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The bug is also reproduced on qemu-kvm-rhev-2.6.0-27.el7.x86_64. +++ This bug was initially created as a clone of Bug #1379929 +++ Description of problem: As subject Version-Release number of selected component (if applicable): nbdkit-1.1.12-1.fc25.x86_64 qemu-img-2.7.0-1.fc26.x86_64 How reproducible: 100% Steps to Reproduce: 1. Create a qcow2 image: # qemu-img create /tmp/xx.qcow2 -f qcow2 100M Formatting '/tmp/xx.qcow2', fmt=qcow2 size=104857600 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16 Export by qemu-nbd: # qemu-nbd -t -p 30001 --format=qcow2 /tmp/xx.qcow2 2. Check the image local and via nbd # qemu-img info /tmp/xx.qcow2 image: /tmp/xx.qcow2 file format: qcow2 virtual size: 100M (104857600 bytes) disk size: 196K cluster_size: 65536 Format specific information: compat: 1.1 lazy refcounts: false refcount bits: 16 corrupt: false # qemu-img info nbd://localhost:30001 image: nbd://localhost:30001 file format: raw virtual size: 100M (104857600 bytes) disk size: unavailable Actual results: As step2 Expected results: qemu-img info via nbd get right result. Additional info: Try to do above steps with nbdkit and the result is right. # nbdkit -p 30000 file file=/tmp/xx.qcow2 # qemu-img info nbd://localhost:30000 image: nbd://localhost:30000 file format: qcow2 virtual size: 100M (104857600 bytes) disk size: unavailable cluster_size: 65536 Format specific information: compat: 1.1 lazy refcounts: false refcount bits: 16 corrupt: false
When you ran 'qemu-nbd' you gave it a --format=qcow2 argument, so it is exporting the *contents* of the qcow2 image, ie a *raw* volume. So the qemu-img info command is correct in reporting that it is raw.
(In reply to Daniel Berrange from comment #2) > When you ran 'qemu-nbd' you gave it a --format=qcow2 argument, so it is > exporting the *contents* of the qcow2 image, ie a *raw* volume. So the > qemu-img info command is correct in reporting that it is raw. I try to use 'qemu-nbd -t -p 30001 --format=raw' and get qcow2 format when using qemu-img info. Well, for the issue in comment0, it is not a bug. But the manual of qemu-nbd is confusing: -f, --format=fmt Force the use of the block driver for format fmt instead of auto-detecting I couldn't get the info of 'exporting the *contents* of the qcow2 image'. Maybe we should improve the manual page and give some examples.
I don't think there's anything that needs fixing here, this behaviour is consistent with elsewhere.