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NAME | DESCRIPTION | SCHEMAS | SERVICE MODELS | DATABASE REPLICATION | CONNECTION METHODS | DATABASE LIFE CYCLE | SEE ALSO | AUTHOR | COPYRIGHT | COLOPHON |
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OVSDB(7) Open vSwitch OVSDB(7)
ovsdb - Open vSwitch Database (Overview)
OVSDB, the Open vSwitch Database, is a network-accessible
database system. Schemas in OVSDB specify the tables in a
database and their columns’ types and can include data,
uniqueness, and referential integrity constraints. OVSDB offers
atomic, consistent, isolated, durable transactions. RFC 7047
specifies the JSON-RPC based protocol that OVSDB clients and
servers use to communicate.
The OVSDB protocol is well suited for state synchronization
because it allows each client to monitor the contents of a whole
database or a subset of it. Whenever a monitored portion of the
database changes, the server tells the client what rows were
added or modified (including the new contents) or deleted. Thus,
OVSDB clients can easily keep track of the newest contents of any
part of the database.
While OVSDB is general-purpose and not particularly specialized
for use with Open vSwitch, Open vSwitch does use it for multiple
purposes. The leading use of OVSDB is for configuring and
monitoring ovs-vswitchd(8), the Open vSwitch switch daemon, using
the schema documented in ovs-vswitchd.conf.db(5). The Open
Virtual Network (OVN) project uses two OVSDB schemas, documented
as part of that project. Finally, Open vSwitch includes the
“VTEP” schema, documented in vtep(5) that many third-party
hardware switches support for configuring VXLAN, although OVS
itself does not directly use this schema.
The OVSDB protocol specification allows independent,
interoperable implementations of OVSDB to be developed. Open
vSwitch includes an OVSDB server implementation named
ovsdb-server(1), which supports several protocol extensions
documented in its manpage, and a basic command-line OVSDB client
named ovsdb-client(1), as well as OVSDB client libraries for C
and for Python. Open vSwitch documentation often speaks of these
OVSDB implementations in Open vSwitch as simply “OVSDB,” even
though that is distinct from the OVSDB protocol; we make the
distinction explicit only when it might otherwise be unclear from
the context.
In addition to these generic OVSDB server and client tools, Open
vSwitch includes tools for working with databases that have
specific schemas: ovs-vsctl works with the ovs-vswitchd
configuration database and vtep-ctl works with the VTEP database.
RFC 7047 specifies the OVSDB protocol but it does not specify an
on-disk storage format. Open vSwitch includes ovsdb-tool(1) for
working with its own on-disk database formats. The most notable
feature of this format is that ovsdb-tool(1) makes it easy for
users to print the transactions that have changed a database
since the last time it was compacted. This feature is often
useful for troubleshooting.
Schemas in OVSDB have a JSON format that is specified in RFC
7047. They are often stored in files with an extension
.ovsschema. An on-disk database in OVSDB includes a schema and
data, embedding both into a single file. The Open vSwitch
utility ovsdb-tool has commands that work with schema files and
with the schemas embedded in database files.
An Open vSwitch schema has three important identifiers. The
first is its name, which is also the name used in JSON-RPC calls
to identify a database based on that schema. For example, the
schema used to configure Open vSwitch has the name Open_vSwitch.
Schema names begin with a letter or an underscore, followed by
any number of letters, underscores, or digits. The ovsdb-tool
commands schema-name and db-name extract the schema name from a
schema or database file, respectively.
An OVSDB schema also has a version of the form x.y.z e.g. 1.2.3.
Schemas managed within the Open vSwitch project manage version
numbering in the following way (but OVSDB does not mandate this
approach). Whenever we change the database schema in a
non-backward compatible way (e.g. when we delete a column or a
table), we increment <x> and set <y> and <z> to 0. When we
change the database schema in a backward compatible way (e.g.
when we add a new column), we increment <y> and set <z> to 0.
When we change the database schema cosmetically (e.g. we reindent
its syntax), we increment <z>. The ovsdb-tool commands
schema-version and db-version extract the schema version from a
schema or database file, respectively.
Very old OVSDB schemas do not have a version, but RFC 7047
mandates it.
