Provided by: ctdb_4.20.4+dfsg-1ubuntu1_amd64 
NAME
ctdb - Clustered TDB
DESCRIPTION
CTDB is a clustered database component in clustered Samba that provides a
high-availability load-sharing CIFS server cluster.
The main functions of CTDB are:
• Provide a clustered version of the TDB database with automatic rebuild/recovery of the
databases upon node failures.
• Monitor nodes in the cluster and services running on each node.
• Manage a pool of public IP addresses that are used to provide services to clients.
Alternatively, CTDB can be used with LVS.
Combined with a cluster filesystem CTDB provides a full high-availablity (HA) environment
for services such as clustered Samba, NFS and other services.
In addition to the CTDB manual pages there is much more information available at
https://wiki.samba.org/index.php/CTDB_and_Clustered_Samba.
ANATOMY OF A CTDB CLUSTER
A CTDB cluster is a collection of nodes with 2 or more network interfaces. All nodes
provide network (usually file/NAS) services to clients. Data served by file services is
stored on shared storage (usually a cluster filesystem) that is accessible by all nodes.
CTDB provides an "all active" cluster, where services are load balanced across all nodes.
CLUSTER LEADER
CTDB uses a cluster leader and follower model of cluster management. All nodes in a
cluster elect one node to be the leader. The leader node coordinates privileged operations
such as database recovery and IP address failover.
CTDB previously referred to the leader as the recovery master or recmaster. References to
these terms may still be found in documentation and code.
CLUSTER LOCK
CTDB uses a cluster lock to assert its privileged role in the cluster. This node takes the
cluster lock when it becomes leader and holds the lock until it is no longer leader. The
cluster lock helps CTDB to avoid a split brain, where a cluster becomes partitioned and
each partition attempts to operate independently. Issues that can result from a split
brain include file data corruption, because file locking metadata may not be tracked
correctly.
CTDB previously referred to the cluster lock as the recovery lock. The abbreviation
reclock is still used - just "clock" would be confusing.
CTDB is unable configure a default cluster lock, because this would depend on factors such
as cluster filesystem mountpoints. However, running CTDB without a cluster lock is not
recommended as there will be no split brain protection.
When a cluster lock is configured it is used as the election mechanism. Nodes race to take
the cluster lock and the winner is the cluster leader. This avoids problems when a node
wins an election but is unable to take the lock - this can occur if a cluster becomes
partitioned (for example, due to a communication failure) and a different leader is
elected by the nodes in each partition, or if the cluster filesystem has a high failover
latency.
By default, the cluster lock is implemented using a file (specified by cluster lock in the
[cluster] section of ctdb.conf(5)) residing in shared storage (usually) on a cluster
filesystem. To support a cluster lock the cluster filesystem must support lock coherence.
See ping_pong(1) for more details.
The cluster lock can also be implemented using an arbitrary cluster mutex helper (or
call-out). This is indicated by using an exclamation point ('!') as the first character of
the cluster lock parameter. For example, a value of !/usr/bin/myhelper cluster would run
the given helper with the specified arguments. The helper will continue to run as long as
it holds its mutex. See ctdb/doc/cluster_mutex_helper.txt in the source tree, and related
code, for clues about writing helpers.
When a file is specified for the cluster lock parameter (i.e. no leading '!') the file
lock is implemented by a default helper (/usr/libexec/ctdb/ctdb_mutex_fcntl_helper). This
helper has arguments as follows:
ctdb_mutex_fcntl_helper FILE [RECHECK-INTERVAL]
ctdb_mutex_fcntl_helper will take a lock on FILE and then check every RECHECK-INTERVAL
seconds to ensure that FILE still exists and that its inode number is unchanged from when
the lock was taken. The default value for RECHECK-INTERVAL is 5.
CTDB does sanity checks to ensure that the cluster lock is held as expected.
PRIVATE VS PUBLIC ADDRESSES
Each node in a CTDB cluster has multiple IP addresses assigned to it:
• A single private IP address that is used for communication between nodes.
• One or more public IP addresses that are used to provide NAS or other services.
Private address
Each node is configured with a unique, permanently assigned private address. This address
is configured by the operating system. This address uniquely identifies a physical node in
the cluster and is the address that CTDB daemons will use to communicate with the CTDB
daemons on other nodes.
Private addresses are listed in the file /etc/ctdb/nodes). This file contains the list of
private addresses for all nodes in the cluster, one per line. This file must be the same
on all nodes in the cluster.
Some users like to put this configuration file in their cluster filesystem. A symbolic
link should be used in this case.
Private addresses should not be used by clients to connect to services provided by the
cluster.
