plucky (1) nbdkit-cow-filter.1.gz

NAME

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

SYNOPSIS

        nbdkit --filter=cow plugin [plugin-args...]
                                   [cow-block-size=N]
                                   [cow-on-cache=false|true]
                                   [cow-on-read=false|true|/PATH]

DESCRIPTION

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

       The underlying plugin is opened read-only.  This filter does not pass any writes or write-like operations
       (like trim and zero) through to the underlying plugin.

       Note that anything written is thrown away as soon as nbdkit exits.  If you want to save changes, either
       copy out the whole disk using a tool like nbdcopy(1), or use the method described in "NOTES" below to
       create a diff.

       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-block-size=N
           Set the block size used by the filter.  This has to be a power of two and the minimum block size is
           4K.  The maximum block size depends on the plugin, but a block size larger than a few megabytes is
           not usually a good idea.

           The default is 64K.

       cow-on-cache=false
           Do not save data from cache (prefetch) requests in the overlay.  This leaves the overlay as small as
           possible.  This is the default.

       cow-on-cache=true
           When the client issues a cache (prefetch) request, preemptively save the data from the plugin into
           the overlay.

       cow-on-read=false
           Do not save data from read requests in the overlay.  This leaves the overlay as small as possible.
           This is the default.

       cow-on-read=true
           When the client issues a read request, copy the data into the overlay so that the same data can be
           served more quickly later.

       cow-on-read=/PATH
           When /PATH (which must be an absolute path) exists, this behaves like "cow-on-read=true", and when it
           does not exist like "cow-on-read=false".  This allows you to control the "cow-on-read" behaviour
           while nbdkit is running.

EXAMPLES

   nbdkit --filter=cow file disk.img
       Serve the file disk.img, allowing writes, but do not save any changes into the file.

   nbdkit --filter=cow --filter=xz file disk.xz cow-on-read=true
       nbdkit-xz-filter(1) only supports read access, but you can provide temporary write access by using the
       command above.  Because xz decompression is slow, using "cow-on-read=true" causes reads to be cached as
       well as writes, improving performance at the expense of using more temporary space.  Note that writes are
       thrown away when nbdkit exits and do not get saved into the file.

NOTES

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

   Compared to nbd-server -c option
       All connections to the nbdkit instance see the same view of the disk.  This is different from
       nbd-server(1) -c option 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 as above.

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-cache-filter(1), nbdkit-cacheextents-filter(1),
       nbdkit-xz-filter(1), nbdkit-filter(3), nbdcopy(1), qemu-img(1).

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

LICENSE

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       that the following conditions are met:

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       •   Neither the name of Red Hat nor the names of its contributors may be used to endorse or promote
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