An OVSDB schema optionally has a “checksum.” RFC 7047 does not
specify the use of the checksum and recommends that clients
ignore it. Open vSwitch uses the checksum to remind developers
to update the version: at build time, if the schema’s embedded
checksum, ignoring the checksum field itself, does not match the
schema’s content, then it fails the build with a recommendation
to update the version and the checksum. Thus, a developer who
changes the schema, but does not update the version, receives an
automatic reminder. In practice this has been an effective way
to ensure compliance with the version number policy. The
ovsdb-tool commands schema-cksum and db-cksum extract the schema
checksum from a schema or database file, respectively.
OVSDB supports four service models for databases: standalone,
active-backup, relay and clustered. The service models provide
different compromises among consistency, availability, and
partition tolerance. They also differ in the number of servers
required and in terms of performance. The standalone and
active-backup database service models share one on-disk format,
and clustered databases use a different format, but the OVSDB
programs work with both formats. ovsdb(5) documents these file
formats. Relay databases have no on-disk storage.
RFC 7047, which specifies the OVSDB protocol, does not mandate or
specify any particular service model.
The following sections describe the individual service models.
Standalone Database Service Model
A standalone database runs a single server. If the server stops
running, the database becomes inaccessible, and if the server’s
storage is lost or corrupted, the database’s content is lost.
This service model is appropriate when the database controls a
process or activity to which it is linked via “fate-sharing.”
For example, an OVSDB instance that controls an Open vSwitch
virtual switch daemon, ovs-vswitchd, is a standalone database
because a server failure would take out both the database and the
virtual switch.
To set up a standalone database, use ovsdb-tool create to create
a database file, then run ovsdb-server to start the database
service.
To configure a client, such as ovs-vswitchd or ovs-vsctl, to use
a standalone database, configure the server to listen on a
“connection method” that the client can reach, then point the
client to that connection method. See Connection Methods below
for information about connection methods.
Active-Backup Database Service Model
An active-backup database runs two servers (on different hosts).
At any given time, one of the servers is designated with the
active role and the other the backup role. An active server
behaves just like a standalone server. A backup server makes an
OVSDB connection to the active server and uses it to continuously
replicate its content as it changes in real time. OVSDB clients
can connect to either server but only the active server allows
data modification or lock transactions.
Setup for an active-backup database starts from a working
standalone database service, which is initially the active
server. On another node, to set up a backup server, create a
database file with the same schema as the active server. The
initial contents of the database file do not matter, as long as
the schema is correct, so ovsdb-tool create will work, as will
copying the database file from the active server. Then use
ovsdb-server --sync-from=<active> to start the backup server,
where <active> is an OVSDB connection method (see Connection
Methods below) that connects to the active server. At that
point, the backup server will fetch a copy of the active database
and keep it up-to-date until it is killed.
When the active server in an active-backup server pair fails, an
administrator can switch the backup server to an active role with
the ovs-appctl command
ovsdb-server/disconnect-active-ovsdb-server. Clients then have
read/write access to the now-active server. Of course,
administrators are slow to respond compared to software, so in
practice external management software detects the active server’s
failure and changes the backup server’s role. For example, the
“Integration Guide for Centralized Control” in the OVN
documentation describes how to use Pacemaker for this purpose in
OVN.
Suppose an active server fails and its backup is promoted to
active. If the failed server is revived, it must be started as a
backup server. Otherwise, if both servers are active, then they
may start out of sync, if the database changed while the server
was down, and they will continue to diverge over time. This also
happens if the software managing the database servers cannot
reach the active server and therefore switches the backup to
active, but other hosts can reach both servers. These
“split-brain” problems are unsolvable in general for server
pairs.
Compared to a standalone server, the active-backup service model
somewhat increases availability, at a risk of split-brain. It
adds generally insignificant performance overhead. On the other
hand, the clustered service model, discussed below, requires at
least 3 servers and has greater performance overhead, but it
avoids the need for external management software and eliminates
the possibility of split-brain.
Open vSwitch 2.6 introduced support for the active-backup service
model.
IMPORTANT:
There was a change of a database file format in version 2.15.
To upgrade/downgrade the ovsdb-server processes across this
version follow the instructions described under Upgrading from
version 2.14 and earlier to 2.15 and later and Downgrading
from version 2.15 and later to 2.14 and earlier.
