Upstream qemu can access a variety of remote disks, like NBD and Ceph. This feature is exposed in libguestfs so you can easily mount remote storage.
However in RHEL 7 many of these drivers are disabled, because they’re not stable enough to support. I was asked exactly how this works, and this post is my answer — as it’s not as simple as it sounds.
There are (at least) five separate layers involved:
| qemu code |
What block drivers are compiled into qemu, and which ones are compiled out completely. |
| qemu block driver r/o whitelist |
A whitelist of drivers that qemu allows you to use read-only. |
| qemu block driver r/w whitelist |
A whitelist of drivers that qemu allows you to use for read and write. |
| libvirt |
What libvirt enables (not covered in this discussion). |
| libguestfs |
In RHEL we patch out some qemu remote storage types using a custom patch. |
Starting at the bottom of the stack, in RHEL we use ./configure --disable-* flags to disable a few features: Ceph is disabled on !x86_64 and 9pfs is disabled everywhere. This means the qemu binary won’t even contain code for those features.
If you run qemu-img --help in RHEL 7, you’ll see the drivers which are compiled into the binary:
$ rpm -qf /usr/bin/qemu-img
qemu-img-1.5.3-92.el7.x86_64
$ qemu-img --help
[...]
Supported formats: vvfat vpc vmdk vhdx vdi ssh
sheepdog rbd raw host_cdrom host_floppy host_device
file qed qcow2 qcow parallels nbd iscsi gluster dmg
tftp ftps ftp https http cloop bochs blkverify blkdebug
Although you can use all of those in qemu-img, not all of those drivers work in qemu (the hypervisor). qemu implements two whitelists. The RHEL 7 qemu-kvm.spec file looks like this:
./configure [...]
--block-drv-rw-whitelist=qcow2,raw,file,host_device,blkdebug,nbd,iscsi,gluster,rbd \
--block-drv-ro-whitelist=vmdk,vhdx,vpc,ssh,https
The --block-drv-rw-whitelist parameter configures the drivers for which full read and write access is permitted and supported in RHEL 7. It’s quite a short list!
Even shorter is the --block-drv-ro-whitelist parameter — drivers for which only read-only access is allowed. You can’t use qemu to open these files for write. You can use these as (read-only) backing files, but you can’t commit to those backing files.
In practice what happens is you get an error if you try to use non-whitelisted block drivers:
$ /usr/libexec/qemu-kvm -drive file=test.vpc
qemu-kvm: -drive file=test.vpc: could not open disk image
test.vpc: Driver 'vpc' can only be used for read-only devices
$ /usr/libexec/qemu-kvm -drive file=test.qcow1
qemu-kvm: -drive file=test.qcow1: could not open disk
image test.qcow1: Driver 'qcow' is not whitelisted
Note that’s a qcow v1 (ancient format) file, not modern qcow2.
Side note: Only qemu (the hypervisor) enforces the whitelist. Tools like qemu-img ignore it.
At the top of the stack, libguestfs has a patch which removes support for many remote protocols. Currently (RHEL 7.2/7.3) we disable: ftp, ftps, http, https, tftp, gluster, iscsi, sheepdog, ssh. That leaves only: local file, rbd (Ceph) and NBD enabled.
virt-v2v uses a mixture of libguestfs and qemu-img to convert VMware and Xen guests to run on KVM. To access VMware we need to use https and to access Xen we use ssh. Both of those drivers are disabled in libguestfs, and only available read-only in the qemu whitelist. However that’s sufficient for virt-v2v, since all it does is add the https or ssh driver as a read-only backing file. (If you are interested in finding out more about how virt-v2v works, then I gave a talk about it at the KVM Forum which is available online).
In summary — it’s complicated.
