How fast SHOULD it move?

Destination:

Quite often, people purchase a new backup application with the idea that it will help them improve the throughput of data between their system and their backup device. Quite often, however, they are disappointed to discover that the new application is no faster than tar or cpio. The resaon for this usually relates directly to the performance of the backup device they are using.

For example, an old 60MB 1/4" tape drive only accepts data at a rate of around 100 kilobytes per second. Therefore, the user should not expect more than around 6 megabytes per minute of data backed up to the tape. On even the slowest 386 running SCO XENIX, you can stream this device with tar, so buying a new backup application will not improve the speed of the backup process. Here are a few standard I/O rates for the more common backup devices available today.


1/4" Cartridges (DC 6000)
With QIC-02 interface card
    QIC-24/120/150 	(60, 125, 150/250MB)	100kps	(6MB/min) 
With SCSI (QIC-104) interface
    QIC-24/120/150 	(60, 125, 150/250MB)	120kps	(7.2MB/min) 
    QIC-525		(525MB)			230kps  (14MB/min)  
    QIC-1000		(1GB)			200kps  (12MB/min)
    QIC-1350		(1.35GB)		600kps  (36MB/min)
    SLR3                (1.2GB)                 300kps  (18MB/min)
    SLR4                (2.5GB)                 300kps  (18MB/min)
    SLR5                (4.0GB)                 380kps  (22MB/min)
1/4" Floppy Tape Drives (QIC-80)
    On a 500Kbps Floppy controller 		 17kps	(1MB/min)
    On a 1Mbps/2Mbps Floppy Controller		 80kps	(4.8MB/min)
TR-3 or Iomega 2G Tape drives
    On a 1Mbps Floppy controller                 80kps  (4.8MB/min)
    On a 2Mbps Accelerator                      145kps  (8.7MB/min)

Travan NS8 (was called TR4)
    SCSI or IDE/ATAPI                           596Kps  (34MB/min)

4mm DAT (SCSI)
    DDS-1		(uncompressed 2.0GB)	183kps	(11MB/min)
    DDS-2		(uncompressed 4.0GB)	366kps  (22MB/min)
    DDS-3               (uncompressed 12.0GB)  1000kps  (60MB/min
    Notes: compression drives provide a speed improvement equal
           to the compression ratio.  2:1 compression on a DDS-2
           drive will provide 736kps effective throughput.

           You must use the correct tape for DDS-2 & DDS-3 drives
           in order the obtain the speeds indicated.  Using older
           tapes (i.e., 120m in DDS-3) will run at slower kps.

8mm Exabyte (SCSI)
    EXB-8200/8205		2.3GB		255kps  (15MB/min)
    EXB-8500/8505		5.0GB		500kps	(30MB/min)

Metrum VHS (SCSI)
    RSP-2150			20GB		2048kps (120MB/min)
    RSP-2150i	               ~40GB	       ~4096kps (~240MB/min)	

1/2" Digital Linear Tape (DLT)
    800kps on the 2/6GB (64MB/min)
    800kps on the 10GB  (64MB/min)
   1200kps on the 20GB  (72MB/min)

Actually, it probably does. If you are running a high-end system like a Sun SparcServer 2000 with Fiber Channel SCSI and a DDS-2 DAT with no compression, you're probably already maxing the drive out at the rated 366kps. Even lower performance x86 platforms can drive a tape at that speed.

If, however, you have a high-end Exabyte mechanism or a new DLT, you may be able to eek a few more kps out of the system by using a better buffering scheme or using a utility like BRU that provides double buffered I/O. With double buffered I/O, BRU fills one buffer while another is being written to tape. This way, your system maintains a steady stream of data to the backup device.

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Last modified: Wednesday, 22-Mar-2000 15:34:51 MST