Another change happened in version 3.2. To upgrade/downgrade
the ovsdb-server processes across this version follow the
instructions described under Upgrading from version 3.1 and
earlier to 3.2 and later and Downgrading from version 3.2 and
later to 3.1 and earlier.
Clustered Database Service Model
A clustered database runs across 3 or 5 or more database servers
(the cluster) on different hosts. Servers in a cluster
automatically synchronize writes within the cluster. A 3-server
cluster can remain available in the face of at most 1 server
failure; a 5-server cluster tolerates up to 2 failures. Clusters
larger than 5 servers will also work, with every 2 added servers
allowing the cluster to tolerate 1 more failure, but write
performance decreases. The number of servers should be odd: a 4-
or 6-server cluster cannot tolerate more failures than a 3- or
5-server cluster, respectively.
To set up a clustered database, first initialize it on a single
node by running ovsdb-tool create-cluster, then start
ovsdb-server. Depending on its arguments, the create-cluster
command can create an empty database or copy a standalone
database’s contents into the new database.
To configure a client to use a clustered database, first
configure all of the servers to listen on a connection method
that the client can reach, then point the client to all of the
servers’ connection methods, comma-separated. See Connection
Methods, below, for more detail.
Open vSwitch 2.9 introduced support for the clustered service
model.
How to Maintain a Clustered Database
To add a server to a cluster, run ovsdb-tool join-cluster on the
new server and start ovsdb-server. To remove a running server
from a cluster, use ovs-appctl to invoke the cluster/leave
command. When a server fails and cannot be recovered, e.g.
because its hard disk crashed, or to otherwise remove a server
that is down from a cluster, use ovs-appctl to invoke
cluster/kick to make the remaining servers kick it out of the
cluster.
The above methods for adding and removing servers only work for
healthy clusters, that is, for clusters with no more failures
than their maximum tolerance. For example, in a 3-server
cluster, the failure of 2 servers prevents servers joining or
leaving the cluster (as well as database access). To prevent
data loss or inconsistency, the preferred solution to this
problem is to bring up enough of the failed servers to make the
cluster healthy again, then if necessary remove any remaining
failed servers and add new ones. If this cannot be done, though,
use ovs-appctl to invoke cluster/leave --force on a running
server. This command forces the server to which it is directed
to leave its cluster and form a new single-node cluster that
contains only itself. The data in the new cluster may be
inconsistent with the former cluster: transactions not yet
replicated to the server will be lost, and transactions not yet
applied to the cluster may be committed. Afterward, any servers
in its former cluster will regard the server to have failed.
Once a server leaves a cluster, it may never rejoin it. Instead,
create a new server and join it to the cluster.
The servers in a cluster synchronize data over a cluster
management protocol that is specific to Open vSwitch; it is not
the same as the OVSDB protocol specified in RFC 7047. For this
purpose, a server in a cluster is tied to a particular IP address
and TCP port, which is specified in the ovsdb-tool command that
creates or joins the cluster. The TCP port used for clustering
must be different from that used for OVSDB clients. To change
the port or address of a server in a cluster, first remove it
from the cluster, then add it back with the new address.
To upgrade the ovsdb-server processes in a cluster from one
version of Open vSwitch to another, upgrading them one at a time
will keep the cluster healthy during the upgrade process. (This
is different from upgrading a database schema, which is covered
later under Upgrading or Downgrading a Database.)
IMPORTANT:
There was a change of a database file format in version 2.15.
To upgrade/downgrade the ovsdb-server processes across this
version follow the instructions described under Upgrading from
version 2.14 and earlier to 2.15 and later and Downgrading
from version 2.15 and later to 2.14 and earlier.
Another change happened in version 3.2. To upgrade/downgrade
the ovsdb-server processes across this version follow the
instructions described under Upgrading from version 3.1 and
earlier to 3.2 and later and Downgrading from version 3.2 and
later to 3.1 and earlier.
Clustered OVSDB does not support the OVSDB “ephemeral columns”
feature. ovsdb-tool and ovsdb-client change ephemeral columns
into persistent ones when they work with schemas for clustered
databases. Future versions of OVSDB might add support for this
feature.
