1OVSDB(7)                         Open vSwitch                         OVSDB(7)
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NAME

6       ovsdb - Open vSwitch Database (Overview)
7

DESCRIPTION

9       OVSDB, the Open vSwitch Database, is a network-accessible database sys‐
10       tem.  Schemas in OVSDB specify the tables in a database and their  col‐
11       umns’ types and can include data, uniqueness, and referential integrity
12       constraints.  OVSDB offers atomic, consistent, isolated, durable trans‐
13       actions.   RFC  7047  specifies  the JSON-RPC based protocol that OVSDB
14       clients and servers use to communicate.
15
16       The OVSDB protocol is well suited for state synchronization because  it
17       allows  each  client  to  monitor the contents of a whole database or a
18       subset of it.  Whenever a monitored portion of  the  database  changes,
19       the server tells the client what rows were added or modified (including
20       the new contents) or deleted.  Thus,  OVSDB  clients  can  easily  keep
21       track of the newest contents of any part of the database.
22
23       While OVSDB is general-purpose and not particularly specialized for use
24       with Open vSwitch, Open vSwitch does use it for multiple purposes.  The
25       leading use of OVSDB is for configuring and monitoring ovs-vswitchd(8),
26       the  Open  vSwitch  switch  daemon,  using  the  schema  documented  in
27       ovs-vswitchd.conf.db(5).   The  Open Virtual Network (OVN) project uses
28       two OVSDB schemas, documented as part of that project.   Finally,  Open
29       vSwitch  includes  the  “VTEP”  schema, documented in vtep(5) that many
30       third-party hardware switches support for configuring  VXLAN,  although
31       OVS itself does not directly use this schema.
32
33       The  OVSDB protocol specification allows independent, interoperable im‐
34       plementations of OVSDB to be developed.  Open vSwitch includes an OVSDB
35       server  implementation  named  ovsdb-server(1),  which supports several
36       protocol extensions documented in its manpage, and a basic command-line
37       OVSDB  client  named ovsdb-client(1), as well as OVSDB client libraries
38       for C and for Python.  Open vSwitch documentation often speaks of these
39       OVSDB  implementations  in  Open vSwitch as simply “OVSDB,” even though
40       that is distinct from the OVSDB protocol; we make the  distinction  ex‐
41       plicit only when it might otherwise be unclear from the context.
42
43       In  addition  to  these  generic  OVSDB  server  and client tools, Open
44       vSwitch includes tools for working with databases  that  have  specific
45       schemas:  ovs-vsctl  works with the ovs-vswitchd configuration database
46       and vtep-ctl works with the VTEP database.
47
48       RFC 7047 specifies the OVSDB  protocol  but  it  does  not  specify  an
49       on-disk  storage format.  Open vSwitch includes ovsdb-tool(1) for work‐
50       ing with its own on-disk database formats.  The most notable feature of
51       this  format is that ovsdb-tool(1) makes it easy for users to print the
52       transactions that have changed a database since the last  time  it  was
53       compacted.  This feature is often useful for troubleshooting.
54

SCHEMAS

56       Schemas  in  OVSDB  have  a  JSON format that is specified in RFC 7047.
57       They are often stored  in  files  with  an  extension  .ovsschema.   An
58       on-disk  database  in  OVSDB includes a schema and data, embedding both
59       into a single file.  The Open vSwitch utility ovsdb-tool  has  commands
60       that  work  with schema files and with the schemas embedded in database
61       files.
62
63       An Open vSwitch schema has three important identifiers.  The  first  is
64       its  name,  which is also the name used in JSON-RPC calls to identify a
65       database based on that schema.  For example, the schema used to config‐
66       ure  Open vSwitch has the name Open_vSwitch.  Schema names begin with a
67       letter or an underscore, followed by  any  number  of  letters,  under‐
68       scores, or digits.  The ovsdb-tool commands schema-name and db-name ex‐
69       tract the schema name from a schema or database file, respectively.
70
71       An OVSDB schema also has a  version  of  the  form  x.y.z  e.g.  1.2.3.
72       Schemas  managed within the Open vSwitch project manage version number‐
73       ing in the following way (but OVSDB does not  mandate  this  approach).
74       Whenever we change the database schema in a non-backward compatible way
75       (e.g. when we delete a column or a table), we increment <x> and set <y>
76       and <z> to 0.  When we change the database schema in a backward compat‐
77       ible way (e.g. when we add a new column), we increment <y> and set  <z>
78       to  0.   When we change the database schema cosmetically (e.g. we rein‐
79       dent  its  syntax),  we  increment  <z>.    The   ovsdb-tool   commands
80       schema-version  and db-version extract the schema version from a schema
81       or database file, respectively.
82
83       Very old OVSDB schemas do not have a version, but RFC 7047 mandates it.
84
85       An OVSDB schema optionally has a “checksum.”  RFC 7047 does not specify
86       the  use  of  the checksum and recommends that clients ignore it.  Open
87       vSwitch uses the checksum to remind developers to update  the  version:
88       at build time, if the schema’s embedded checksum, ignoring the checksum
89       field itself, does not match the schema’s content, then  it  fails  the
90       build  with  a  recommendation  to update the version and the checksum.
91       Thus, a developer who changes the schema, but does not update the  ver‐
92       sion, receives an automatic reminder.  In practice this has been an ef‐
93       fective way to ensure compliance with the version number  policy.   The
94       ovsdb-tool commands schema-cksum and db-cksum extract the schema check‐
95       sum from a schema or database file, respectively.
96