It is strongly recommended that the private addresses are configured on a private network
that is separate from client networks. This is because the CTDB protocol is both
unauthenticated and unencrypted. If clients share the private network then steps need to
be taken to stop injection of packets to relevant ports on the private addresses. It is
also likely that CTDB protocol traffic between nodes could leak sensitive information if
it can be intercepted.
Example /etc/ctdb/nodes for a four node cluster:
192.168.1.1
192.168.1.2
192.168.1.3
192.168.1.4
Public addresses
Public addresses are used to provide services to clients. Public addresses are not
configured at the operating system level and are not permanently associated with a
particular node. Instead, they are managed by CTDB and are assigned to interfaces on
physical nodes at runtime.
The CTDB cluster will assign/reassign these public addresses across the available healthy
nodes in the cluster. When one node fails, its public addresses will be taken over by one
or more other nodes in the cluster. This ensures that services provided by all public
addresses are always available to clients, as long as there are nodes available capable of
hosting this address.
The public address configuration is stored in /etc/ctdb/public_addresses on each node.
This file contains a list of the public addresses that the node is capable of hosting, one
per line. Each entry also contains the netmask and the interface to which the address
should be assigned. If this file is missing then no public addresses are configured.
Some users who have the same public addresses on all nodes like to put this configuration
file in their cluster filesystem. A symbolic link should be used in this case.
Example /etc/ctdb/public_addresses for a node that can host 4 public addresses, on 2
different interfaces:
10.1.1.1/24 eth1
10.1.1.2/24 eth1
10.1.2.1/24 eth2
10.1.2.2/24 eth2
In many cases the public addresses file will be the same on all nodes. However, it is
possible to use different public address configurations on different nodes.
Example: 4 nodes partitioned into two subgroups:
Node 0:/etc/ctdb/public_addresses
10.1.1.1/24 eth1
10.1.1.2/24 eth1
Node 1:/etc/ctdb/public_addresses
10.1.1.1/24 eth1
10.1.1.2/24 eth1
Node 2:/etc/ctdb/public_addresses
10.1.2.1/24 eth2
10.1.2.2/24 eth2
Node 3:/etc/ctdb/public_addresses
10.1.2.1/24 eth2
10.1.2.2/24 eth2
In this example nodes 0 and 1 host two public addresses on the 10.1.1.x network while
nodes 2 and 3 host two public addresses for the 10.1.2.x network.
Public address 10.1.1.1 can be hosted by either of nodes 0 or 1 and will be available to
clients as long as at least one of these two nodes are available.
If both nodes 0 and 1 become unavailable then public address 10.1.1.1 also becomes
unavailable. 10.1.1.1 can not be failed over to nodes 2 or 3 since these nodes do not have
this public address configured.
The ctdb ip command can be used to view the current assignment of public addresses to
physical nodes.
NODE STATUS
The current status of each node in the cluster can be viewed by the ctdb status command.
A node can be in one of the following states:
OK
This node is healthy and fully functional. It hosts public addresses to provide
services.
DISCONNECTED
This node is not reachable by other nodes via the private network. It is not currently
participating in the cluster. It does not host public addresses to provide services.
It might be shut down.
DISABLED
This node has been administratively disabled. This node is partially functional and
participates in the cluster. However, it does not host public addresses to provide
services.
UNHEALTHY
A service provided by this node has failed a health check and should be investigated.
This node is partially functional and participates in the cluster. However, it does
not host public addresses to provide services. Unhealthy nodes should be investigated
and may require an administrative action to rectify.
BANNED
CTDB is not behaving as designed on this node. For example, it may have failed too
many recovery attempts. Such nodes are banned from participating in the cluster for a
configurable time period before they attempt to rejoin the cluster. A banned node does
not host public addresses to provide services. All banned nodes should be investigated
and may require an administrative action to rectify.
STOPPED
This node has been administratively exclude from the cluster. A stopped node does no
participate in the cluster and does not host public addresses to provide services.
This state can be used while performing maintenance on a node.
PARTIALLYONLINE
A node that is partially online participates in a cluster like a healthy (OK) node.
Some interfaces to serve public addresses are down, but at least one interface is up.
See also ctdb ifaces.
CAPABILITIES
Cluster nodes can have several different capabilities enabled. These are listed below.
LEADER
Indicates that a node can become the CTDB cluster leader. The current leader is
decided via an election held by all active nodes with this capability.
Default is YES.
LMASTER
Indicates that a node can be the location master (LMASTER) for database records. The
LMASTER always knows which node has the latest copy of a record in a volatile
database.
Default is YES.