Upgrading from version 2.14 and earlier to 2.15 and later
There is a change of a database file format in version 2.15 that
doesn’t allow older versions of ovsdb-server to read the database
file modified by the ovsdb-server version 2.15 or later. This
also affects runtime communications between servers in
active-backup and cluster service models. To upgrade the
ovsdb-server processes from one version of Open vSwitch (2.14 or
earlier) to another (2.15 or higher) instructions below should be
followed. (This is different from upgrading a database schema,
which is covered later under Upgrading or Downgrading a
Database.)
In case of standalone service model no special handling during
upgrade is required.
For the active-backup service model, administrator needs to
update backup ovsdb-server first and the active one after that,
or shut down both servers and upgrade at the same time.
For the cluster service model recommended upgrade strategy is
following:
1. Upgrade processes one at a time. Each ovsdb-server process
after upgrade should be started with
--disable-file-column-diff command line argument.
2. When all ovsdb-server processes upgraded, use ovs-appctl to
invoke ovsdb/file/column-diff-enable command on each of them
or restart all ovsdb-server processes one at a time without
--disable-file-column-diff command line option.
Downgrading from version 2.15 and later to 2.14 and earlier
Similar to upgrading covered under Upgrading from version 2.14
and earlier to 2.15 and later, downgrading from the ovsdb-server
version 2.15 and later to 2.14 and earlier requires additional
steps. (This is different from upgrading a database schema, which
is covered later under Upgrading or Downgrading a Database.)
For all service models it’s required to:
1. Stop all ovsdb-server processes (single process for standalone
service model, all involved processes for active-backup and
cluster service models).
2. Compact all database files with ovsdb-tool compact command.
3. Downgrade and restart ovsdb-server processes.
Upgrading from version 3.1 and earlier to 3.2 and later
There is another change of a database file format in version 3.2
that doesn’t allow older versions of ovsdb-server to read the
database file modified by the ovsdb-server version 3.2 or later.
This also affects runtime communications between servers in
cluster service models. To upgrade the ovsdb-server processes
from one version of Open vSwitch (3.1 or earlier) to another (3.2
or higher) instructions below should be followed. (This is
different from upgrading a database schema, which is covered
later under Upgrading or Downgrading a Database.)
In case of standalone or active-backup service model no special
handling during upgrade is required.
For the cluster service model recommended upgrade strategy is
following:
1. Upgrade processes one at a time. Each ovsdb-server process
after upgrade should be started with
--disable-file-no-data-conversion command line argument.
2. When all ovsdb-server processes upgraded, use ovs-appctl to
invoke ovsdb/file/no-data-conversion-enable command on each of
them or restart all ovsdb-server processes one at a time
without --disable-file-no-data-conversion command line option.
Downgrading from version 3.2 and later to 3.1 and earlier
Similar to upgrading covered under Upgrading from version 3.1 and
earlier to 3.2 and later, downgrading from the ovsdb-server
version 3.2 and later to 3.1 and earlier requires additional
steps. (This is different from upgrading a database schema, which
is covered later under Upgrading or Downgrading a Database.)
For all service models it’s required to:
1. Compact all database files via ovsdb-server/compact command
with ovs-appctl utility. This should be done for each
involved ovsdb-server process separately (single process for
standalone service model, all involved processes for
active-backup and cluster service models).
2. Stop all ovsdb-server processes. Make sure that no database
schema conversion operations were performed between steps 1
and 2. For standalone and active-backup service models, the
database compaction can be performed after stopping all the
processes instead with the ovsdb-tool compact command.
3. Downgrade and restart ovsdb-server processes.
Understanding Cluster Consistency
To ensure consistency, clustered OVSDB uses the Raft algorithm
described in Diego Ongaro’s Ph.D. thesis, “Consensus: Bridging
Theory and Practice”. In an operational Raft cluster, at any
given time a single server is the “leader” and the other nodes
are “followers”. Only the leader processes transactions, but a
transaction is only committed when a majority of the servers
confirm to the leader that they have written it to persistent
storage.
In most database systems, read and write access to the database
happens through transactions. In such a system, Raft allows a
cluster to present a strongly consistent transactional interface.