SERVICE MODELS

98       OVSDB supports three service  models  for  databases:  standalone,  ac‐
99       tive-backup,  and clustered.  The service models provide different com‐
100       promises among  consistency,  availability,  and  partition  tolerance.
101       They also differ in the number of servers required and in terms of per‐
102       formance.  The standalone and  active-backup  database  service  models
103       share  one on-disk format, and clustered databases use a different for‐
104       mat, but the OVSDB programs work with both formats.  ovsdb(5) documents
105       these file formats.
106
107       RFC 7047, which specifies the OVSDB protocol, does not mandate or spec‐
108       ify any particular service model.
109
110       The following sections describe the individual service models.
111
112   Standalone Database Service Model
113       A standalone database runs a single server.  If the server  stops  run‐
114       ning, the database becomes inaccessible, and if the server’s storage is
115       lost or corrupted, the database’s content is lost.  This service  model
116       is  appropriate  when  the  database  controls a process or activity to
117       which it is linked via “fate-sharing.”  For example, an OVSDB  instance
118       that controls an Open vSwitch virtual switch daemon, ovs-vswitchd, is a
119       standalone database because a server failure would take  out  both  the
120       database and the virtual switch.
121
122       To  set  up  a  standalone  database, use ovsdb-tool create to create a
123       database file, then run ovsdb-server to start the database service.
124
125       To configure a client, such as ovs-vswitchd  or  ovs-vsctl,  to  use  a
126       standalone  database,  configure  the server to listen on a “connection
127       method” that the client can reach, then point the client to  that  con‐
128       nection  method.   See  Connection  Methods below for information about
129       connection methods.
130
131   Active-Backup Database Service Model
132       An active-backup database runs two servers (on  different  hosts).   At
133       any  given  time, one of the servers is designated with the active role
134       and the other the backup role.  An active server behaves  just  like  a
135       standalone  server.   A  backup server makes an OVSDB connection to the
136       active server and uses it to continuously replicate its content  as  it
137       changes  in  real time.  OVSDB clients can connect to either server but
138       only the active server allows data modification or lock transactions.
139
140       Setup for an active-backup database starts from  a  working  standalone
141       database  service,  which  is  initially the active server.  On another
142       node, to set up a backup server, create a database file with  the  same
143       schema as the active server.  The initial contents of the database file
144       do not matter, as long as the schema is correct, so  ovsdb-tool  create
145       will  work,  as  will copying the database file from the active server.
146       Then use ovsdb-server --sync-from=<active> to start the backup  server,
147       where  <active>  is  an OVSDB connection method (see Connection Methods
148       below) that connects to the active server.  At that point,  the  backup
149       server  will fetch a copy of the active database and keep it up-to-date
150       until it is killed.
151
152       When the active server in an active-backup server pair fails, an admin‐
153       istrator  can  switch  the  backup  server  to  an active role with the
154       ovs-appctl     command     ovsdb-server/disconnect-active-ovsdb-server.
155       Clients  then  have  read/write  access  to  the now-active server.  Of
156       course, administrators are slow to respond compared to software, so  in
157       practice external management software detects the active server’s fail‐
158       ure and changes the backup server’s role.  For example,  the  “Integra‐
159       tion  Guide for Centralized Control” in the OVN documentation describes
160       how to use Pacemaker for this purpose in OVN.
161
162       Suppose an active server fails and its backup is  promoted  to  active.
163       If the failed server is revived, it must be started as a backup server.
164       Otherwise, if both servers are active, then they may start out of sync,
165       if  the  database changed while the server was down, and they will con‐