The LEADER and LMASTER capabilities can be disabled when CTDB is used to create a cluster
spanning across WAN links. In this case CTDB acts as a WAN accelerator.
LVS
LVS is a mode where CTDB presents one single IP address for the entire cluster. This is an
alternative to using public IP addresses and round-robin DNS to loadbalance clients across
the cluster.
This is similar to using a layer-4 loadbalancing switch but with some restrictions.
One extra LVS public address is assigned on the public network to each LVS group. Each LVS
group is a set of nodes in the cluster that presents the same LVS address public address
to the outside world. Normally there would only be one LVS group spanning an entire
cluster, but in situations where one CTDB cluster spans multiple physical sites it might
be useful to have one LVS group for each site. There can be multiple LVS groups in a
cluster but each node can only be member of one LVS group.
Client access to the cluster is load-balanced across the HEALTHY nodes in an LVS group. If
no HEALTHY nodes exists then all nodes in the group are used, regardless of health status.
CTDB will, however never load-balance LVS traffic to nodes that are BANNED, STOPPED,
DISABLED or DISCONNECTED. The ctdb lvs command is used to show which nodes are currently
load-balanced across.
In each LVS group, one of the nodes is selected by CTDB to be the LVS leader. This node
receives all traffic from clients coming in to the LVS public address and multiplexes it
across the internal network to one of the nodes that LVS is using. When responding to the
client, that node will send the data back directly to the client, bypassing the LVS leader
node. The command ctdb lvs leader will show which node is the current LVS leader.
The path used for a client I/O is:
1. Client sends request packet to LVS leader.
2. LVS leader passes the request on to one node across the internal network.
3. Selected node processes the request.
4. Node responds back to client.
This means that all incoming traffic to the cluster will pass through one physical node,
which limits scalability. You can send more data to the LVS address that one physical node
can multiplex. This means that you should not use LVS if your I/O pattern is
write-intensive since you will be limited in the available network bandwidth that node can
handle. LVS does work very well for read-intensive workloads where only smallish READ
requests are going through the LVS leader bottleneck and the majority of the traffic
volume (the data in the read replies) goes straight from the processing node back to the
clients. For read-intensive i/o patterns you can achieve very high throughput rates in
this mode.
Note: you can use LVS and public addresses at the same time.
If you use LVS, you must have a permanent address configured for the public interface on
each node. This address must be routable and the cluster nodes must be configured so that
all traffic back to client hosts are routed through this interface. This is also required
in order to allow samba/winbind on the node to talk to the domain controller. This LVS IP
address can not be used to initiate outgoing traffic.
Make sure that the domain controller and the clients are reachable from a node before you
enable LVS. Also ensure that outgoing traffic to these hosts is routed out through the
configured public interface.
Configuration
To activate LVS on a CTDB node you must specify the CTDB_LVS_PUBLIC_IFACE,
CTDB_LVS_PUBLIC_IP and CTDB_LVS_NODES configuration variables. CTDB_LVS_NODES specifies a
file containing the private address of all nodes in the current node's LVS group.
Example:
CTDB_LVS_PUBLIC_IFACE=eth1
CTDB_LVS_PUBLIC_IP=10.1.1.237
CTDB_LVS_NODES=/etc/ctdb/lvs_nodes
Example /etc/ctdb/lvs_nodes:
192.168.1.2
192.168.1.3
192.168.1.4
Normally any node in an LVS group can act as the LVS leader. Nodes that are highly loaded
due to other demands maybe flagged with the "follower-only" option in the CTDB_LVS_NODES
file to limit the LVS functionality of those nodes.
LVS nodes file that excludes 192.168.1.4 from being the LVS leader node:
192.168.1.2
192.168.1.3
192.168.1.4 follower-only
TRACKING AND RESETTING TCP CONNECTIONS
CTDB tracks TCP connections from clients to public IP addresses, on known ports. When an
IP address moves from one node to another, all existing TCP connections to that IP address
are reset. The node taking over this IP address will also send gratuitous ARPs (for IPv4,
or neighbour advertisement, for IPv6). This allows clients to reconnect quickly, rather
than waiting for TCP timeouts, which can be very long.
It is important that established TCP connections do not survive a release and take of a
public IP address on the same node. Such connections can get out of sync with sequence and
ACK numbers, potentially causing a disruptive ACK storm.
NAT GATEWAY
NAT gateway (NATGW) is an optional feature that is used to configure fallback routing for
nodes. This allows cluster nodes to connect to external services (e.g. DNS, AD, NIS and
LDAP) when they do not host any public addresses (e.g. when they are unhealthy).