OVSDB uses conventional transactions for writes, but clients
often effectively do reads a different way, by asking the server
to “monitor” a database or a subset of one on the client’s
behalf. Whenever monitored data changes, the server
automatically tells the client what changed, which allows the
client to maintain an accurate snapshot of the database in its
memory. Of course, at any given time, the snapshot may be
somewhat dated since some of it could have changed without the
change notification yet being received and processed by the
client.
Given this unconventional usage model, OVSDB also adopts an
unconventional clustering model. Each server in a cluster acts
independently for the purpose of monitors and read-only
transactions, without verifying that data is up-to-date with the
leader. Servers forward transactions that write to the database
to the leader for execution, ensuring consistency. This has the
following consequences:
• Transactions that involve writes, against any server in the
cluster, are linearizable if clients take care to use correct
prerequisites, which is the same condition required for
linearizability in a standalone OVSDB. (Actually,
“at-least-once” consistency, because OVSDB does not have a
session mechanism to drop duplicate transactions if a
connection drops after the server commits it but before the
client receives the result.)
• Read-only transactions can yield results based on a stale
version of the database, if they are executed against a
follower. Transactions on the leader always yield fresh
results. (With monitors, as explained above, a client can
always see stale data even without clustering, so clustering
does not change the consistency model for monitors.)
• Monitor-based (or read-heavy) workloads scale well across a
cluster, because clustering OVSDB adds no additional work or
communication for reads and monitors.
• A write-heavy client should connect to the leader, to avoid the
overhead of followers forwarding transactions to the leader.
• When a client conducts a mix of read and write transactions
across more than one server in a cluster, it can see
inconsistent results because a read transaction might read
stale data whose updates have not yet propagated from the
leader. By default, utilities such as ovn-sbctl (in OVN)
connect to the cluster leader to avoid this issue.
The same might occur for transactions against a single follower
except that the OVSDB server ensures that the results of a
write forwarded to the leader by a given server are visible at
that server before it replies to the requesting client.
• If a client uses a database on one server in a cluster, then
another server in the cluster (perhaps because the first server
failed), the client could observe stale data. Clustered OVSDB
clients, however, can use a column in the _Server database to
detect that data on a server is older than data that the client
previously read. The OVSDB client library in Open vSwitch uses
this feature to avoid servers with stale data.
Relay Service Model
A relay database is a way to scale out read-mostly access to the
existing database working in any service model including relay.
Relay database creates and maintains an OVSDB connection with
another OVSDB server. It uses this connection to maintain an
in-memory copy of the remote database (a.k.a. the relay source)
keeping the copy up-to-date as the database content changes on
the relay source in the real time.
The purpose of relay server is to scale out the number of
database clients. Read-only transactions and monitor requests
are fully handled by the relay server itself. For the
transactions that request database modifications, relay works as
a proxy between the client and the relay source, i.e. it forwards
transactions and replies between them.
Compared to the clustered and active-backup models, relay service
model provides read and write access to the database similarly to
a clustered database (and even more scalable), but with generally
insignificant performance overhead of an active-backup model. At
the same time it doesn’t increase availability that needs to be
covered by the service model of the relay source.
Relay database has no on-disk storage and therefore cannot be
converted to any other service model.
If there is already a database started in any service model, to
start a relay database server use ovsdb-server
relay:<DB_NAME>:<relay source>, where <DB_NAME> is the database
name as specified in the schema of the database that existing
server runs, and <relay source> is an OVSDB connection method
(see Connection Methods below) that connects to the existing
database server. <relay source> could contain a comma-separated
list of connection methods, e.g. to connect to any server of the
clustered database. Multiple relay servers could be started for
the same relay source.
Since the way relays handle read and write transactions is very
similar to the clustered model where “cluster” means “set of
relay servers connected to the same relay source”, “follower”
means “relay server” and the “leader” means “relay source”, same
consistency consequences as for the clustered model applies to
relay as well (See Understanding Cluster Consistency above).
Open vSwitch 2.16 introduced support for relay service model.
OVSDB can layer replication on top of any of its service models.
Replication, in this context, means to make, and keep up-to-date,
a read-only copy of the contents of a database (the replica).
One use of replication is to keep an up-to-date backup of a
database. A replica used solely for backup would not need to
support clients of its own. A set of replicas that do serve
clients could be used to scale out read access to the primary
database, however Relay Service Model is more suitable for that
purpose.