166       tinue to diverge over time.  This also happens if the software managing
167       the  database  servers  cannot  reach  the  active server and therefore
168       switches the backup to active, but other hosts can reach both  servers.
169       These  “split-brain”  problems  are  unsolvable  in  general for server
170       pairs.
171
172       Compared to a standalone server, the active-backup service model  some‐
173       what  increases availability, at a risk of split-brain.  It adds gener‐
174       ally insignificant performance overhead.  On the other hand, the  clus‐
175       tered  service  model, discussed below, requires at least 3 servers and
176       has greater performance overhead, but it avoids the need  for  external
177       management software and eliminates the possibility of split-brain.
178
179       Open  vSwitch  2.6  introduced  support  for  the active-backup service
180       model.
181
182       IMPORTANT:
183          There was a change of a database file format in  version  2.15.   To
184          upgrade/downgrade  the  ovsdb-server  processes  across this version
185          follow the instructions described under Upgrading from version  2.14
186          and  earlier to 2.15 and later and Downgrading from version 2.15 and
187          later to 2.14 and earlier.
188
189   Clustered Database Service Model
190       A clustered database runs across 3 or 5 or more database  servers  (the
191       cluster)  on  different hosts.  Servers in a cluster automatically syn‐
192       chronize writes within the cluster.   A  3-server  cluster  can  remain
193       available  in  the face of at most 1 server failure; a 5-server cluster
194       tolerates up to 2 failures.  Clusters larger than 5 servers  will  also
195       work,  with  every  2  added servers allowing the cluster to tolerate 1
196       more failure, but write performance decreases.  The number  of  servers
197       should  be  odd: a 4- or 6-server cluster cannot tolerate more failures
198       than a 3- or 5-server cluster, respectively.
199
200       To set up a clustered database, first initialize it on a single node by
201       running  ovsdb-tool create-cluster, then start ovsdb-server.  Depending
202       on its arguments, the create-cluster command can create an empty  data‐
203       base or copy a standalone database’s contents into the new database.
204
205       To  configure a client to use a clustered database, first configure all
206       of the servers to listen on a connection method  that  the  client  can
207       reach, then point the client to all of the servers’ connection methods,
208       comma-separated.  See Connection Methods, below, for more detail.
209
210       Open vSwitch 2.9 introduced support for the clustered service model.
211
212   How to Maintain a Clustered Database
213       To add a server to a cluster, run ovsdb-tool join-cluster  on  the  new
214       server and start ovsdb-server.  To remove a running server from a clus‐
215       ter, use ovs-appctl to invoke the cluster/leave command.  When a server
216       fails  and  cannot be recovered, e.g. because its hard disk crashed, or
217       to otherwise remove a server that is down from a cluster,  use  ovs-ap‐
218       pctl  to  invoke cluster/kick to make the remaining servers kick it out
219       of the cluster.
220
221       The above methods for adding and removing servers only work for healthy
222       clusters,  that is, for clusters with no more failures than their maxi‐
223       mum tolerance.  For example, in a 3-server cluster, the  failure  of  2
224       servers  prevents  servers  joining  or leaving the cluster (as well as
225       database access).  To prevent data loss or inconsistency, the preferred
226       solution to this problem is to bring up enough of the failed servers to
227       make the cluster healthy again, then if necessary remove any  remaining
228       failed  servers  and add new ones.  If this cannot be done, though, use
229       ovs-appctl to invoke cluster/leave --force on a running  server.   This
230       command  forces the server to which it is directed to leave its cluster
231       and form a new single-node cluster that contains only itself.  The data
232       in  the new cluster may be inconsistent with the former cluster: trans‐
233       actions not yet replicated to the server will be lost, and transactions
