- bullseye 1.24.1-2
nbdkit-tar-plugin - read and write files inside tar files without unpacking
nbdkit tar tar=FILENAME.tar file=PATH_INSIDE_TAR
Serve a single file inside a tarball¶
nbdkit tar 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 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.
The "tar" and "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.
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.
Richard W.M. Jones.
Based on the virt-v2v OVA importer written by Tomáš Golembiovský.
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