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Tapes fill up prematurely

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My scheduled backup asks for a new tape member before the first member is full. I am using 2 backup sets alternating daily, Normal backup. Retrospect Server is running on an NT4 server and backing up itself and a Clients container consisting of 2 AppleShare servers on a different subnet. The tape drive is a Sony SDT-10000 DDS-4 and claims hardware compression capability. Both backup sets request a second member at anywhere from 10gb to 22gb. All tape media is new 20/40gb Fuji.


Your KnowledgeBase says I can expect less than the claimed capacity amount for a variety of reasons and that hardware compression could help. 50% less than the stated uncompressed media capacity seems excessive.

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Tape capacity is dictated by several factors, none of which involve software settings or anything you can set as options. Tape drives are streaming mechanisms, meaning that tapes move at a constant speed during backups and do not wait for data to arrive to them. As such, the following factors can influence how much you fit on a tape:




1) Transfer Speeds- the speed of the data arriving to the tape drive is affected by the speed of the processor on the computer running Retrospect, the speed in which data arrives over a network cable, or the speed in which data from a local drive is sent through the SCSI bus to the tape drive which can be seriously hampered by a troubled drive or a fragmented drive.




2) Local vs. network backups- partly covered in 'speed'; you'll find that local backups fit more on a tape than network backups due to the intrinsic speed differences. When doing troubleshooting tests, it's often good to see how a tape drive performs on local backups only so you don't have to account for network issues at all initially. If a local backup also has substandard capacity on tapes, it's time to troubleshoot the hardware and SCSI bus, assuming you've already checked the local disks for any problems.




3) Compression- Keep in mind that compression (if supported by your drive) relies on the types of files you're backing up. If you back up a lot of text files, these compress very well and you'll get a larger capacity per tape. If you back up a lot of application/video/audio/graphics files, these are already compressed and will not compress any further. Your capacity will therefore suffer.




4) SCSI Hardware- a bad tape or dirty heads can undermine how much data is written on a tape. Poor tape qualify can cause underrunning, a phenomenon in which the tape drive's heads write the same bit of data multiple times as error correction on the tape, wasting long portions of tape that go unwritten. Dirty heads likewise can cause similar problems. Also, SCSI interference on the bus from a bad cable or from other devices can also impede the flow of data traffic to the tape drive. Isolate the bus and remove all non-essential devices and run test backups to the tape drive. Swap out the SCSI cable on the tape drive. Clean the heads with a cleaning cartridge, no matter how new or how recently you might have already done this.




Essentially, you want to first rule out a bad tape and dirty heads. Having done this, you want to test the sanity of your tape drive by trying it as the only device on a different computer, running local backups to a tape to see how much it takes to fill it up. If you can reproduce this under-capacity problem on two different computers, you most likely have a hardware problem with the tape drive. Contact your vendor for repairs or replacement possibilities.




You can review Retrospect's operations log for transfer rates as well. Extremely slow transfer rates on larger sized backups (e.g., 300MB or greater) could indicate problems elsewhere like a CPU, network, or SCSI card. Do not expect high figures on smaller backups as they will naturally be slower due to the size of the data being copied.

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  • 3 months later...





Is there any way to scan the tape and find out how the media has been used?




I'm the same sort of problem in my Sony AIT drive. The tapes are rated for a 25GB native capacity and should hold 50GB compressed. I'm only getting about 30GB on a tape. All of the larger backups run at about 30MB/Minute. The drive is rated to run at 180MB/Minute. I've seen log entries that indicate compare speeds of 210MB/Minute.




Does the compare speed indicate anything about how the tape was written?




How much extra space is used by Retrospect to store file lists, etc? Is this space in the total of the tape usage?







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