Proxmox Cluster File System (pmxcfs)

The Proxmox Cluster file system (“pmxcfs”) is a database-driven file system for storing configuration files, replicated in real time to all cluster nodes using corosync. We use this to store all Proxmox VE related configuration files.

Although the file system stores all data inside a persistent database on disk, a copy of the data resides in RAM. This imposes restrictions on the maximum size, which is currently 128 MiB. This is still enough to store the configuration of several thousand virtual machines.

This system provides the following advantages:

• Seamless replication of all configuration to all nodes in real time

• Provides strong consistency checks to avoid duplicate VM IDs

• Read-only when a node loses quorum

• Automatic updates of the corosync cluster configuration to all nodes

• Includes a distributed locking mechanism

POSIX Compatibility

The file system is based on FUSE, so the behavior is POSIX like. But some feature are simply not implemented, because we do not need them:

• You can just generate normal files and directories, but no symbolic links, …

• You can’t rename non-empty directories (because this makes it easier to guarantee that VMIDs are unique).

• You can’t change file permissions (permissions are based on paths)

• O_EXCL creates were not atomic (like old NFS)

• O_TRUNC creates are not atomic (FUSE restriction)

File Access Rights

All files and directories are owned by user root and have group www-data. Only root has write permissions, but group www-data can read most files. Files below the following paths are only accessible by root:

/etc/pve/priv/
/etc/pve/nodes/${NAME}/priv/

Technology

We use the Corosync Cluster Engine for cluster communication, and SQlite for the database file. The file system is implemented in user space using FUSE.

File System Layout

The file system is mounted at:

/etc/pve

Files

authkey.pub	Public key used by the ticket system
ceph.conf	Ceph configuration file (note: /etc/ceph/ceph.conf is a symbolic link to this)
corosync.conf	Corosync cluster configuration file (prior to Proxmox VE 4.x, this file was called cluster.conf)
datacenter.cfg	Proxmox VE datacenter-wide configuration (keyboard layout, proxy, …)
domains.cfg	Proxmox VE authentication domains
firewall/cluster.fw	Firewall configuration applied to all nodes
firewall/<NAME>.fw	Firewall configuration for individual nodes
firewall/<VMID>.fw	Firewall configuration for VMs and containers
ha/crm_commands	Displays HA operations that are currently being carried out by the CRM
ha/manager_status	JSON-formatted information regarding HA services on the cluster
ha/resources.cfg	Resources managed by high availability, and their current state
nodes/<NAME>/config	Node-specific configuration
nodes/<NAME>/lxc/<VMID>.conf	VM configuration data for LXC containers
nodes/<NAME>/openvz/	Prior to Proxmox VE 4.0, used for container configuration data (deprecated, removed soon)
nodes/<NAME>/pve-ssl.key	Private SSL key for pve-ssl.pem
nodes/<NAME>/pve-ssl.pem	Public SSL certificate for web server (signed by cluster CA)
nodes/<NAME>/pveproxy-ssl.key	Private SSL key for pveproxy-ssl.pem (optional)
nodes/<NAME>/pveproxy-ssl.pem	Public SSL certificate (chain) for web server (optional override for pve-ssl.pem)
nodes/<NAME>/qemu-server/<VMID>.conf	VM configuration data for KVM VMs
priv/authkey.key	Private key used by ticket system
priv/authorized_keys	SSH keys of cluster members for authentication
priv/ceph*	Ceph authentication keys and associated capabilities
priv/known_hosts	SSH keys of the cluster members for verification
priv/lock/*	Lock files used by various services to ensure safe cluster-wide operations
priv/pve-root-ca.key	Private key of cluster CA
priv/shadow.cfg	Shadow password file for PVE Realm users
priv/storage/<STORAGE-ID>.pw	Contains the password of a storage in plain text
priv/tfa.cfg	Base64-encoded two-factor authentication configuration
priv/token.cfg	API token secrets of all tokens
pve-root-ca.pem	Public certificate of cluster CA
pve-www.key	Private key used for generating CSRF tokens
sdn/*	Shared configuration files for Software Defined Networking (SDN)
status.cfg	Proxmox VE external metrics server configuration
storage.cfg	Proxmox VE storage configuration
user.cfg	Proxmox VE access control configuration (users/groups/…)
virtual-guest/cpu-models.conf	For storing custom CPU models
vzdump.cron	Cluster-wide vzdump backup-job schedule

Symbolic links

Certain directories within the cluster file system use symbolic links, in order to point to a node’s own configuration files. Thus, the files pointed to in the table below refer to different files on each node of the cluster.

local	nodes/<LOCAL_HOST_NAME>
lxc	nodes/<LOCAL_HOST_NAME>/lxc/
openvz	nodes/<LOCAL_HOST_NAME>/openvz/ (deprecated, removed soon)
qemu-server	nodes/<LOCAL_HOST_NAME>/qemu-server/

Special status files for debugging (JSON)

.version	File versions (to detect file modifications)
.members	Info about cluster members
.vmlist	List of all VMs
.clusterlog	Cluster log (last 50 entries)
.rrd	RRD data (most recent entries)

Enable/Disable debugging

You can enable verbose syslog messages with:

echo "1" >/etc/pve/.debug

And disable verbose syslog messages with:

echo "0" >/etc/pve/.debug

Recovery

If you have major problems with your Proxmox VE host, for example hardware issues, it could be helpful to copy the pmxcfs database file /var/lib/pve-cluster/config.db, and move it to a new Proxmox VE host. On the new host (with nothing running), you need to stop the pve-cluster service and replace the config.db file (required permissions 0600). Following this, adapt /etc/hostname and /etc/hosts according to the lost Proxmox VE host, then reboot and check (and don’t forget your VM/CT data).

Remove Cluster Configuration

The recommended way is to reinstall the node after you remove it from your cluster. This ensures that all secret cluster/ssh keys and any shared configuration data is destroyed.

In some cases, you might prefer to put a node back to local mode without reinstalling, which is described in Separate A Node Without Reinstalling

Recovering/Moving Guests from Failed Nodes

For the guest configuration files in nodes/<NAME>/qemu-server/ (VMs) and nodes/<NAME>/lxc/ (containers), Proxmox VE sees the containing node <NAME> as the owner of the respective guest. This concept enables the usage of local locks instead of expensive cluster-wide locks for preventing concurrent guest configuration changes.

As a consequence, if the owning node of a guest fails (for example, due to a power outage, fencing event, etc.), a regular migration is not possible (even if all the disks are located on shared storage), because such a local lock on the (offline) owning node is unobtainable. This is not a problem for HA-managed guests, as Proxmox VE’s High Availability stack includes the necessary (cluster-wide) locking and watchdog functionality to ensure correct and automatic recovery of guests from fenced nodes.

If a non-HA-managed guest has only shared disks (and no other local resources which are only available on the failed node), a manual recovery is possible by simply moving the guest configuration file from the failed node’s directory in /etc/pve/ to an online node’s directory (which changes the logical owner or location of the guest).

For example, recovering the VM with ID 100 from an offline node1 to another node node2 works by running the following command as root on any member node of the cluster:

mv /etc/pve/nodes/node1/qemu-server/100.conf /etc/pve/nodes/node2/qemu-server/

Before manually recovering a guest like this, make absolutely sure that the failed source node is really powered off/fenced. Otherwise Proxmox VE’s locking principles are violated by the mv command, which can have unexpected consequences.

Guests with local disks (or other local resources which are only available on the offline node) are not recoverable like this. Either wait for the failed node to rejoin the cluster or restore such guests from backups.