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       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,

       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.

          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  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

       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

       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.)

          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 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

       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.

   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

       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:

              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 database 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

              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

       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

       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

       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>.

       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).


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