OVSDB(7)                         Open vSwitch                         OVSDB(7)

       ovsdb - Open vSwitch Database (Overview)

       OVSDB, the Open vSwitch Database, is a network-accessible database sys‐
       tem.  Schemas in OVSDB specify the tables in a database and their  col‐
       umns’ types and can include data, uniqueness, and referential integrity
       constraints.  OVSDB offers atomic, consistent, isolated, durable trans‐
       actions.   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 sev‐
       eral protocol extensions documented in its manpage, and  a  basic  com‐
       mand-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  work‐
       ing 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

       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 config‐
       ure Open vSwitch has the name Open_vSwitch.  Schema names begin with  a
       letter  or  an  underscore,  followed  by any number of letters, under‐
       scores, 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  number‐
       ing  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 compat‐
       ible  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  rein‐
       dent   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 ver‐
       sion, 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 check‐
       sum from a schema or database file, respectively.

       OVSDB   supports   three  service  models  for  databases:  standalone,
       active-backup, 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 per‐
       formance.   The  standalone  and  active-backup database service models
       share one on-disk format, and clustered databases use a different  for‐
       mat, but the OVSDB programs work with both formats.  ovsdb(5) documents
       these file formats.

       RFC 7047, which specifies the OVSDB protocol, does not mandate or spec‐
       ify 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 run‐
       ning, 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 con‐
       nection 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 admin‐
       istrator 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 fail‐
       ure  and  changes the backup server’s role.  For example, the “Integra‐
       tion 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  con‐
       tinue 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

       Compared  to a standalone server, the active-backup service model some‐
       what increases availability, at a risk of split-brain.  It adds  gener‐
       ally  insignificant performance overhead.  On the other hand, the clus‐
       tered 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

          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.

   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  syn‐
       chronize  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 data‐
       base 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 clus‐
       ter, 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  maxi‐
       mum  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:  trans‐
       actions 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, cre‐
       ate 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 spec‐
       ified 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.)

          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.

       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 ver‐
       sions 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) instruc‐
       tions 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 follow‐

       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-col‐
          umn-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 ear‐
       lier 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 dif‐
       ferent 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 ser‐
          vice 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.

   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 com‐
       mitted 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 con‐
       ventional 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  notifica‐
       tion yet being received and processed by the client.

       Given  this  unconventional usage model, OVSDB also adopts an unconven‐
       tional clustering model.  Each server in a cluster  acts  independently
       for the purpose of monitors and read-only transactions, without verify‐
       ing that data is up-to-date with the leader.  Servers forward  transac‐
       tions  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  stand‐
         alone  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

       · 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 over‐
         head 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 for‐
         warded 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.

       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.

       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  pro‐
       moted 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 con‐
       nection 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 connec‐
       tion 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:

              The specified SSL or TLS <port> on the given <host>.

              The specified TCP <port> on the given <host>.

              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.

              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  data‐
              base  cluster, as printed by ovsdb-tool db-cid.  This feature is

       OVSDB supports the following passive connection methods:

              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.

              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.

              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  Cre‐
              ator 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 num‐
       ber.  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  sec‐
       tion, 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  con‐
       sidering.   One  way is to use database replication, discussed above in
       Database Replication which keeps an online, up-to-date copy of a  data‐
       base,  possibly  on a remote system.  This works with all OVSDB service

       A more common backup strategy is to periodically take and store a snap‐
       shot.   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 mod‐
       els, is to use ovsdb-client backup, which connects to a  running  data‐
       base  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  exam‐
       ple, a malicious or malfunctioning OVSDB client deletes or tampers with

   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, espe‐
       cially 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

       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 con‐
       straints (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

       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 struc‐
       ture.  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 dif‐
       fer, even at points when they reflect the same data.

   Truncating History
       It may occasionally be useful to “roll back” a database file to an ear‐
       lier 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>.

       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 data‐
       base 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  con‐
       sidered  secure for use in cryptography, it is acceptable for this pur‐
       pose because it is not used to defend against malicious attackers.

       The first record in a standalone or active-backup database file  speci‐
       fies  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 data‐
       base, 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

       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, The Open vSwitch Development Community

2.15.90                          Feb 04, 2021                         OVSDB(7)