Skip to main content This browser is no longer supported. Show
Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. FILESTREAM (SQL Server)
In this articleApplies to: SQL Server (all supported versions) - Windows onlyFILESTREAM enables SQL Server-based applications to store unstructured data, such as documents and images, on the file system. Applications can use the rich streaming APIs and performance of the file system and at the same time maintain transactional consistency between the unstructured data and corresponding structured data. FILESTREAM integrates the SQL Server Database Engine with an NTFS or ReFS file systems by storing varbinary(max) binary large object (BLOB) data as files on the file system. Transact-SQL statements can insert, update, query, search, and back up FILESTREAM data. Win32 file system interfaces provide streaming access to the data. FILESTREAM uses the NT system cache for caching file data. Caching files in the system cache helps reduce any impact that FILESTREAM data might have on Database Engine performance. The SQL Server buffer pool isn't used; therefore, this memory is available for query processing. FILESTREAM is not automatically enabled when you install or upgrade SQL Server. You must enable FILESTREAM by using SQL Server Configuration Manager and SQL Server Management Studio. To use FILESTREAM, you must create or modify a database to contain a special type of filegroup. Then, create or modify a table so that it contains a varbinary(max) column with the FILESTREAM attribute. After you complete these tasks, you can use Transact-SQL and Win32 to manage the FILESTREAM data. When to Use FILESTREAMIn SQL Server, BLOBs can be standard varbinary(max) data that stores the data in tables, or FILESTREAM varbinary(max) objects that store the data in the file system. The size and use of the data determines whether you should use database storage or file system storage. If the following conditions are true, you should consider using FILESTREAM:
For smaller objects, storing varbinary(max) BLOBs in the database often provides better streaming performance. FILESTREAM StorageFILESTREAM storage is implemented as a varbinary(max) column in which the data is stored as BLOBs in the file system. The sizes of the BLOBs are limited only by the volume size of the file system. The standard varbinary(max) limitation of 2-GB file sizes doesn't apply to BLOBs that are stored in the file system. To specify that a column should store data on the file system, specify the FILESTREAM attribute on a varbinary(max) column. This attribute causes the Database Engine to store all data for that column on the file system, but not in the database file. FILESTREAM data must be stored in FILESTREAM filegroups. A FILESTREAM filegroup is a special filegroup that contains file system directories instead of the files themselves. These file system directories are called data containers. Data containers are the interface between Database Engine storage and file system storage. When you use FILESTREAM storage, consider the following:
Integrated ManagementBecause FILESTREAM is implemented as a varbinary(max) column and integrated directly into the Database Engine, most SQL Server management tools and functions work without modification for FILESTREAM data. For example, you can use all backup and recovery models with FILESTREAM data, and the FILESTREAM data is backed up with the structured data in the database. If you don't want to back up FILESTREAM data with relational data, you can use a partial backup to exclude FILESTREAM filegroups. Integrated SecurityIn SQL Server, FILESTREAM data is secured just like other data is secured: by granting permissions at the table or column levels. If a user has permission to the FILESTREAM column in a table, the user can open the associated files. Note Encryption is not supported on FILESTREAM data. Only the account under which the SQL Server service account runs is granted permissions to the FILESTREAM container. We recommend that no other account be granted permissions on the data container. Note SQL logins will not work with FILESTREAM containers. Only NTFS or ReFS authentication will work with FILESTREAM containers. Accessing BLOB Data with Transact-SQL and File System Streaming AccessAfter you store data in a FILESTREAM column, you can access the files by using Transact-SQL transactions or by using Win32 APIs. Transact-SQL AccessBy using Transact-SQL, you can insert, update, and delete FILESTREAM data:
File System Streaming AccessThe Win32 streaming support works in the context of a SQL Server transaction. Within a transaction, you can use FILESTREAM functions to obtain a logical UNC file system path of a file. You then use the OpenSqlFilestream API to obtain a file handle. This handle can then be used by Win32 file streaming interfaces, such as ReadFile() and WriteFile(), to access and update the file by way of the file system. Because file operations are transactional, you can't delete or rename FILESTREAM files through the file system. Warning The FILESTREAM container is a folder managed by SQL Server. Do not add or remove files in the FILESTREAM folder manually or through other applications. If you do, this will result in backup and inconsistency errors. For more information, see MSSQLSERVER_3056, MSSQLSERVER_7908, and MSSQLSERVER_7906. Statement Model The FILESTREAM file system access models a Transact-SQL statement by using file open and close. The statement starts when a file handle is opened and ends when the handle is closed. For example, when a write handle is closed, any possible AFTER trigger that is registered on the table, fires as if an UPDATE statement is completed. Storage Namespace In FILESTREAM, the Database Engine controls the BLOB physical file system