A database replica is set up in the same way as a backup server
in an active-backup pair, with the difference that the replica is
never promoted to an active role.
A database can have multiple replicas.
Open vSwitch 2.6 introduced support for database replication.
An OVSDB connection method is a string that specifies how to make
a JSON-RPC connection between an OVSDB client and server.
Connection methods are part of the Open vSwitch implementation of
OVSDB and not specified by RFC 7047. ovsdb-server uses
connection methods to specify how it should listen for
connections from clients and ovsdb-client uses them to specify
how it should connect to a server. Connections in the opposite
direction, where ovsdb-server connects to a client that is
configured to listen for an incoming connection, are also
possible.
Connection methods are classified as active or passive. An
active connection method makes an outgoing connection to a remote
host; a passive connection method listens for connections from
remote hosts. The most common arrangement is to configure an
OVSDB server with passive connection methods and clients with
active ones, but the OVSDB implementation in Open vSwitch
supports the opposite arrangement as well.
OVSDB supports the following active connection methods:
ssl:<host>:<port>
The specified SSL or TLS <port> on the given <host>.
tcp:<host>:<port>
The specified TCP <port> on the given <host>.
unix:<file>
On Unix-like systems, connect to the Unix domain server
socket named <file>.
On Windows, connect to a local named pipe that is
represented by a file created in the path <file> to mimic
the behavior of a Unix domain socket.
<method1>,<method2>,…,<methodN>
For a clustered database service to be highly available, a
client must be able to connect to any of the servers in
the cluster. To do so, specify connection methods for
each of the servers separated by commas (and optional
spaces).
In theory, if machines go up and down and IP addresses
change in the right way, a client could talk to the wrong
instance of a database. To avoid this possibility, add
cid:<uuid> to the list of methods, where <uuid> is the
cluster ID of the desired database cluster, as printed by
ovsdb-tool db-cid. This feature is optional.
OVSDB supports the following passive connection methods:
pssl:<port>[:<ip>]
Listen on the given TCP <port> for SSL or TLS connections.
By default, connections are not bound to a particular
local IP address. Specifying <ip> limits connections to
those from the given IP.
ptcp:<port>[:<ip>]
Listen on the given TCP <port>. By default, connections
are not bound to a particular local IP address.
Specifying <ip> limits connections to those from the given
IP.
punix:<file>
On Unix-like systems, listens for connections on the Unix
domain socket named <file>.
On Windows, listens on a local named pipe, creating a
named pipe <file> to mimic the behavior of a Unix domain
socket. The ACLs of the named pipe include LocalSystem,
Administrators, and Creator Owner.
All IP-based connection methods accept IPv4 and IPv6 addresses.
To specify an IPv6 address, wrap it in square brackets, e.g.
ssl:[::1]:6640. Passive IP-based connection methods by default
listen for IPv4 connections only; use [::] as the address to
accept both IPv4 and IPv6 connections, e.g. pssl:6640:[::]. DNS
names are also accepted if built with unbound library. On Linux,
use %<device> to designate a scope for IPv6 link-level addresses,
e.g. ssl:[fe80::1234%eth0]:6653.
The <port> may be omitted from connection methods that use a port
number. The default <port> for TCP-based connection methods is
6640, e.g. pssl: is equivalent to pssl:6640. In Open vSwitch
prior to version 2.4.0, the default port was 6632. To avoid
incompatibility between older and newer versions, we encourage
users to specify a port number.
The ssl and pssl connection methods requires additional
configuration through --private-key, --certificate, and --ca-cert
command line options. Open vSwitch can be built without SSL
support, in which case these connection methods are not
supported.
This section describes how to handle various events in the life
cycle of a database using the Open vSwitch implementation of
OVSDB.
Creating a Database
Creating and starting up the service for a new database was
covered separately for each database service model in the Service
Models section, above.
Backing Up and Restoring a Database
OVSDB is often used in contexts where the database contents are
not particularly valuable. For example, in many systems, the
database for configuring ovs-vswitchd is essentially rebuilt from
scratch at boot time. It is not worthwhile to back up these
databases.
When OVSDB is used for valuable data, a backup strategy is worth
considering. One way is to use database replication, discussed
above in Database Replication which keeps an online, up-to-date
copy of a database, possibly on a remote system. This works with
all OVSDB service models.
