VM storage is provided by a Ceph cluster. Each block device visible in VMs (e.g., /dev/vda or /dev/sda) reflects an underlying Ceph RBD volume.

Disk Layout

Each RBD volume contains a complete virtual disk image consisting of a partition table and one or more filesystems. We use both XFS and ext4 filesystems in VMs.

Growing and shrinking disks

Such a virtual disk is quite easy to grow but not so easy to shrink.

When growing a virtual disk, we enlarge the underlying RBD volume, adapt the partition table and resize the main filesystem. This procedure is fully transpart and can be performed without downtime.

Shrinking a virtual disk is not directly supported. On customer request, we install a filesystem quota to ensure that no more space is used than booked. On newer NixOS VMs with XFS filesystems, tools like df report that reduced size correctly. Older, Gentoo-based VMs with ext4 filesystems are not so smart; df still reports the old size even when a quota is installed. Nonetheless, we will monitor disk usage on basis of the reduced disk size.