What happens during a backup

During a backup, SQL Server creates one reader thread for each volume that the database files reside on.  The reader thread simply reads the contents of the files.  Each time that it reads a portion of the file, it stores them in a buffer.  There are multiple buffers in use, so the reader thread will keep on reading as long as there are free buffers to write to.  SQL Server also creates one writer thread for each backup device to write the contents of the buffers out to disk or tape.  The writer thread writes the data from the buffer to the disk or tape.  Once the data has been written, the buffer can be reused by the reader thread.

Thus, given the following command:

BACKUP DATABASE AdventureWorks TO  DISK = 'M:\backups\AdventureWorks_FULL_01.bak',  DISK = 'N:\backups\AdventureWorks_FULL_02.bak' WITH BUFFERCOUNT = 10, MAXTRANSFERSIZE = 1048576

and assuming that AdventureWorks files are spread across three volumes, the backup process will proceed as follows:

SQL Server does not distinguish between logical and physical volumes.  In the above example, if E:\, F:\ and G:\ are all partitions on the same physical drive, 3 reader threads are still used to read the data files.  If your disk cannot keep up with the read demands, the backup throughput will be adversely affected.

If the BUFFERCOUNT and MAXTRANSFERSIZE parameters are not used, SQL Server will dynamically determine the number of buffers to use and the size of each buffer.  The total memory used for the backup buffers will be BUFFERCOUNT x MAXTRANSFERSIZE (bytes) + some overheads.  This memory is allocated from the  non-buffer pool memory, also known as the MemToLeave memory.  If you specify values that are larger than what is available, SQL Server will raise the following error:

Server: Msg 701, Level 17, State 1, Line 1 There is insufficient system memory to run this query. Server: Msg 3013, Level 16, State 1, Line 1 BACKUP DATABASE is terminating abnormally.

If you want to find out the size and quantity of the buffers used by SQL Server for a backup, you can turn on trace flags 3605 and 3213.  The values used are then recorded in the SQL Server log e.g.

2008-07-14 17:21:20.65 spid51    BufferPoolLimit:          100 MB 2008-07-14 17:21:20.65 spid51    Backup/Restore buffer configuration parameters 2008-07-14 17:21:20.65 spid51    Buffer count:             15 2008-07-14 17:21:20.65 spid51    Max transfer size:        983040 2008-07-14 17:21:20.65 spid51    Total buffer space:       14 MB 2008-07-14 17:21:20.65 spid51    Buffers per read stream:  15 2008-07-14 17:21:20.65 spid51    Buffers per write stream: 5 2008-07-14 17:21:20.65 spid51    Memory Limit:             100 MB

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