nbdkit-cow-filter - Man Page

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:

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

License

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Referenced By

nbdkit(1), nbdkit-cache-filter(1), nbdkit-filter(3), nbdkit-floppy-plugin(1), nbdkit-info-plugin(1), nbdkit-linuxdisk-plugin(1), nbdkit-log-filter(1), nbdkit-release-notes-1.10(1), nbdkit-release-notes-1.14(1), nbdkit-release-notes-1.26(1), nbdkit-truncate-filter(1).

2021-09-14 nbdkit-1.27.9