I have read through main sites but want to know the real experience from the users.
So please let me know the status of the mysql instance when we using xtrabackup practically.
as per the docs. i have learn the below points.
By default, it starts xtrabackup with the --suspend-at-end option, and lets it copy the InnoDB data files. When xtrabackup finishes that, innobackupex sees it create the xtrabackup_suspended_2 file and executes FLUSH TABLES WITH READ LOCK. xtrabackup will use Backup locks where available as a lightweight alternative to FLUSH TABLES WITH READ LOCK. This feature is available in Percona Server 5.6+. Percona XtraBackup uses this automatically to copy non-InnoDB data to avoid blocking DML queries that modify InnoDB tables. Then it begins copying the rest of the files.
innobackupex will then check MySQL variables to determine which features are supported by server. Special interest are backup locks, changed page bitmaps, GTID mode, etc. If everything goes well, the binary is started as a child process.
innobackupex will wait for slaves in a replication setup if the option --safe-slave-backup is set and will flush all tables with READ LOCK, preventing all MyISAM tables from writing (unless option --no-lock is specified).