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 (C) 2018-2021 Red Hat Inc.

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