234       not  yet  applied  to  the  cluster  may  be committed.  Afterward, any
235       servers in its former cluster will regard the server to have failed.
236
237       Once a server leaves a cluster, it may never rejoin it.  Instead,  cre‐
238       ate a new server and join it to the cluster.
239
240       The  servers  in  a  cluster synchronize data over a cluster management
241       protocol that is specific to Open vSwitch; it is not the  same  as  the
242       OVSDB  protocol specified in RFC 7047.  For this purpose, a server in a
243       cluster is tied to a particular IP address and TCP port, which is spec‐
244       ified in the ovsdb-tool command that creates or joins the cluster.  The
245       TCP port used for clustering must be different from that used for OVSDB
246       clients.  To change the port or address of a server in a cluster, first
247       remove it from the cluster, then add it back with the new address.
248
249       To upgrade the ovsdb-server processes in a cluster from one version  of
250       Open  vSwitch  to  another,  upgrading them one at a time will keep the
251       cluster healthy during the upgrade process.  (This  is  different  from
252       upgrading  a database schema, which is covered later under Upgrading or
253       Downgrading a Database.)
254
255       IMPORTANT:
256          There was a change of a database file format in  version  2.15.   To
257          upgrade/downgrade  the  ovsdb-server  processes  across this version
258          follow the instructions described under Upgrading from version  2.14
259          and  earlier to 2.15 and later and Downgrading from version 2.15 and
260          later to 2.14 and earlier.
261
262       Clustered OVSDB does not support the OVSDB “ephemeral columns” feature.
263       ovsdb-tool  and  ovsdb-client  change ephemeral columns into persistent
264       ones when they work with schemas for clustered databases.  Future  ver‐
265       sions of OVSDB might add support for this feature.
266
267   Upgrading from version 2.14 and earlier to 2.15 and later
268       There  is  a  change  of  a  database  file format in version 2.15 that
269       doesn’t allow older versions of ovsdb-server to read the database  file
270       modified  by the ovsdb-server version 2.15 or later.  This also affects
271       runtime communications between servers  in  active-backup  and  cluster
272       service  models. To upgrade the ovsdb-server processes from one version
273       of Open vSwitch (2.14 or earlier) to another (2.15 or higher)  instruc‐
274       tions  below  should  be  followed. (This is different from upgrading a
275       database schema, which is covered later under Upgrading or  Downgrading
276       a Database.)
277
278       In  case of standalone service model no special handling during upgrade
279       is required.
280
281       For the active-backup service  model,  administrator  needs  to  update
282       backup  ovsdb-server  first and the active one after that, or shut down
283       both servers and upgrade at the same time.
284
285       For the cluster service model recommended upgrade strategy  is  follow‐
286       ing:
287
288       1. Upgrade  processes  one  at a time.  Each ovsdb-server process after
289          upgrade should be started  with  --disable-file-column-diff  command
290          line argument.
291
292       2. When  all  ovsdb-server processes upgraded, use ovs-appctl to invoke
293          ovsdb/file/column-diff-enable command on each of them or restart all
294          ovsdb-server  processes  one  at  a time without --disable-file-col‐
295          umn-diff command line option.
296
297   Downgrading from version 2.15 and later to 2.14 and earlier
298       Similar to upgrading covered under Upgrading from version 2.14 and ear‐
299       lier  to 2.15 and later, downgrading from the ovsdb-server version 2.15
300       and later to 2.14 and earlier requires additional steps. (This is  dif‐
301       ferent  from  upgrading a database schema, which is covered later under
302       Upgrading or Downgrading a Database.)
303
304       For all service models it’s required to:
305
306       1. Stop all ovsdb-server processes (single process for standalone  ser‐
307          vice  model,  all  involved  processes for active-backup and cluster
308          service models).
309
310       2. Compact all database files with ovsdb-tool compact command.
311
312       3. Downgrade and restart ovsdb-server processes.
