Proxmox Cluster file system (pmxcfs)

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Yellowpin.svg 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 im RAM. That imposes restriction on the maximal size, which is currently 30MB. This is still enough to store the configuration several thousand virtual machines.

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:

  • just normal files, 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.

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