Proxmox Cluster file system (pmxcfs)
Note: Article about Proxmox VE 2.0 beta |
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
Source code
The source code is available here.