313
314   Understanding Cluster Consistency
315       To ensure consistency, clustered OVSDB  uses  the  Raft  algorithm  de‐
316       scribed in Diego Ongaro’s Ph.D. thesis, “Consensus: Bridging Theory and
317       Practice”.  In an operational Raft cluster, at any given time a  single
318       server  is  the “leader” and the other nodes are “followers”.  Only the
319       leader processes transactions, but a transaction is only committed when
320       a  majority of the servers confirm to the leader that they have written
321       it to persistent storage.
322
323       In most database systems, read and write access to the database happens
324       through  transactions.   In  such  a  system,  Raft allows a cluster to
325       present a strongly consistent transactional interface.  OVSDB uses con‐
326       ventional  transactions  for  writes,  but clients often effectively do
327       reads a different way, by asking the server to “monitor” a database  or
328       a  subset  of  one  on  the  client’s  behalf.  Whenever monitored data
329       changes, the server automatically tells the client what changed,  which
330       allows  the  client to maintain an accurate snapshot of the database in
331       its memory.  Of course, at any given time, the snapshot may be somewhat
332       dated  since some of it could have changed without the change notifica‐
333       tion yet being received and processed by the client.
334
335       Given this unconventional usage model, OVSDB also adopts  an  unconven‐
336       tional  clustering  model.  Each server in a cluster acts independently
337       for the purpose of monitors and read-only transactions, without verify‐
338       ing  that data is up-to-date with the leader.  Servers forward transac‐
339       tions that write to the database to the leader for execution,  ensuring
340       consistency.  This has the following consequences:
341
342       • Transactions  that involve writes, against any server in the cluster,
343         are linearizable if clients take care to use  correct  prerequisites,
344         which  is the same condition required for linearizability in a stand‐
345         alone OVSDB.  (Actually, “at-least-once” consistency,  because  OVSDB
346         does not have a session mechanism to drop duplicate transactions if a
347         connection drops after the server commits it but  before  the  client
348         receives the result.)
349
350       • Read-only  transactions can yield results based on a stale version of
351         the database, if they are executed against a follower.   Transactions
352         on  the  leader  always  yield fresh results.  (With monitors, as ex‐
353         plained above, a client can always see stale data even without  clus‐
354         tering, so clustering does not change the consistency model for moni‐
355         tors.)
356
357       • Monitor-based (or read-heavy) workloads scale well across a  cluster,
358         because clustering OVSDB adds no additional work or communication for
359         reads and monitors.
360
361       • A write-heavy client should connect to the leader, to avoid the over‐
362         head of followers forwarding transactions to the leader.
363
364       • When  a  client  conducts a mix of read and write transactions across
365         more than one server in a cluster, it can  see  inconsistent  results
366         because  a  read transaction might read stale data whose updates have
367         not yet propagated from the leader.  By default,  utilities  such  as
368         ovn-sbctl (in OVN) connect to the cluster leader to avoid this issue.
369
370         The  same  might occur for transactions against a single follower ex‐
371         cept that the OVSDB server ensures that the results of a  write  for‐
372         warded to the leader by a given server are visible at that server be‐
373         fore it replies to the requesting client.
374
375       • If a client uses a database on one server in a cluster, then  another
376         server  in the cluster (perhaps because the first server failed), the
377         client could observe stale data.  Clustered OVSDB  clients,  however,
378         can  use  a  column  in the _Server database to detect that data on a
379         server is older than data that the client previously read.  The OVSDB
380         client  library  in  Open  vSwitch uses this feature to avoid servers
381         with stale data.
382

