Proxmox Cluster file system (pmxcfs)

Note: Article about Proxmox VE 2.0

Introduction

Proxmox Cluster file system (pmxcfs) is a database-driven file system for storing configuration files, replicated in real time on all nodes using corosync. We use this to store all PVE related configuration files. Although the file system stores all data inside a persistent database on disk, a copy of the data resides in RAM. That imposes restriction on the maximal size, which is currently 30MB. This is still enough to store the configuration of several thousand virtual machines.

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 looses quorum
• automatic updates of the corosync cluster configuration to all nodes.

POSIX Compatibility

The file system is based on fuse, so the behavior is POSIX like. But many 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 path)
• O_EXCL creates were not atomic (like old NFS)
• O_TRUNC creates are not atomic (fuse restriction)
• ...

File access rights

All files/dirs 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:

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

are only accessible by root.

Technologie

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

File system layout

The file system is mounted at:

/etc/pve

Files

cluster.conf  => corosync/cman cluster configuration file
storage.cfg   => PVE storage configuration
user.cfg      => PVE access control configuration (users/groups/...)
domains.cfg   => PVE Authentication domains 
authkey.pub   => public key used by ticket system

priv/shadow.cfg  => shadow password file
priv/authkey.key => private key used by ticket system

nodes/${NAME}/pve-ssl.pem                 => public ssl key fo apache
nodes/${NAME}/priv/pve-ssl.key            => private ssl key
nodes/${NAME}/qemu-server/${VMID}.conf    => VM configuration data for KVM VMs
nodes/${NAME}/openvz/${VMID}.conf         => VM configuratin data for OpenVZ containers

Symbolic links

local => nodes/${LOCALNAME}
qemu-server => nodes/${LOCALNAME}/qemu-server/
openvz => nodes/${LOCALNAME}/openvz/

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

# enable verbose syslog messages
echo "1" >/etc/pve/.debug 

# disable verbose syslog messages
echo "0" >/etc/pve/.debug 

Recovery

If you have major problems with your Proxmox VE host, e.g. hardware issues, it could be helpful to just 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 (needed permissions : 600). Second, 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)

Source code

The source code is available here.