I have different values for the filemarks, measured with another program and not amtapetype. Do you know if the filemarks are
I don't have a DDS3 drive, so I just made one up, that was a good approximation. Amanda uses the filemarks to get a better calculation of the tapecapacity. If the filemark is small compared to the tapelength, and you don't have many DLE's to write to tape, they are not that important. The variations in tapelength between tapes or the number of retries when writing with a dirty head could be more important.
application-dependant, tape-dependant, tape-and-taper-dependant ??? The IBM fms software tells that filemarks are
tape-and-taper-dependant. Do you know more? Do you have any opinions
concerning that?
Amtapetype tries to measure the size of a filemark by measuring the difference in capacity when writing 100 files vs. 200 files to tape. The fact that it sometimes even calculates a negative size (reported as 0 too), means that the filemarks are not important in this case.
Some tapedrives do have a significant amount, however.
Just to be sure we are talking about the same thing. When you write data on a tape, you write sequences of "data / mark / data / mark" wehre the marks are here to make a separation between the different flows of data, the "fsf 1" parameter of mt, the "EOD" (End Of Data) mark. A tape contains a sequence like that: BOT.(data.EOD)*.EOT | BOT = Begin Of Tape | data = well... | EOD = End Of Data | EOT = End Of Tape
I'm not a tape hardware specialist, so don't believe me blindly. How a tape separates the different data blocks on a tape could be implemented in different ways. Some tapedrives use a gap, resulting in lost capacity equal to the length of the gap. The gap distance is used to stop the tape, and is dependent of the speed of the tape (could be a few inches, when I used to work with 9inch reel tapes on an IBM mainframe). Others use a distinct pattern. It could even be implemented out-of-band on e.g. a seperate track on the tape, resulting in no capacity loss at all.
EOT is often implemented as twice EOD.
With access to the really low level commands of a tapedrive, you could read the data that was once written behind the EOT mark. Data rescue centers use these techniques, if you pay them enough.
Some tapedrives (QIC tapes) write in "serpentine" tracks to the tape: When hitting the physical end of tape, the heads are shifted a little, and the tape is reversed, writing the data between the previous tracks. I believe some can do 4 passes over the tapelength, each time shifting the heads a little. While writing there is a seperate head that erases the tape. The "erasing" head erases the tape over the complete width of the tape, so writing a quarter of such a tape erases it completely.
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