DATABASE REPLICATION

384       OVSDB can layer replication on  top  of  any  of  its  service  models.
385       Replication,  in  this  context,  means to make, and keep up-to-date, a
386       read-only copy of the contents of a database (the replica).  One use of
387       replication  is  to keep an up-to-date backup of a database.  A replica
388       used solely for backup would not need to support clients of its own.  A
389       set  of  replicas that do serve clients could be used to scale out read
390       access to the primary database.
391
392       A database replica is set up in the same way as a backup server  in  an
393       active-backup  pair, with the difference that the replica is never pro‐
394       moted to an active role.
395
396       A database can have multiple replicas.
397
398       Open vSwitch 2.6 introduced support for database replication.
399

CONNECTION METHODS

401       An OVSDB connection method is a string that specifies  how  to  make  a
402       JSON-RPC  connection  between  an  OVSDB client and server.  Connection
403       methods are part of the Open vSwitch implementation of  OVSDB  and  not
404       specified by RFC 7047.  ovsdb-server uses connection methods to specify
405       how it should listen for connections from clients and ovsdb-client uses
406       them  to specify how it should connect to a server.  Connections in the
407       opposite direction, where ovsdb-server connects to  a  client  that  is
408       configured to listen for an incoming connection, are also possible.
409
410       Connection methods are classified as active or passive.  An active con‐
411       nection method makes an outgoing connection to a remote host; a passive
412       connection  method listens for connections from remote hosts.  The most
413       common arrangement is to configure an OVSDB server with passive connec‐
414       tion methods and clients with active ones, but the OVSDB implementation
415       in Open vSwitch supports the opposite arrangement as well.
416
417       OVSDB supports the following active connection methods:
418
419       ssl:<host>:<port>
420              The specified SSL or TLS <port> on the given <host>.
421
422       tcp:<host>:<port>
423              The specified TCP <port> on the given <host>.
424
425       unix:<file>
426              On Unix-like systems, connect to the Unix domain  server  socket
427              named <file>.
428
429              On Windows, connect to a local named pipe that is represented by
430              a file created in the path <file> to mimic  the  behavior  of  a
431              Unix domain socket.
432
433       <method1>,<method2>,…,<methodN>
434              For  a  clustered  database  service  to  be highly available, a
435              client must be able to connect to any  of  the  servers  in  the
436              cluster.   To  do so, specify connection methods for each of the
437              servers separated by commas (and optional spaces).
438
439              In theory, if machines go up and down and IP addresses change in
440              the  right  way,  a client could talk to the wrong instance of a
441              database.  To avoid this possibility, add cid:<uuid> to the list
442              of  methods, where <uuid> is the cluster ID of the desired data‐
443              base cluster, as printed by ovsdb-tool db-cid.  This feature  is
444              optional.
445
446       OVSDB supports the following passive connection methods:
447
448       pssl:<port>[:<ip>]
449              Listen  on  the given TCP <port> for SSL or TLS connections.  By
450              default, connections are not bound to a particular local IP  ad‐
451              dress.   Specifying  <ip>  limits  connections to those from the
452              given IP.
453
454       ptcp:<port>[:<ip>]
455              Listen on the given TCP <port>.  By default, connections are not
456              bound  to a particular local IP address.  Specifying <ip> limits
457              connections to those from the given IP.
458
459       punix:<file>
460              On Unix-like systems, listens for connections on the Unix domain
461              socket named <file>.
462
463              On Windows, listens on a local named pipe, creating a named pipe
464              <file> to mimic the behavior of a Unix domain socket.  The  ACLs
465              of  the named pipe include LocalSystem, Administrators, and Cre‐
466              ator Owner.
467
468       All IP-based connection methods accept IPv4  and  IPv6  addresses.   To
469       specify   an   IPv6   address,   wrap   it  in  square  brackets,  e.g.
470       ssl:[::1]:6640.  Passive IP-based connection methods by default  listen
471       for  IPv4 connections only; use [::] as the address to accept both IPv4
472       and IPv6 connections, e.g. pssl:6640:[::].  DNS names are also accepted
473       if  built with unbound library.  On Linux, use %<device> to designate a
474       scope for IPv6 link-level addresses, e.g. ssl:[fe80::1234%eth0]:6653.
475
476       The <port> may be omitted from connection methods that use a port  num‐
477       ber.  The default <port> for TCP-based connection methods is 6640, e.g.
478       pssl: is equivalent to pssl:6640.  In Open  vSwitch  prior  to  version
479       2.4.0,  the  default  port  was 6632.  To avoid incompatibility between
480       older and newer versions, we encourage users to specify a port number.
481
482       The ssl and pssl connection methods requires  additional  configuration
483       through  --private-key,  --certificate,  and --ca-cert command line op‐
484       tions.  Open vSwitch can be built without SSL support,  in  which  case
485       these connection methods are not supported.
486

