The Art of SQL Server FILESTREAM - Red Gate Software

Chapter 7: FILESTREAM Database AdministrationFILESTREAM and Garbage FilesSQL Server does not support in-place updates of the disk file storing the BLOB data for aFILESTREAM column. A disk file is created for every non-NULL FILESTREAM data valuestored in every FILESTREAM column of every table in the database. When you updatethe BLOB value stored in a FILESTREAM cell, the associated disk file also needs to beupdated. What SQL Server does internally is to create a new file with the modified value.The old version of the file will lie around in the folder until the file is no longer needed.Unwanted files are later deleted by an asynchronous garbage collection process that runsin the background. An old version of a FILESTREAM data file is available for garbagecollection when a database CHECKPOINT occurs after the transaction log is truncated pastthe LSN (Log Sequence Number) which generated the new file.For this reason, FILESTREAM storage is not ideal for cases where the BLOB data storedin the FILESTREAM column is frequently updated. Every time the FILESTREAM valueis updated (from a non-NULL value to another non-NULL value) SQL Server will haveto create a new file with the modified value. If the frequency of such update operationsis high, you might experience severe performance problems. In such a case, storing theBLOB data in the file system and storing the path to the disk file in a table might be moreefficient than using a FILESTREAM column.Garbage files and recovery modelsUnder the SIMPLE recovery model, the FILESTREAM garbage collector is triggered whena database CHECKPOINT occurs. You can issue a CHECKPOINT to trigger the garbagecollection thread to clean up the unwanted files in the FILESTREAM data container.Under the FULL and BULK_LOGGED recovery models, you might notice that issuing aCHECKPOINT does not clean up the unwanted files in the FILESTREAM data container.A CHECKPOINT may still run the garbage collector, but it may not remove the file as youwould normally expect. This is because the transaction log virtual log file (VLF) that279