flag to nbdkit.

       •   Note that the use of this filter rounds the image size down to a multiple of the
           caching granularity (4096), to ease the implementation. If you need to round the image
           size up instead to access the last few bytes, combine this filter with
           nbdkit-truncate-filter(1).

       Limitations of the filter include:

       •   The underlying file/device must not be resized.

       •   The underlying plugin must behave “normally”, meaning that it must serve the same data
           to each client.

PARAMETERS

       cow-on-cache=true
           Treat a client cache request as a shortcut for copying unmodified data from the plugin
           to the overlay, rather than the default of passing cache requests on to the plugin.
           This parameter defaults to false (which leaves the overlay as small as possible), but
           setting it can be useful for converting cache commands into a form of copy-on-read
           behavior, in addition to the filter's normal copy-on-write semantics.

EXAMPLES

       Serve the file disk.img, allowing writes, but do not save any changes into the file:

        nbdkit --filter=cow file disk.img

       nbdkit-xz-filter(1) only supports read access, but you can provide temporary write access
       by doing (although this does not save changes to the file):

        nbdkit --filter=cow --filter=xz file disk.xz

CREATING A DIFF WITH QEMU-IMG

       Although nbdkit-cow-filter itself cannot save the differences, it is possible to do this
       using an obscure feature of qemu-img(1).  nbdkit must remain continuously running during
       the whole operation, otherwise all changes will be lost.

       Run nbdkit:

        nbdkit --filter=cow file disk.img

       and then connect with a client and make whatever changes you need.  At the end, disconnect
       the client.

       Run these "qemu-img" commands to construct a qcow2 file containing the differences:

        qemu-img create -f qcow2 -b nbd:localhost diff.qcow2
        qemu-img rebase -b disk.img diff.qcow2

       diff.qcow2 now contains the differences between the base (disk.img) and the changes stored
       in nbdkit-cow-filter.  "nbdkit" can now be killed.

ENVIRONMENT VARIABLES

       "TMPDIR"
           The copy-on-write changes are stored in a temporary file located in /var/tmp by
           default.  You can override this location by setting the "TMPDIR" environment variable
           before starting nbdkit.

FILES

       $filterdir/nbdkit-cow-filter.so
           The filter.

           Use "nbdkit --dump-config" to find the location of $filterdir.

VERSION

       "nbdkit-cow-filter" first appeared in nbdkit 1.2.

SEE ALSO

       nbdkit(1), nbdkit-file-plugin(1), nbdkit-truncate-filter(1), nbdkit-xz-filter(1),
       nbdkit-filter(3), qemu-img(1).

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2018 Red Hat Inc.

LICENSE

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