DATABASE LIFE CYCLE

488       This  section  describes how to handle various events in the life cycle
489       of a database using the Open vSwitch implementation of OVSDB.
490
491   Creating a Database
492       Creating and starting up the service for a  new  database  was  covered
493       separately  for  each database service model in the Service Models sec‐
494       tion, above.
495
496   Backing Up and Restoring a Database
497       OVSDB is often used in contexts where the  database  contents  are  not
498       particularly  valuable.  For example, in many systems, the database for
499       configuring ovs-vswitchd is essentially rebuilt from  scratch  at  boot
500       time.  It is not worthwhile to back up these databases.
501
502       When  OVSDB  is used for valuable data, a backup strategy is worth con‐
503       sidering.  One way is to use database replication, discussed  above  in
504       Database  Replication which keeps an online, up-to-date copy of a data‐
505       base, possibly on a remote system.  This works with all  OVSDB  service
506       models.
507
508       A more common backup strategy is to periodically take and store a snap‐
509       shot.  For the standalone and active-backup service  models,  making  a
510       copy of the database file, e.g. using cp, effectively makes a snapshot,
511       and because OVSDB database files are append-only, it works even if  the
512       database  is  being  modified  when the snapshot takes place.  This ap‐
513       proach does not work for clustered databases.
514
515       Another way to make a backup, which works with all OVSDB  service  mod‐
516       els,  is  to use ovsdb-client backup, which connects to a running data‐
517       base server and outputs an atomic snapshot of its schema  and  content,
518       in the same format used for standalone and active-backup databases.
519
520       Multiple  options  are  also available when the time comes to restore a
521       database from a backup.  For the standalone and  active-backup  service
522       models, one option is to stop the database server or servers, overwrite
523       the database file with the backup (e.g. with cp), and then restart  the
524       servers.   Another  way,  which works with any service model, is to use
525       ovsdb-client restore, which connects to a running database  server  and
526       replaces  the data in one of its databases by a provided snapshot.  The
527       advantage of ovsdb-client restore is that it causes zero  downtime  for
528       the database and its server.  It has the downside that UUIDs of rows in
529       the restored database will differ from those in the  snapshot,  because
530       the OVSDB protocol does not allow clients to specify row UUIDs.
531
532       None  of  these  approaches saves and restores data in columns that the
533       schema designates as ephemeral.  This is by design: the designer  of  a
534       schema  only  marks  a  column as ephemeral if it is acceptable for its
535       data to be lost when a database server restarts.
536
537       Clustering and backup serve different purposes.   Clustering  increases
538       availability,  but  it does not protect against data loss if, for exam‐
539       ple, a malicious or malfunctioning OVSDB client deletes or tampers with
540       data.
541
542   Changing Database Service Model
543       Use  ovsdb-tool  create-cluster to create a clustered database from the
544       contents of a standalone database.  Use ovsdb-client backup to create a
545       standalone  database from the contents of a running clustered database.
546       When the cluster is down and cannot  be  revived,  ovsdb-client  backup
547       will not work.
548
549       Use  ovsdb-tool  cluster-to-standalone to convert clustered database to
550       standalone database when the cluster is down and cannot be revived.
551
552   Upgrading or Downgrading a Database
553       The evolution of a piece of software can require changes to the schemas
554       of the databases that it uses.  For example, new features might require
555       new tables or new columns in existing  tables,  or  conceptual  changes
556       might  require  a  database  to  be reorganized in other ways.  In some
557       cases, the easiest way to deal with a change in a database schema is to
558       delete the existing database and start fresh with the new schema, espe‐
559       cially if the data in the database is easy to reconstruct.  But in many
560       other  cases,  it  is better to convert the database from one schema to
561       another.
562
563       The OVSDB implementation in Open vSwitch has built-in support for  some
564       simple cases of converting a database from one schema to another.  This
565       support can handle changes that add or remove database columns  or  ta‐
566       bles or that eliminate constraints (for example, changing a column that
567       must have exactly one value into one that has one or more values).   It
568       can also handle changes that add constraints or make them stricter, but
569       only if the existing data in the database satisfies the new constraints
570       (for example, changing a column that has one or more values into a col‐
571       umn with exactly one value, if every row in the column has exactly  one
572       value).   The  built-in conversion can cause data loss in obvious ways,
573       for example if the new schema removes tables or columns, or indirectly,
574       for example by deleting unreferenced rows in tables that the new schema
575       marks for garbage collection.
576
577       Converting a database can lose data, so it is wise to make a backup be‐
578       forehand.
579
580       To use OVSDB’s built-in support for schema conversion with a standalone
581       or active-backup database, first stop the database server  or  servers,
582       then  use  ovsdb-tool convert to convert it to the new schema, and then
583       restart the database server.
584
585       OVSDB also supports online database schema conversion for  any  of  its
586       database   service   models.    To   convert  a  database  online,  use
