nbdkit --filter=cow plugin [plugin-args...]
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.
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.
Serve the file disk.img, allowing writes, but do not save any changes into the file:
nbdkit --filter=cow file disk.img
nbdkit-xz-plugin(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 xz 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.
nbdkit --filter=cow file disk.img
and then connect with a client and make whatever changes you need. At the end, disconnect the client.
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.
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
TMPDIRenvironment variable before starting nbdkit.
nbdkit --dump-configto find the location of
nbdkit-cow-filter first appeared in nbdkit 1.2.
nbdkit(1), nbdkit-file-plugin(1), nbdkit-xz-plugin(1), nbdkit-truncate-filter(1), nbdkit-filter(3), qemu-img(1).
Richard W.M. Jones
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nbdkit(1), nbdkit-filter(3), 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-truncate-filter(1).