Rocky Linux 10 ships with QEMU 9.x and libvirt 10.x — significantly newer than the versions found on Rocky Linux 9. This combination introduces stricter XML validation and removes block driver support that older tooling relied upon. This article documents a concrete failure encountered when creating a VM in virt-manager and explains the underlying cause and fix.

The Error

When attempting to create a new VM via the virt-manager wizard on Rocky Linux 10, the installation fails with:

Unable to complete install: 'unsupported configuration: storage type 'dir' requires use of storage format 'fat''

The traceback points to libvirt.libvirtError: unsupported configuration: storage type 'dir' requires use of storage format 'fat' raised during virDomainCreateXML().

The virt-manager wizard had generated the following CD-ROM disk entry in the domain XML, pointing at a directory rather than an ISO file:

<disk type="dir" device="cdrom">
  <source dir="/home/howard/torrents/Rocky-9.8-x86_64-dvd"/>
  <target dev="sda" bus="sata"/>
  <readonly/>
</disk>

Why libvirt Rejects This

libvirt 10.x added strict validation: when a disk source uses type="dir", the driver format must be declared as fat. This is because the only way QEMU can present a host directory to a guest is as a virtual FAT filesystem. Earlier libvirt versions were more permissive and would silently pass the configuration through; libvirt 10.x refuses to do so.

Adding <driver name="qemu" type="fat"/> to satisfy the validation check produces a second, harder failure:

Unable to complete install: 'internal error: process exited while connecting to monitor:
qemu-kvm: -blockdev {"driver":"vvfat","dir":"/home/howard/torrents/Rocky-9.8-x86_64-dvd",...}:
Unknown driver 'vvfat''

Why the Second Error Occurs

libvirt translates type="dir" with format="fat" into a QEMU -blockdev argument using the vvfat (Virtual VFAT) block driver. QEMU 9.x removed the vvfat driver entirely. There is no workaround that preserves type="dir" — the driver simply does not exist in the version of QEMU shipped with Rocky Linux 10.

The Root Cause

The Rocky Linux torrent downloads the ISO file into a named subdirectory alongside its checksums, rather than placing the ISO directly in the download root. The directory contents look like this:

Rocky-9.8-x86_64-dvd/
├── CHECKSUM
├── Rocky-9.8-x86_64-dvd.iso
├── Rocky-9.8-x86_64-dvd.iso.CHECKSUM
├── Rocky-9.8-x86_64-dvd.iso.CHECKSUM.asc
└── Rocky-9.8-x86_64-dvd.iso.manifest

When the path provided to virt-manager is the directory rather than the .iso file inside it, virt-manager generates type="dir" for the CD-ROM disk entry. On Rocky Linux 9 and earlier hosts this would have worked via vvfat; on Rocky Linux 10 it does not.

The Fix

Point the CD-ROM disk entry directly at the ISO file. Change the disk entry from:

<disk type="dir" device="cdrom">
  <driver name="qemu" type="fat"/>
  <source dir="/home/howard/torrents/Rocky-9.8-x86_64-dvd"/>
  <target dev="sda" bus="sata"/>
  <readonly/>
</disk>

To:

<disk type="file" device="cdrom">
  <driver name="qemu" type="raw"/>
  <source file="/home/howard/torrents/Rocky-9.8-x86_64-dvd/Rocky-9.8-x86_64-dvd.iso"/>
  <target dev="sda" bus="sata"/>
  <readonly/>
</disk>

In virt-manager, this can be corrected before starting the installation by enabling Customise configuration before install on the final step of the New VM wizard, navigating to the SATA CD-ROM device, and updating the source path to point to the .iso file directly.

Storage Pool Configuration in virt-manager

A broader way to avoid this class of problem entirely is to ensure that the virt-manager storage pool for ISO images is pointed at the directory that actually contains the .iso files, not a parent directory that contains per-torrent subdirectories.

Rocky Linux torrents (and many others) place the ISO inside a named subdirectory rather than directly in the download root:

~/torrents/
├── Rocky-9.8-x86_64-dvd/
│   └── Rocky-9.8-x86_64-dvd.iso   ← the actual ISO
└── Rocky-10.2-x86_64-dvd1/
    └── Rocky-10.2-x86_64-dvd1.iso  ← the actual ISO

If the storage pool is set to ~/torrents/, browsing it in virt-manager will show the subdirectories, and selecting one of them produces a type="dir" disk entry. If the pool is instead configured to point directly at the subdirectory containing the ISO (e.g. ~/torrents/Rocky-9.8-x86_64-dvd/), the ISO file is visible and selectable directly, producing the correct type="file" entry.

To add or update a storage pool in virt-manager, go to Edit → Connection Details → Storage, click the + button to add a new pool of type dir, and set the target path to the directory that directly contains the .iso files. This is especially worth doing if torrenting ISOs, where the download structure routinely adds an extra directory layer.

Summary

Component Change in Rocky Linux 10
libvirt 10.x Requires format="fat" when type="dir" is used for a disk source
QEMU 9.x Removed the vvfat block driver entirely
Effect Directory-based CD-ROM sources no longer work under any configuration
Fix Always point the CD-ROM source at an ISO file (type="file")

Updated: