nbdkit tar [tar=]FILENAME.tar file=PATH_INSIDE_TAR
The tar plugin is deprecated in nbdkit ≥ 1.22.16 and will be removed in nbdkit 1.26. It has been replaced with a filter with the same functionality, see nbdkit-tar-filter(1). You can use the filter like this:
nbdkit file FILENAME.tar --filter=tar tar-entry=PATH_INSIDE_TAR
Serve a single file inside a tarball
nbdkit tar file.tar file=some/disk.img guestfish --format=raw -a nbd://localhost
Opening a disk image inside an OVA file
The popular “Open Virtual Appliance” (OVA) format is really an uncompressed tar file containing (usually) VMDK-format files, so you could access one file in an OVA like this:
$ tar tf rhel.ova rhel.ovf rhel-disk1.vmdk rhel.mf $ nbdkit -r tar rhel.ova file=rhel-disk1.vmdk $ guestfish --ro --format=vmdk -a nbd://localhost
In this case the tarball is opened readonly (-r option). The plugin supports write access, but writing to the VMDK file in the tarball does not change data checksums stored in other files (the
rhel.mf file in this example), and as these will become incorrect you probably won't be able to open the file with another tool afterwards.
nbdkit-tar-plugin is a plugin which can read and writes files inside an uncompressed tar file without unpacking the tar file.
file parameters are required, specifying the name of the uncompressed tar file and the exact path of the file within the tar file to access as a disk image.
This plugin will not work on compressed tar files.
Use the nbdkit -r flag to open the file readonly. This is the safest option because it guarantees that the tar file will not be modified. Without -r writes will modify the tar file.
The disk image cannot be resized.
The path of the tarball. This parameter is required.
tar=is a magic config key and may be omitted in most cases. See “Magic parameters” in nbdkit(1).
The path of the file inside the tarball to serve. This parameter is required. It must exactly match the name stored in the tarball, so use
tar tf filename.tar
Alternatives to the tar plugin
The tar plugin ought to be a filter so that you can extract files from within tarballs hosted elsewhere (eg. using nbdkit-curl-plugin(1)). However this is hard to implement given the way that the tar(1) command works.
Nevertheless you can apply the same technique even to tarballs hosted remotely, provided you can run tar(1) on them first. The trick is to use the
tar -tRvf options to find the block number of the file of interest. In tar files, blocks are 512 bytes in size, and there is one hidden block used for the header, so you have to take the block number, add 1, and multiply by 512.
$ tar -tRvf disk.tar block 2: -rw-r--r-- rjones/rjones 105923072 2020-03-28 20:34 disk └──┬──┘ └───┬───┘ offset = (2+1)*512 = 1536 range
You can then apply the offset filter:
nbdkit --filter=offset \ curl https://example.com/disk.tar \ offset=1536 range=105923072
If the remote file is compressed then add nbdkit-xz-filter(1):
nbdkit --filter=offset --filter=xz \ curl https://example.com/disk.tar.xz \ offset=1536 range=105923072
nbdkit-tar-plugin first appeared in nbdkit 1.2.
nbdkit(1), nbdkit-offset-filter(1), nbdkit-plugin(3), nbdkit-tar-filter(1), nbdkit-xz-filter(1), tar(1).
Richard W.M. Jones.
Based on the virt-v2v OVA importer written by Tomáš Golembiovský.
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