nbdkit-extentlist-filter - place extent list over a plugin
nbdkit -r --filter=extentlist plugin extentlist=FILENAMEnbdkit-extentlist-filter is an nbdkit filter that lets you place a static list of extents on top of an existing plugin. Extents record whether or not specific parts of the disk are allocated or sparse.
You can use this with plugins which cannot get extent information themselves, but where you can get this information from another source. One place where it is useful is with nbdkit-ssh-plugin(1) because the sftp protocol does not support reading sparseness information, but you may be able to get this information directly from the source disk on the remote server using commands such as xfs_bmap(8). A similar situation applies to nbdkit-curl-plugin(1).
Note that the extent list is read-only. This filter does not monitor writes and trims in order to update the extent list. What can happen is that you would write to a “hole” in the disk, but would not be able to read it back because the NBD client would still think that part of the disk is a hole. So it is generally only safe to use this filter in read-only mode (-r option).
The list of extents is specified in a text file. There is one extent specified per line. Each line has the format:
offset length typeThe offset and length fields may use any format understood by nbdkit_parse_size(3). The optional type field may be an integer, missing (same as 0), or a comma-separated list of the words hole and zero. (The fields correspond to the inputs of the nbdkit_add_extent function, see nbdkit-plugin(3)).
An example of a valid set of extents covering a 10M disk where the first megabyte only is allocated data:
0 1M
1M 9M hole,zeroOr you could omit the hole,zero extent since any gaps are assumed to be holes with that type:
0 1MThe extent list need not cover the whole disk, and does not need to be in ascending order, but it must not contain overlapping extents.
Specify the file containing the extent list, in the format described in "FILE FORMAT" above.
The filter.
Use nbdkit --dump-config to find the location of $filterdir.
nbdkit-extentlist-filter first appeared in nbdkit 1.18.
nbdkit(1), nbdkit-cacheextents-filter(1), nbdkit-noextents-filter(1), nbdkit-filter(3), nbdkit-plugin(3).
Richard W.M. Jones
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