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State Snapshot Transfer

State Snapshot Transfer (SST) is a full data copy from one node (donor) to the joining node (joiner). It’s used when a new node joins the cluster. In order to be synchronized with the cluster, the new node has to receive data from a node that is already part of the cluster.

There are three methods of SST available in Percona XtraDB Cluster:

  • mysqldump
  • rsync
  • xtrabackup

The downside of mysqldump and rsync is that the donor node becomes READ-ONLY while data is being copied. Xtrabackup SST, on the other hand, uses backup locks, which means the Galera provider is not paused at all as with FTWRL (Flush Tables with Read Lock) earlier. The SST method can be configured using the wsrep_sst_method variable.

Note

If the gcs.sync_donor variable is set to Yes (default is No), the whole cluster will get blocked if the donor is blocked by SST.

Choosing the SST Donor

If there are no nodes available that can safely perform incremental state transfer (IST), the cluster defaults to SST.

If there are nodes available that can perform IST, the cluster prefers a local node over remote nodes to serve as the donor.

If there are no local nodes available that can perform IST, the cluster chooses a remote node to serve as the donor.

If there are several local and remote nodes that can perform IST, the cluster chooses the node with the highest seqno to serve as the donor.

Using Percona Xtrabackup

The default SST method is xtrabackup-v2 which uses Percona XtraBackup. This is the least blocking method that leverages backup locks. XtraBackup is run locally on the donor node, so it’s important that the correct user credentials are set up on the donor node. In order for Percona XtraDB Cluster to perform SST using XtraBackup, credentials for connecting to the donor node need to be set up in the wsrep_sst_auth variable. Besides the credentials, the datadir needs to be specified in the server configuration file my.cnf, otherwise the transfer process will fail.

For more information about the required credentials, see the XtraBackup manual.

To test if the credentials will work, run innobackupex on the donor node with the username and password specified in the wsrep_sst_auth variable. For example, if the value of wsrep_sst_auth is root:Passw0rd, the innobackupex command should look like this:

innobackupex --user=root --password=Passw0rd /tmp/

Detailed information on this method is provided in Percona XtraBackup SST Configuration documentation.

Using mysqldump

This method uses the standard mysqldump utility to dump all the databases from the donor node and import them to the joining node. For this method to work, the wsrep_sst_auth variable needs to be set up with the root credentials. This method is the slowest and it performs a global lock during SST, which blocks writes to the donor node.

The script used for this method is /usr/bin/wsrep_sst_mysqldump and it is included in the Percona XtraDB Cluster binary packages.

Using rsync

This method uses rsync to copy files from donor to the joining node. In some cases, this can be faster than using XtraBackup, but it requires a global data lock, which will block writes to the donor node. This method doesn’t require root credentials to be set up in the wsrep_sst_auth variable.

The script used for this method is /usr/bin/wsrep_sst_rsync and it is included in the Percona XtraDB Cluster binary packages.

SST for tables with tablespaces that are not in the data directory

For example:

CREATE TABLE t1 (c1 INT PRIMARY KEY) DATA DIRECTORY = '/alternative/directory';

The result depends on the SST method:

  • SST using rsync

    SST will report success, however the table’s data will not be copied over, since rsync just copies the files. You will not be able to access the table on the joiner node:

    mysql> select * from t1;
    ERROR 1812 (HY000): Tablespace is missing for table `sbtest`.`t1`.
    
  • SST using mysqldump

    Works as expected. If the file does not exist, it will be created. Otherwise it will attempt to use the file (if the file doesn’t have the expected format, an error is returned).

  • SST using Percona XtraBackup

    XtraBackup will restore the table to the same location on the joiner node. If the target directory does not exist, it will be created. If the target file already exists, an error will be returned, because XtraBackup cannot clear tablespaces not in the data directory.

This documentation is developed in Launchpad as part of the Percona XtraDB Cluster 5.7.
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