A more common backup strategy is to periodically take and store a
snapshot. For the standalone and active-backup service models,
making a copy of the database file, e.g. using cp, effectively
makes a snapshot, and because OVSDB database files are
append-only, it works even if the database is being modified when
the snapshot takes place. This approach does not work for
clustered databases.
Another way to make a backup, which works with all OVSDB service
models, is to use ovsdb-client backup, which connects to a
running database server and outputs an atomic snapshot of its
schema and content, in the same format used for standalone and
active-backup databases.
Multiple options are also available when the time comes to
restore a database from a backup. For the standalone and
active-backup service models, one option is to stop the database
server or servers, overwrite the database file with the backup
(e.g. with cp), and then restart the servers. Another way, which
works with any service model, is to use ovsdb-client restore,
which connects to a running database server and replaces the data
in one of its databases by a provided snapshot. The advantage of
ovsdb-client restore is that it causes zero downtime for the
database and its server. It has the downside that UUIDs of rows
in the restored database will differ from those in the snapshot,
because the OVSDB protocol does not allow clients to specify row
UUIDs.
None of these approaches saves and restores data in columns that
the schema designates as ephemeral. This is by design: the
designer of a schema only marks a column as ephemeral if it is
acceptable for its data to be lost when a database server
restarts.
Clustering and backup serve different purposes. Clustering
increases availability, but it does not protect against data loss
if, for example, a malicious or malfunctioning OVSDB client
deletes or tampers with data.
Changing Database Service Model
Use ovsdb-tool create-cluster to create a clustered database from
the contents of a standalone database. Use ovsdb-client backup
to create a standalone database from the contents of a running
clustered database. When the cluster is down and cannot be
revived, ovsdb-client backup will not work.
Use ovsdb-tool cluster-to-standalone to convert clustered
database to standalone database when the cluster is down and
cannot be revived.
Upgrading or Downgrading a Database
The evolution of a piece of software can require changes to the
schemas of the databases that it uses. For example, new features
might require new tables or new columns in existing tables, or
conceptual changes might require a database to be reorganized in
other ways. In some cases, the easiest way to deal with a change
in a database schema is to delete the existing database and start
fresh with the new schema, especially if the data in the database
is easy to reconstruct. But in many other cases, it is better to
convert the database from one schema to another.
The OVSDB implementation in Open vSwitch has built-in support for
some simple cases of converting a database from one schema to
another. This support can handle changes that add or remove
database columns or tables or that eliminate constraints (for
example, changing a column that must have exactly one value into
one that has one or more values). It can also handle changes
that add constraints or make them stricter, but only if the
existing data in the database satisfies the new constraints (for
example, changing a column that has one or more values into a
column with exactly one value, if every row in the column has
exactly one value). The built-in conversion can cause data loss
in obvious ways, for example if the new schema removes tables or
columns, or indirectly, for example by deleting unreferenced rows
in tables that the new schema marks for garbage collection.
Converting a database can lose data, so it is wise to make a
backup beforehand.
To use OVSDB’s built-in support for schema conversion with a
standalone or active-backup database, first stop the database
server or servers, then use ovsdb-tool convert to convert it to
the new schema, and then restart the database server.
OVSDB also supports online database schema conversion for any of
its database service models. To convert a database online, use
ovsdb-client convert. The conversion is atomic, consistent,
isolated, and durable. ovsdb-server disconnects any clients
connected when the conversion takes place (except clients that
use the set_db_change_aware Open vSwitch extension RPC). Upon
reconnection, clients will discover that the schema has changed.
Schema versions and checksums (see Schemas above) can give hints
about whether a database needs to be converted to a new schema.
If there is any question, though, the needs-conversion command on
ovsdb-tool and ovsdb-client can provide a definitive answer.
Working with Database History
Both on-disk database formats that OVSDB supports are organized
as a stream of transaction records. Each record describes a
change to the database as a list of rows that were inserted or
deleted or modified, along with the details. Therefore, in
normal operation, a database file only grows, as each change
causes another record to be appended at the end. Usually, a user
has no need to understand this file structure. This section
covers some exceptions.
