The Art of SQL Server FILESTREAM - Red Gate Software

Chapter 1: Storing and ManagingUnstructured DataOne of the challenges of designing data storage of SQL Server-driven applications isthat not all data fits into the relational model. The relational model expects each tableto describe a discrete entity and each column in the table to define exactly one aspect ofthat entity. Much business data is less structured in nature; it is becoming increasinglycommon to need to store images, documents, and speech data (PDF, MP3, and so on)alongside the relational data.It's been possible to store this sort of unstructured data in SQL Server for a number ofyears, but there have always been associated difficulties. With older, large object (LOB)data types, such as TEXT, NTEXT, and IMAGE, the data was generally held out of row, i.e.on a different page from the rest of the row data. Reading and modifying the data, usingT-SQL functions such as READTEXT and WRITETEXT was painfully slow. The introductionof the large-value (MAX) data types, namely VARCHAR(MAX), NVARCHAR(MAX)and VARBINARY(MAX) improved matters somewhat and made it easier to develop applicationsthat used these LOB data types. However, it was still preferable to store theseLOBs on the file system, with only a link to the data stored in the database, in order toavoid the buffer pool pollution that arose from having such data stored inline, and alsoto take advantage of high-performance streaming, lower rates of fragmentation duringmodifications, and so on, that are offered by file system storage.Nevertheless, storing LOBs on the file system, separately from the rest of the businessdata, was not without problems, including the fact that the data wasn't included indatabase backups and, perhaps most significantly, having the data outside the transactionalcontrol of the database meant that it was possible for the LOB data, if it weresubject to modification, to become out of sync with the structured, SQL Server-baseddata to which it related. In addition, when LOB data is stored in the file system, outsidethe control of the database administrator, any user or application with sufficient23