This also applies to node startup because CTDB marks nodes as UNHEALTHY until they have
passed a "monitor" event. In this context, NAT gateway helps to avoid a "chicken and egg"
situation where a node needs to access an external service to become healthy.
Another way of solving this type of problem is to assign an extra static IP address to a
public interface on every node. This is simpler but it uses an extra IP address per node,
while NAT gateway generally uses only one extra IP address.
Operation
One extra NATGW public address is assigned on the public network to each NATGW group. Each
NATGW group is a set of nodes in the cluster that shares the same NATGW address to talk to
the outside world. Normally there would only be one NATGW group spanning an entire
cluster, but in situations where one CTDB cluster spans multiple physical sites it might
be useful to have one NATGW group for each site.
There can be multiple NATGW groups in a cluster but each node can only be member of one
NATGW group.
In each NATGW group, one of the nodes is selected by CTDB to be the NATGW leader and the
other nodes are consider to be NATGW followers. NATGW followers establish a fallback
default route to the NATGW leader via the private network. When a NATGW follower hosts no
public IP addresses then it will use this route for outbound connections. The NATGW leader
hosts the NATGW public IP address and routes outgoing connections from follower nodes via
this IP address. It also establishes a fallback default route.
Configuration
NATGW is usually configured similar to the following example configuration:
CTDB_NATGW_NODES=/etc/ctdb/natgw_nodes
CTDB_NATGW_PRIVATE_NETWORK=192.168.1.0/24
CTDB_NATGW_PUBLIC_IP=10.0.0.227/24
CTDB_NATGW_PUBLIC_IFACE=eth0
CTDB_NATGW_DEFAULT_GATEWAY=10.0.0.1
Normally any node in a NATGW group can act as the NATGW leader. Some configurations may
have special nodes that lack connectivity to a public network. In such cases, those nodes
can be flagged with the "follower-only" option in the CTDB_NATGW_NODES file to limit the
NATGW functionality of those nodes.
See the NAT GATEWAY section in ctdb-script.options(5) for more details of NATGW
configuration.
Implementation details
When the NATGW functionality is used, one of the nodes is selected to act as a NAT gateway
for all the other nodes in the group when they need to communicate with the external
services. The NATGW leader is selected to be a node that is most likely to have usable
networks.
The NATGW leader hosts the NATGW public IP address CTDB_NATGW_PUBLIC_IP on the configured
public interfaces CTDB_NATGW_PUBLIC_IFACE and acts as a router, masquerading outgoing
connections from follower nodes via this IP address. If CTDB_NATGW_DEFAULT_GATEWAY is set
then it also establishes a fallback default route to the configured this gateway with a
metric of 10. A metric 10 route is used so it can co-exist with other default routes that
may be available.
A NATGW follower establishes its fallback default route to the NATGW leader via the
private network CTDB_NATGW_PRIVATE_NETWORKwith a metric of 10. This route is used for
outbound connections when no other default route is available because the node hosts no
public addresses. A metric 10 routes is used so that it can co-exist with other default
routes that may be available when the node is hosting public addresses.
CTDB_NATGW_STATIC_ROUTES can be used to have NATGW create more specific routes instead of
just default routes.
This is implemented in the 11.natgw eventscript. Please see the eventscript file and the
NAT GATEWAY section in ctdb-script.options(5) for more details.
POLICY ROUTING
Policy routing is an optional CTDB feature to support complex network topologies. Public
addresses may be spread across several different networks (or VLANs) and it may not be
possible to route packets from these public addresses via the system's default route.
Therefore, CTDB has support for policy routing via the 13.per_ip_routing eventscript. This
allows routing to be specified for packets sourced from each public address. The routes
are added and removed as CTDB moves public addresses between nodes.
Configuration variables
There are 4 configuration variables related to policy routing: CTDB_PER_IP_ROUTING_CONF,
CTDB_PER_IP_ROUTING_RULE_PREF, CTDB_PER_IP_ROUTING_TABLE_ID_LOW,
CTDB_PER_IP_ROUTING_TABLE_ID_HIGH. See the POLICY ROUTING section in ctdb-
script.options(5) for more details.
Configuration
The format of each line of CTDB_PER_IP_ROUTING_CONF is:
<public_address> <network> [ <gateway> ]
Leading whitespace is ignored and arbitrary whitespace may be used as a separator. Lines
that have a "public address" item that doesn't match an actual public address are ignored.
This means that comment lines can be added using a leading character such as '#', since
this will never match an IP address.
A line without a gateway indicates a link local route.
