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Old 06-12-2008, 10:37 AM   #1
Registered: Apr 2003
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Wink Benchmarking a tape drive, time to copy takes forever, time to append takes longer?

I'm benchmarking a tape drive, and when I run
time tar cvf /dev/st0 /mnt/junk1
, where junk1 is a 250 MB file, it takes 43 seconds. Then when I run
time tar rvf /dev/st0 /mnt/junk2
, where junk2 is also 250 MB, it takes like 1m 15 secs. Is it normal for appending to take so much longer than the initial? Also, any ideas why this new tape drive takes so long to transfer 250 MB? I'm looking into different block sizes, so ideas besides that are welcome (unless you have a recommended block size for an IBM Ultrium LTO 4 ).

Last edited by jeriryan; 06-12-2008 at 10:38 AM.
Old 06-14-2008, 09:01 PM   #2
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Try using some tape utilities. Also sdparam and other SCSI disk utilities can be used.

In order to send 120 MB per second to a drive, you will have to make sure your file setup can handle it. If your setup can handle 240 MB per second, I would say it should handle it easily.

I think you are using EXT3. It is not as fast as XFS or JFS on terms of throughput, but it is more reliable. You will have to pick to be reliable or have a high throughput. I mainly use XFS for my desktops and notebook, but only if RAM is at least 512 MB or more.

I have not used a tape drive yet, but I am just looking at the data throughput point of view.
Old 06-15-2008, 12:15 AM   #3
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This sounds like typical tape performance. Remember that you cannot seek and change things on a tape the same way as a HD. For example, unless a large blank space was left at the end of a file, you cannot easily add anything to the end of the file. One trick used to make things go faster is to copy the file to the 'end' of the current data on tape, put in the additions, and mark the old copy so that it will be skipped. You can imagine that after a few operations only, the tape is fragmented. Yet another trick is to mark the tape to say "this file is continued at THIS location", but you can imagine that on restoring that file the tape will read a tiny bit then do a long wind to get to the other bit.

Dealing with tape (whether paper or magnetic) is an art of its own; numerous articles have been written over the decades on how "best" to do whatever on tape.


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