Symantec NetBackup Option NDMP - ( v. 7.1 ) - Essential Support

Data is written to tape in a series of fragments, which are separated with a logical element called a FileMark. NetBackup has a specific format for the fragments it writes, consisting of small blocks of data to represent the source of the following data (a list of backup ids). Following the list of backup ids is a much larger set of data blocks that contain the backup data itself. A single backup will consist of at least one fragment, and may be made up of thousands of fragments. As tape drive technology has advanced, the time required to write data has significantly decreased, while the time required to write a FileMark has remained about the same. For customers writing many small images, such as database transaction logs, this means that the maintenance tasks involved with writing a NetBackup image to tape now take a substantial amount of the time required to write the image.

When a tape drive receives a request to write a standard (synchronous) FileMark, it must first flush all buffered write data to tape before writing the file mark and reporting success, which can take 1 to 2 seconds to perform. It is possible to use a flag for the write filemark command to the tape drive, called the Immediate flag, that allows the write FileMark operation to be buffered (asynchronous) on the drive, in which case the operation only takes 3 ms, and does not require the drive to flush buffered data to tape.

NetBackup 7.1 will still use the synchronous form of the write FileMark command for checkpoints, and when a client has finished sending data for an image. NetBackup will use the asynchronous form of the command when there is still more data to be written for an image, such as TIR, maximum fragment size, next image start, and single image failure in an MPX group.