Compacting Databases
If OVSDB database files were truly append-only, then over time
they would grow without bound. To avoid this problem, OVSDB can
compact a database file, that is, replace it by a new version
that contains only the current database contents, as if it had
been inserted by a single transaction. From time to time,
ovsdb-server automatically compacts a database that grows much
larger than its minimum size.
Because ovsdb-server automatically compacts databases, it is
usually not necessary to compact them manually, but OVSDB still
offers a few ways to do it. First, ovsdb-tool compact can
compact a standalone or active-backup database that is not
currently being served by ovsdb-server (or otherwise locked for
writing by another process). To compact any database that is
currently being served by ovsdb-server, use ovs-appctl to send
the ovsdb-server/compact command. Each server in an
active-backup or clustered database maintains its database file
independently, so to compact all of them, issue this command
separately on each server.
Viewing History
The ovsdb-tool utility’s show-log command displays the
transaction records in an OVSDB database file in a human-readable
format. By default, it shows minimal detail, but adding the
option -m once or twice increases the level of detail. In
addition to the transaction data, it shows the time and date of
each transaction and any “comment” added to the transaction by
the client. The comments can be helpful for quickly
understanding a transaction; for example, ovs-vsctl adds its
command line to the transactions that it makes.
The show-log command works with both OVSDB file formats, but the
details of the output format differ. For active-backup and
clustered databases, the sequence of transactions in each
server’s log will differ, even at points when they reflect the
same data.
Truncating History
It may occasionally be useful to “roll back” a database file to
an earlier point. Because of the organization of OVSDB records,
this is easy to do. Start by noting the record number <i> of the
first record to delete in ovsdb-tool show-log output. Each
record is two lines of plain text, so trimming the log is as
simple as running head -n <j>, where <j> = 2 * <i>.
Corruption
When ovsdb-server opens an OVSDB database file, of any kind, it
reads as many transaction records as it can from the file until
it reaches the end of the file or it encounters a corrupted
record. At that point it stops reading and regards the data that
it has read to this point as the full contents of the database
file, effectively rolling the database back to an earlier point.
Each transaction record contains an embedded SHA-1 checksum,
which the server verifies as it reads a database file. It
detects corruption when a checksum fails to verify. Even though
SHA-1 is no longer considered secure for use in cryptography, it
is acceptable for this purpose because it is not used to defend
against malicious attackers.
The first record in a standalone or active-backup database file
specifies the schema. ovsdb-server will refuse to work with a
database where this record is corrupted, or with a clustered
database file with corruption in the first few records. Delete
and recreate such a database, or restore it from a backup.
When ovsdb-server adds records to a database file in which it
detected corruption, it first truncates the file just after the
last good record.
RFC 7047, “The Open vSwitch Database Management Protocol.”
Open vSwitch implementations of generic OVSDB functionality:
ovsdb-server(1), ovsdb-client(1), ovsdb-tool(1).
Tools for working with databases that have specific OVSDB
schemas: ovs-vsctl(8), vtep-ctl(8), and (in OVN) ovn-nbctl(8),
ovn-sbctl(8).
OVSDB schemas for Open vSwitch and related functionality:
ovs-vswitchd.conf.db(5), vtep(5), and (in OVN) ovn-nb(5),
ovn-sb(5).
The Open vSwitch Development Community
2016-2021, The Open vSwitch Development Community
This page is part of the Open vSwitch (a distributed virtual
multilayer switch) project. Information about the project can be
found at ⟨http://openvswitch.org/⟩. If you have a bug report for
this manual page, send it to bugs@openvswitch.org. This page was
obtained from the project's upstream Git repository
⟨https://github.com/openvswitch/ovs.git⟩ on 2023-12-22. (At that
time, the date of the most recent commit that was found in the
repository was 2023-12-21.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
3.2.90 Dec 22, 2023 OVSDB(7)
Pages that refer to this page: ovsdb-client(1), ovsdb-server(1), ovsdb-tool(1), ovsdb(5), ovn-controller(8), ovn-ic(8), ovn-ic-nbctl(8), ovn-ic-sbctl(8), ovn-nbctl(8), ovn-northd(8), ovn-sbctl(8), ovn-trace(8), ovs-vsctl(8), ovs-vswitchd(8), vtep-ctl(8)
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