pmxcfs: language and style fixup

[pve-docs.git] / pmxcfs.adoc

[[chapter_pmxcfs]]
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pmxcfs(8)
=========
:pve-toplevel:

NAME
----

pmxcfs - Proxmox Cluster File System

SYNOPSIS
--------

include::pmxcfs.8-synopsis.adoc[]

DESCRIPTION
-----------
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Proxmox Cluster File System (pmxcfs)
====================================
:pve-toplevel:
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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 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. This imposes restrictions
on the maximum size, which is currently 30MB. 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 https://www.corosync.org[Corosync Cluster Engine] for
cluster communication, and https://www.sqlite.org[SQlite] for the
database file. The file system is implemented in user space using
https://github.com/libfuse/libfuse[FUSE].

File System Layout
------------------

The file system is mounted at:

 /etc/pve

Files
~~~~~

[width="100%",cols="m,d"]
|=======
|`corosync.conf`                        | Corosync cluster configuration file (previous to {pve} 4.x this file was called cluster.conf)
|`storage.cfg`                          | {pve} storage configuration
|`datacenter.cfg`                       | {pve} datacenter wide configuration (keyboard layout, proxy, ...)
|`user.cfg`                             | {pve} access control configuration (users/groups/...)
|`domains.cfg`                          | {pve} authentication domains
|`status.cfg`                           | {pve} external metrics server configuration
|`authkey.pub`                          | Public key used by ticket system
|`pve-root-ca.pem`                      | Public certificate of cluster CA
|`priv/shadow.cfg`                      | Shadow password file
|`priv/authkey.key`                     | Private key used by ticket system
|`priv/pve-root-ca.key`                 | Private key of cluster CA
|`nodes/<NAME>/pve-ssl.pem`             | Public SSL certificate for web server (signed by cluster CA)
|`nodes/<NAME>/pve-ssl.key`             | Private SSL key for `pve-ssl.pem`
|`nodes/<NAME>/pveproxy-ssl.pem`        | Public SSL certificate (chain) for web server (optional override for `pve-ssl.pem`)
|`nodes/<NAME>/pveproxy-ssl.key`        | Private SSL key for `pveproxy-ssl.pem` (optional)
|`nodes/<NAME>/qemu-server/<VMID>.conf` | VM configuration data for KVM VMs
|`nodes/<NAME>/lxc/<VMID>.conf`         | VM configuration data for LXC containers
|`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
|=======


Symbolic links
~~~~~~~~~~~~~~

[width="100%",cols="m,m"]
|=======
|`local`         | `nodes/<LOCAL_HOST_NAME>`
|`qemu-server`   | `nodes/<LOCAL_HOST_NAME>/qemu-server/`
|`lxc`           | `nodes/<LOCAL_HOST_NAME>/lxc/`
|=======


Special status files for debugging (JSON)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[width="100%",cols="m,d"]
|=======
|`.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 {pve} 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 {pve}
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 {pve} 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
<<pvecm_separate_node_without_reinstall,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), {pve} 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 {pve}'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/

WARNING: Before manually recovering a guest like this, make absolutely sure
that the failed source node is really powered off/fenced. Otherwise {pve}'s
locking principles are violated by the `mv` command, which can have unexpected
consequences.

WARNING: 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.

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