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Xtrabackup SST Configuration

XtraBackup SST works in two stages:

  • Stage I on joiner checks if it is SST or IST based on presence of xtrabackup_ist file.
  • In Stage II it starts the data transfer, if it’s SST, it empties the data directory sans few files (galera.cache, sst_in_progress, grastate.dat) and then proceed with the SST or if it’s IST, proceeds as before.


To maintain compatibility with Percona XtraDB Cluster older than 5.5.33-23.7.6, use xtrabackup as SST method, else xtrabackup-v2 is recommended. xtrabackup-v2 is also the default SST method now.

Latest Percona XtraBackup 2.1.x is strongly recommended for Xtrabackup SST. Refer to Incompatibilities for possible caveats.

Following SST specific options are allowed in my.cnf under [sst]


In following options:

  • Non-integer options which have no default are disabled if not set.
  • ”:Match: Yes” implies that options should match on donor and joiner (in their cnf).
  • ”:Recommended: Yes” implies the value which is recommended.
  • In following options, path always means full path.
  • Also, these variables are cnf only (thus not mysqld runtime variables), and can be modified in config till the SST script runs on either donor or joiner (ie. config is read by SST script during SST time and not during mysqld startup).
  • Only the main mysqld my.cnf is used. (so providing in ~/.my.cnf etc. won’t work).
  • All the following options are to be provided under [sst] in cnf unless indicated otherwise.
option streamfmt
Values:xbstream, tar

Xbstream is highly recommended. Refer to Xbstream v/s Tar for details and caveats of using tar v/s xbstream for SST.

option transferfmt
Values:socat, nc

socat is recommended because it allows for socket options like transfer buffer sizes. Refer to socat(1) for details.

option tca
Description:CA file for openssl based encryption with socat.
Type:Full path to CRT file (.crt).
option tcert
Description:PEM for openssl based encryption with socat.
Type:Full path to PEM (.pem).


For tca and tcert, refer to http://www.dest-unreach.org/socat/doc/socat-openssltunnel.html for an example. The tca is essentially the self-signed certificate in that example, and tcert is the PEM file generated after concatenation of the key and the certificate generated earlier. The names of options were chosen so as to be compatible with socat’s parameter’ names as well as with MySQL’s SSL authentication. For testing you can also download certificates from launchpad. Note that irrespective of what is shown in the example, you can use same crt and pem files on all nodes and it will work, since there is no server-client paradigm here but a cluster with homogeneous nodes.

option encrypt

Decides whether encryption is to be done or not, if this is zero, no encryption is done. encrypt=2 is recommended if your nodes are over WAN and security constraints are higher, while encrypt=1 (Xtrabackup-based symmetric encryption) is easier to setup.

  • Xtrabackup based encryption with encrypt=1.
  • OpenSSL based encryption with encrypt=2. Socat must be built with openSSL for encryption: socat -V | grep OPENSSL.
  • Support for SSL encryption for just the key and crt files as implemented in Galera can be enabled with encrypt=3 option. Information on this option can be found here.

Refer to this document when enabling with encrypt=1.

option encrypt-algo

This option is only considered when encrypt is equal to 1. Refer to this before setting this. This option takes the same value as encrypt option here.

option sockopt

Comma separated key/value pairs of socket options. Must begin with a comma. You can use tcpwrap option here to blacklist/whitelist the clients. Refer to socat manual for further details.


You can also enable SSL based compression with sockopt. This can be used in place of compress option of Xtrabackup.

option progress

If equal to:

  • 1 it writes to mysql stderr
  • path/to/file writes to that file. If this is a fifo, it needs to exist and be open on reader end before itself, otherwise wsrep_sst_xtrabackup will block indefinitely.


Value of 0 is not valid.

option rebuild

Used only on joiner. 1 implies rebuild indexes. Note that this is independent of compaction, though compaction enables it. Rebuild of indexes may be used as an optimization. Note that #1192834 affects this, hence use of compact and rebuild are recommended after that is fixed in Percona Xtrabackup and released.

option time

Enabling it instruments key stages of backup/restore in SST.

option rlimit

Ratelimit to x kilobytes, megabytes etc. Refer to pv(1) for details. Note this rate-limiting happens on donor. The rationale behind this is to not allow SST to saturate the donor’s regular cluster operations and/or to ratelimit for other purposes.

option incremental

To be set on joiner only, supersedes IST if set. Currently requires manual setup. Hence, not supported currently.

option use_extra

If set to 1, SST will use the thread pool’s extra_port. Make sure that thread pool is enabled and extra_port option is set in my.cnf before you turn on this option.

option cpat

During the SST, the datadir is cleaned up so that state of other node can be restored cleanly. This option provides the ability to define the files that need to be deleted before the SST. It can be set like:


NOTE: This option can only be used when wsrep_sst_method is set to xtrabackup-v2.

option sst_special_dirs

In order for XtraBackup SST to support innodb_data_home_dir and innodb_log_home_dir variables in the configuration file this option was introduced in Percona XtraDB Cluster 5.6.15-25.2. This requires sst-special-dirs to be set under [sst] in the configuration file to either 0 or 1. Also, innodb-data-home-dir and/or innodb-log-group-home-dir need to be defined in my.cnf under [mysqld]. Percona XtraBackup 2.1.6 or higher is required in order for this to work.

