State Snapshot Transfer 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, new node has to transfer data from the node that is already part of the cluster. There are three methods of SST available in Percona XtraDB Cluster: mysqldump, rsync and xtrabackup. The downside of mysqldump and rsync is that the donor node becomes READ-ONLY while data is being copied from one node to another. Xtrabackup SST, on the other hand, uses backup locks, which means galera provider is not paused at all as with FTWRL (Flush Tables with Read Lock) earlier. State snapshot transfer method can be configured with the wsrep_sst_method variable.
If the variable gcs.sync_donor is set to Yes (default No), whole cluster will get blocked if the donor is blocked by the State Snapshot Transfer and not just the donor node.
If there are no nodes available that can safely perform an incremental state transfer, the cluster defaults to a state snapshot transfer. If there are nodes available that can safely perform an incremental state transfer, the cluster prefers a local node over remote nodes to serve as the donor. If there are no local nodes available that can safely perform an incremental state transfer, the cluster chooses a remote node to serve as the donor. Where there are several local or remote nodes available that can safely perform an incremental state transfer, the cluster chooses the node with the highest seqno to serve as the donor.
This is the default SST method (version 2 of it: xtrabackup-v2). This is the least blocking method as it uses 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 PXC to perform the SST using the XtraBackup, credentials for connecting to the donor node need to be set up in the variable wsrep_sst_auth. Beside the credentials, one more important thing is that the datadir needs to be specified in the server configuration file my.cnf, otherwise the transfer process will fail.
More information about the required credentials can be found in the XtraBackup manual. Easy way to test if the credentials will work is to run the innobackupex on the donor node with the username and password specified in the variable wsrep_sst_auth. For example, if the value of the wsrep_sst_auth is root:Passw0rd innobackupex command should look like:
innobackupex --user=root --password=Passw0rd /tmp/
Detailed information on this method are provided in Xtrabackup SST Configuration documentation.
This method uses the standard mysqldump to dump all the databases from the donor node and import them to the joining node. For this method to work wsrep_sst_auth needs to be set up with the root credentials. This method is the slowest one and it also performs the global lock while doing the SST which will block writes to the donor node.
Script used for this method can be found in /usr/bin/wsrep_sst_mysqldump and it’s provided with the Percona XtraDB Cluster binary packages.
This method uses rsync to copy files from donor to the joining node. In some cases this can be faster than using the XtraBackup but requires the global data lock which will block writes to the donor node. This method doesn’t require username/password credentials to be set up in the variable wsrep_sst_auth.
Script used for this method can be found in /usr/bin/wsrep_sst_rsync and it’s provided with the Percona XtraDB Cluster binary packages.
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