For example, consider the configuration line:
192.168.1.99 192.168.1.0/24
If the corresponding public_addresses line is:
192.168.1.99/24 eth2,eth3
CTDB_PER_IP_ROUTING_RULE_PREF is 100, and CTDB adds the address to eth2 then the following
routing information is added:
ip rule add from 192.168.1.99 pref 100 table ctdb.192.168.1.99
ip route add 192.168.1.0/24 dev eth2 table ctdb.192.168.1.99
This causes traffic from 192.168.1.99 to 192.168.1.0/24 go via eth2.
The ip rule command will show (something like - depending on other public addresses and
other routes on the system):
0: from all lookup local
100: from 192.168.1.99 lookup ctdb.192.168.1.99
32766: from all lookup main
32767: from all lookup default
ip route show table ctdb.192.168.1.99 will show:
192.168.1.0/24 dev eth2 scope link
The usual use for a line containing a gateway is to add a default route corresponding to a
particular source address. Consider this line of configuration:
192.168.1.99 0.0.0.0/0 192.168.1.1
In the situation described above this will cause an extra routing command to be executed:
ip route add 0.0.0.0/0 via 192.168.1.1 dev eth2 table ctdb.192.168.1.99
With both configuration lines, ip route show table ctdb.192.168.1.99 will show:
192.168.1.0/24 dev eth2 scope link
default via 192.168.1.1 dev eth2
Sample configuration
Here is a more complete example configuration.
/etc/ctdb/public_addresses:
192.168.1.98 eth2,eth3
192.168.1.99 eth2,eth3
/etc/ctdb/policy_routing:
192.168.1.98 192.168.1.0/24
192.168.1.98 192.168.200.0/24 192.168.1.254
192.168.1.98 0.0.0.0/0 192.168.1.1
192.168.1.99 192.168.1.0/24
192.168.1.99 192.168.200.0/24 192.168.1.254
192.168.1.99 0.0.0.0/0 192.168.1.1
The routes local packets as expected, the default route is as previously discussed, but
packets to 192.168.200.0/24 are routed via the alternate gateway 192.168.1.254.
NOTIFICATIONS
When certain state changes occur in CTDB, it can be configured to perform arbitrary
actions via notifications. For example, sending SNMP traps or emails when a node becomes
unhealthy or similar.
The notification mechanism runs all executable files ending in ".script" in
/etc/ctdb/events/notification/, ignoring any failures and continuing to run all files.
CTDB currently generates notifications after CTDB changes to these states:
init
setup
startup
healthy
unhealthy
LOG LEVELS
Valid log levels, in increasing order of verbosity, are:
ERROR
WARNING
NOTICE
INFO
DEBUG
REMOTE CLUSTER NODES
It is possible to have a CTDB cluster that spans across a WAN link. For example where you
have a CTDB cluster in your datacentre but you also want to have one additional CTDB node
located at a remote branch site. This is similar to how a WAN accelerator works but with
the difference that while a WAN-accelerator often acts as a Proxy or a MitM, in the ctdb
remote cluster node configuration the Samba instance at the remote site IS the genuine
server, not a proxy and not a MitM, and thus provides 100% correct CIFS semantics to
clients.
See the cluster as one single multihomed samba server where one of the NICs (the remote
node) is very far away.
NOTE: This does require that the cluster filesystem you use can cope with WAN-link
latencies. Not all cluster filesystems can handle WAN-link latencies! Whether this will
provide very good WAN-accelerator performance or it will perform very poorly depends
entirely on how optimized your cluster filesystem is in handling high latency for data and
metadata operations.
To activate a node as being a remote cluster node you need to set the following two
parameters in /etc/ctdb/ctdb.conf for the remote node:
[legacy]
lmaster capability = false
leader capability = false
Verify with the command "ctdb getcapabilities" that that node no longer has the leader or
the lmaster capabilities.
SEE ALSO
ctdb(1), ctdbd(1), ctdb_diagnostics(1), ltdbtool(1), onnode(1), ping_pong(1),
ctdb.conf(5), ctdb-script.options(5), ctdb.sysconfig(5), ctdb-statistics(7), ctdb-
tunables(7), https://wiki.samba.org/index.php/CTDB_and_Clustered_Samba,
http://ctdb.samba.org/
AUTHOR
This documentation was written by Ronnie Sahlberg, Amitay Isaacs, Martin Schwenke
COPYRIGHT
Copyright © 2007 Andrew Tridgell, Ronnie Sahlberg
This program is free software; you can redistribute it and/or modify it under the terms of
the GNU General Public License as published by the Free Software Foundation; either
version 3 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program;
if not, see http://www.gnu.org/licenses.