NOTE: This option can only be used when wsrep_sst_method is set to xtrabackup-v2.

option compressor/decompressor
Values:command-lines to compressor/decompressor
Default:Not set, hence not enabled.
Example:compressor=’gzip’, decompressor=’gzip -dc’

This option introduces stream-based compression/decompression. When these options are set, compression/decompression are done on stream, in contrast to earlier PXB-based one where decompression was done after streaming to disk, involving additional I/O; hence I/O is saved here (almost halved on joiner). You can use any compression utility which works on stream - gzip, pigz (which is multi-threaded and hence, recommended) etc. Also, note that, compressor has to be set on donor and decompressor on joiner (though you can have decompressor set on donor and vice-versa for config homogeneity, it won’t affect that particular SST). To use Xtrabackup-based compression as before use compress under [xtrabackup] as before, also having both enabled won’t cause any failure (though you will be wasting CPU cycles with this).

option inno-backup-opts, inno-apply-opts, inno-move-opts
Type:Quoted String

These group of options can be used to pass options to backup, apply, move stages of innobackupex. Note, this option is to be used to pass only those options which are innobackupex-only and thus cannot be provided in my.cnf. Otherwise, it is strongly recommended to pass xtrabackup options through my.cnf (under [xtrabackup]).

option sst-initial-timeout
Values:0 (Disabled)

This option is use to configure initial timeout (in seconds) to receive a first packet via SST. This has been implemented, so that if donor dies somewhere in between, joiner doesn’t hang and wait forever. Thus joiner won’t wait for more than 100 seconds to get a donor. The default should be sufficient, however, it is configurable, so you can set it appropriately for your cluster.


If you are using wsrep_sst_donor, and you want joiner to strictly wait for donors listed in the variable and not fallback (ie. without a terminating comma at the end), and there is a possibility of all nodes in that variable to be unavailable, set this variable to 0 to disable it or set it to a higher value (maximum threshold that you want joiner to wait). You can also disable (or set higher) this if you believe all other nodes in the cluster can be potentially unavailable at any point in time (mostly in small clusters) or there is a high network latency / network disturbance (which can cause donor selection to take longer than 100 seconds).

Tar against xbstream

  • Features - encryption, compression, parallel streaming, streaming incremental backups, compaction - won’t work with tar. Refer to xbstream docs for more.

Xtrabackup SST Dependencies

Following are optional dependencies of PXC introduced by wsrep_sst_xtrabackup: (obvious and direct dependencies are not provided here)

  • qpress for decompression. It is an optional dependency of Percona XtraBackup 2.1.4 and it is available in our software repositories.
  • my_print_defaults to extract values from my.cnf. Provided by the server package.
  • openbsd-netcat or socat for transfer. socat is a direct dependency of Percona XtraDB Cluster and it is the default.
  • xbstream/tar for streaming. tar is default.
  • pv. Required for progress and rlimit. Provided by pv.
  • mkfifo. Required for progress. Provided by coreutils.
  • mktemp. Required for incremental. Provided by coreutils.

Galera compatible encryption

Support for SSL encryption for just the key and crt files as implemented in Galera can be enabled with encrypt=3 option. This has been implemented in 5.5.34-23.7.6 for compatibility with Galera. NOTE: This option does not provide certificate validation. In order to work correctly paths to the key and cert files need to be specified as well, like:


NOTE: This option can only be used when wsrep_sst_method is set to xtrabackup-v2 (which is default now).

Xtrabackup-based encryption

This is enabled when encrypt is set to 1 under [sst]. However, due to bug #1190335, it will also be enabled when you specify any of the following options under [xtrabackup] in my.cnf:

  • encrypt
  • encrypt-key
  • encrypt-key-file

There is no way to disallow encryption from innobackupex if the above are in my.cnf under [xtrabackup]. For that reason, do the following:

  1. If you want to use xtrabackup based encryption for SST but not otherwise, use encrypt=1 under [sst] and provide xtrabackup_encrypt_options under [sst]. Details of those options can be found here.
  2. If you want to use xtrabackup based encryption always, use encrypt=1 under [sst] and have those xtrabackup_encrypt_options either under [sst] or [xtrabackup].
  3. If you don’t want xtrabackup based encryption for SST but want it otherwise, use encrypt=0 or encrypt=2 and do NOT provide xtrabackup_encrypt_options under [xtrabackup]. You can still have them under [sst] though. You will need to provide those options on innobackupex commandline then.
  4. If you don’t want to use xtrabackup based encryption at all (or only the openssl-based for SST with encrypt=2), then you don’t need worry about these options! (just don’t provide them in my.cnf)


The encrypt under [sst] is different from under [xtrabackup]. The former is for disabling/changing encryption mode, latter is to provide encryption algorithm. To disambiguate, if you need to provide latter under [sst] (which you need to, for points #1 and #2 above) then it should be specified as encrypt-algo.


An implication of the above is that if you specify xtrabackup_encrypt_options but encrypt=0 under [sst], it will STILL be encrypted and SST will fail. Look at point#3 above for resolution.

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