587       ovsdb-client convert.  The conversion is atomic, consistent,  isolated,
588       and  durable.   ovsdb-server disconnects any clients connected when the
589       conversion takes place (except clients that use the set_db_change_aware
590       Open  vSwitch extension RPC).  Upon reconnection, clients will discover
591       that the schema has changed.
592
593       Schema versions and checksums (see Schemas above) can give hints  about
594       whether  a database needs to be converted to a new schema.  If there is
595       any question, though, the needs-conversion command  on  ovsdb-tool  and
596       ovsdb-client can provide a definitive answer.
597
598   Working with Database History
599       Both  on-disk  database  formats that OVSDB supports are organized as a
600       stream of transaction records.  Each record describes a change  to  the
601       database  as  a list of rows that were inserted or deleted or modified,
602       along with the details.  Therefore, in  normal  operation,  a  database
603       file only grows, as each change causes another record to be appended at
604       the end.  Usually, a user has no need to understand  this  file  struc‐
605       ture.  This section covers some exceptions.
606
607   Compacting Databases
608       If  OVSDB  database  files  were truly append-only, then over time they
609       would grow without bound.  To avoid this problem, OVSDB can  compact  a
610       database  file, that is, replace it by a new version that contains only
611       the current database contents, as if it had been inserted by  a  single
612       transaction.   From time to time, ovsdb-server automatically compacts a
613       database that grows much larger than its minimum size.
614
615       Because ovsdb-server automatically compacts databases,  it  is  usually
616       not  necessary  to  compact them manually, but OVSDB still offers a few
617       ways to do it.  First, ovsdb-tool compact can compact a  standalone  or
618       active-backup   database   that   is  not  currently  being  served  by
619       ovsdb-server (or otherwise locked for writing by another process).   To
620       compact  any  database  that is currently being served by ovsdb-server,
621       use ovs-appctl to send the ovsdb-server/compact command.   Each  server
622       in  an  active-backup or clustered database maintains its database file
623       independently, so to compact all of them, issue this command separately
624       on each server.
625
626   Viewing History
627       The  ovsdb-tool  utility’s  show-log  command  displays the transaction
628       records in an OVSDB database file in a human-readable format.   By  de‐
629       fault,  it shows minimal detail, but adding the option -m once or twice
630       increases the level of detail.  In addition to the transaction data, it
631       shows  the time and date of each transaction and any “comment” added to
632       the transaction by the client.  The comments can be helpful for quickly
633       understanding  a  transaction;  for example, ovs-vsctl adds its command
634       line to the transactions that it makes.
635
636       The show-log command works with both OVSDB file formats,  but  the  de‐
637       tails  of  the  output  format differ.  For active-backup and clustered
638       databases, the sequence of transactions in each server’s log will  dif‐
639       fer, even at points when they reflect the same data.
640
641   Truncating History
642       It may occasionally be useful to “roll back” a database file to an ear‐
643       lier point.  Because of the organization of OVSDB records, this is easy
644       to  do.   Start  by noting the record number <i> of the first record to
645       delete in ovsdb-tool show-log output.  Each  record  is  two  lines  of
646       plain  text,  so  trimming the log is as simple as running head -n <j>,
647       where <j> = 2 * <i>.
648
649   Corruption
650       When ovsdb-server opens an OVSDB database file, of any kind,  it  reads
651       as  many  transaction  records as it can from the file until it reaches
652       the end of the file or it encounters a corrupted record.  At that point
653       it stops reading and regards the data that it has read to this point as
654       the full contents of the database file, effectively rolling  the  data‐
655       base back to an earlier point.
656
657       Each  transaction record contains an embedded SHA-1 checksum, which the
658       server verifies as it reads a database  file.   It  detects  corruption
659       when  a  checksum fails to verify.  Even though SHA-1 is no longer con‐
660       sidered secure for use in cryptography, it is acceptable for this  pur‐
661       pose because it is not used to defend against malicious attackers.
662
663       The  first record in a standalone or active-backup database file speci‐
664       fies the schema.  ovsdb-server will refuse  to  work  with  a  database
665       where  this record is corrupted, or with a clustered database file with
666       corruption in the first few records.  Delete and recreate such a  data‐
667       base, or restore it from a backup.
668
669       When  ovsdb-server adds records to a database file in which it detected
670       corruption, it first truncates  the  file  just  after  the  last  good
671       record.
672

SEE ALSO

674       RFC 7047, “The Open vSwitch Database Management Protocol.”
675
676       Open   vSwitch   implementations   of   generic   OVSDB  functionality:
677       ovsdb-server(1), ovsdb-client(1), ovsdb-tool(1).
678
679       Tools for working with databases  that  have  specific  OVSDB  schemas:
680       ovs-vsctl(8), vtep-ctl(8), and (in OVN) ovn-nbctl(8), ovn-sbctl(8).
681
682       OVSDB   schemas   for   Open   vSwitch   and   related   functionality:
683       ovs-vswitchd.conf.db(5), vtep(5), and (in OVN) ovn-nb(5), ovn-sb(5).
684

AUTHOR

686       The Open vSwitch Development Community
687
689       2021, The Open vSwitch Development Community
690
691
692
693
6942.15                             Feb 21, 2021                         OVSDB(7)
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