namespace. A new intrinsic function, PathName, provides the logical UNC path of the BLOB that corresponds to each FILESTREAM cell in the table. The application uses this logical path to obtain the Win32 handle and operate on the BLOB data by using regular Win32 file system interfaces. The function returns NULL if the value of the FILESTREAM column is NULL. Transacted File System Access A new intrinsic function, GET_FILESTREAM_TRANSACTION_CONTEXT(), provides the token that represents the current transaction that the session is associated with. The transaction must have been started and not yet aborted or committed. By obtaining a token, the application binds the FILESTREAM file system streaming operations with a started transaction. The function returns NULL in case of no explicitly started transaction. All file handles must be closed before the transaction commits or aborts. If a handle is left open beyond the transaction scope, additional reads against the handle will cause a failure; additional writes against the handle will succeed, but the actual data won't be written to disk. Similarly, if the database or instance of the Database Engine shuts down, all open handles are invalidated. Transactional Durability With FILESTREAM, upon transaction commit, the Database Engine ensures transaction durability for FILESTREAM BLOB data that is modified from the file system streaming access. Isolation Semantics The isolation semantics are governed by Database Engine transaction isolation levels. Read-committed isolation level is supported for Transact-SQL and file system access. Repeatable read operations, serializable and snapshot isolation levels are supported. Dirty read isn't supported. The file system access open operations do not wait for any locks. Instead, the open operations fail immediately if they can't access the data because of transaction isolation. The streaming API calls fail with ERROR_SHARING_VIOLATION if the open operation cannot continue because of isolation violation. To allow for partial updates to be made, the application can issue a device FS control (FSCTL_SQL_FILESTREAM_FETCH_OLD_CONTENT) to fetch the old content into the file that the opened handle references. This will trigger a server-side old content copy. For better application performance and to avoid running into potential time-outs when you are working with very large files, we recommend that you use asynchronous I/O. If the FSCTL is issued after the handle has been written to, the last write operation will persist, and prior writes that were made to the handle are lost. File System APIs and Supported Isolation Levels When a file system API cannot open a file because of an isolation violation, an ERROR_SHARING_VIOLATION exception is returned. This isolation violation occurs when two transactions try to access the same file. The outcome of the access operation depends on the mode the file was opened in and the version of SQL Server that the transaction is running on. The following table outlines the possible outcomes for two transactions that are accessing the same file.
Write-Through from Remote Clients Remote file system access to FILESTREAM data is enabled over the Server Message Block (SMB) protocol. If the client is remote, no write operations are cached by the client side. The write operations will always be sent to the server. The data can be cached on the server side. We recommend that applications that are running on remote clients consolidate small write operations into larger size operations. The goal is to perform fewer writes. Creating memory mapped views (memory mapped I/O) by using a FILESTREAM handle is not supported. If memory mapping is used for FILESTREAM data, the Database Engine cannot guarantee consistency and durability of the data or the integrity of the database. Recommendations and guidelines for improving FILESTREAM performanceThe SQL Server FILESTREAM feature allows you to store varbinary(max) binary large object data as files in the file system. When you have a large number of rows in FILESTREAM containers, which are the underlying storage for both FILESTREAM columns and FileTables, you can end up with a file system volume that contains large number of files. To achieve best performance when processing the integrated data from the database and the file system, it is important to ensure the file system is tuned optimally. The following are some of the tuning options that are available from a file system perspective:
Enable and Configure FILESTREAM Related ContentFILESTREAM
Compatibility with Other SQL Server Features FeedbackSubmit and view feedback for Which type of file access jumps directly to a piece of data in the file without having to read all the data that comes before it quizlet?When working with a direct access file, you can jump directly to any piece of data in the file without reading the data that comes before it.
What type of access is used to read a file from beginning to end?When working with a sequential access file, you access its data from the beginning of the file to the end of the file. A direct access file, on the other hand, allows you to access its data from any point within the file that you choose.
Which type of data file access starts from the beginning?The direct access file is similar to the way an MP3 player works: the player starts from the beginning. *What is the terminology used when a task is temporarily interrupted as a control variable reaches a specific value? True/False: The term "output file" is used to describe a file that data is read from.
What type of data file can be opened and viewed in an editor such as Notepad?TEXT FILES AND NON-TEXT FILES
Text files can be directly read using a text editor such as Notepad in Microsoft Windows. These files often have extension .
|