When you reindex a suffix, the server examines all of the entries the suffix contains and rebuilds the index files. During reindexing, the contents of the suffix are read-only. Because the server must scan the entire suffix for every attribute that is reindexed, this process might take up to several hours for suffixes with millions of entries. The length of time also depends on the indexes you configure. In addition, while the suffix is being reindexed, it is not available.
You can use DSCC to perform this task. For information, see Directory Service Control Center Interface and the DSCC online help.