nbdkit-qcow2dec-filter1

Usage

Overview

"nbdkit-qcow2dec-filter" is a filter for nbdkit(1) that can decode some qcow2 files. You can use it on top of plugins that provide qcow2 data, such as nbdkit-file-plugin(1) or nbdkit-curl-plugin(1). For example to expose a local qcow2 file as raw format over NBD:

 nbdkit -r --filter=qcow2dec file image.qcow2

This filter can only decode qcow2 files, it cannot write to them, and it has other limitations. See "LIMITATIONS" below. The NBD export will be read-only unless you use nbdkit-cow-filter(1) on top.

qemu-nbd(8) is an alternate NBD server with native support for qcow2.

Limitations

This filter has several limitations, and some of them we never intend to implement. For general qcow2 access, especially creating and writing to qcow2 files, it is always better to use qemu and tools such as qemu-img(1) and qemu-nbd(8).

Read-only

This filter only supports decoding qcow2 into raw format. You cannot write to the qcow2 file. The NBD export will be read-only.

Versions 2 and 3 only

Only qcow2 subversions 2 and 3 (a.k.a. "compat=0.10" and "compat=1.1") are supported.

Deflate or Zstd compression only

The most common types of qcow2 compression are supported if nbdkit was built with the correct libraries. To find out which types of compression are supported, use:

 $ nbdkit null --filter=qcow2dec --dump-plugin
 ...
 qcow2dec_deflate=yes       # printed if Deflate (zlib) is supported
 qcow2dec_zstd=yes          # printed if Zstd is supported

No snapshots

We may add the ability to extract an internal snapshot in future.

No bitmaps

It may be possible to support bitmaps in future.

No backing files

No dirty or corrupted qcow2 files

No encryption

No extended L2 entries

No external data files

These features are not currently supported and unlikely to be supported in future.

Parameters

There are no parameters specific to this filter. Parameters are passed through to the underlying plugin.

Files

$filterdir/nbdkit-qcow2dec-filter.so

The filter.

Use "nbdkit --dump-config" to find the location of $filterdir.

Version

"nbdkit-qcow2dec-filter" first appeared in nbdkit 1.36.