Online functionality
The underlying table cannot be modified, truncated, or dropped while an online index operation is in process.
The online option setting (ON or OFF) specified when you create or drop a clustered index is applied to any nonclustered indexes that must be rebuilt. For example, if the clustered index is built online by using CREATE INDEX WITH DROP_EXISTING, ONLINE=ON, all associated nonclustered indexes are re-created online also.
When you create or rebuild a UNIQUE index online, the index builder and a concurrent user transaction may try to insert the same key, therefore violating uniqueness. If a row entered by a user is inserted into the new index (target) before the original row from the source table is moved to the new index, the online index operation will fail.
Although not common, the online index operation can cause a deadlock when it interacts with database updates because of user or application activities. In these rare cases, the SQL Server Database Engine will select the user or application activity as a deadlock victim.
You can perform concurrent online index DDL operations on the same table or view only when you are creating multiple new nonclustered indexes, or reorganizing nonclustered indexes. All other online index operations performed at the same time fail. For example, you cannot create a new index online while rebuilding an existing index online on the same table.
An online operation cannot be performed when an index contains a column of the large object type, and in the same transaction there are update operations before this online operation. To work around this issue, place the online operation outside the transaction or place it before any updates in the transaction.
Disk Space Considerations
Generally, disk space requirements are the same for online and offline index operations. An exception is additional disk space required by the temporary mapping index. This temporary index is used in online index operations that create, rebuild, or drop a clustered index. Dropping a clustered index online requires as much space as creating a clustered index online. For more information, see Disk Space Requirements for Index DDL Operations.
Performance Considerations
Although online index operations permit concurrent user update activity, the index operations will take longer if the update activity is very heavy. Typically, online index operations will be slower than equivalent offline index operations regardless of the concurrent update activity level.
Because both the source and target structures are maintained during the online index operation, the resource usage for insert, update, and delete transactions is increased, potentially up to double. This could cause a decrease in performance and greater resource usage, especially CPU time, during the index operation. Online index operations are fully logged.
Although we recommend online operations, you should evaluate your environment and specific requirements. It may be optimal to run index operations offline. In doing this, users have restricted access to the data during the operation, but the operation finishes faster and uses fewer resources.
On multiprocessor computers that are running SQL Server 2012, index statements may use more processors to perform the scan and sort operations associated with the index statement just like other queries do. You can use the MAXDOP index option to control the number of processors dedicated to the online index operation. In this way, you can balance the resources that are used by index operation with those of the concurrent users. For more information, see Configure Parallel Index Operations. For more information about the editions of SQL Server that support Parallel indexed operations, see Features Supported by the Editions of SQL Server 2012 (http://go.microsoft.com/fwlink/?linkid=232473).
Because an S-lock or Sch-M lock is held in the final phase of the index operation, be careful when you run an online index operation inside an explicit user transaction, such as BEGIN TRANSACTION...COMMIT block. Doing this causes the lock to be held until the end of the transaction, therefore impeding user concurrency.
Online index rebuilding may increase fragmentation when it is allowed to run with MAX DOP > 1 and ALLOW_PAGE_LOCKS = OFF options. For more information, see How It Works: Online Index Rebuild - Can Cause Increased Fragmentation.
Transaction Log Considerations
Large-scale index operations, performed offline or online, can generate large data loads that can cause the transaction log to quickly fill. To make sure that the index operation can be rolled back, the transaction log cannot be truncated until the index operation has been completed; however, the log can be backed up during the index operation. Therefore, the transaction log must have sufficient space to store both the index operation transactions and any concurrent user transactions for the duration of the index operation. For more information, see Transaction Log Disk Space for Index Operations.
参考文档：
https://msdn.microsoft.com/en-us/library/ms190981(v=sql.110).aspx