The Ftape Installation and Usage Guide - File System Interface
From the user's point of view there is a single -- possibly severe -- compatibility problem w.r.t. ftape-3.04d:
The compression support has vanished!
Why the hell? Well, ftape-4.03 adds a major new feature to the ftape driver (besides supporting additional hardware):
ftape-4.03 comes with hard write error recovery!
`But why does this new feature require that zftape's compression facility vanishes
' I hear you cry. Well. I thought about it several
hours and gave up. Implementing hard write error recovery would have
been very hard if I had kept the compression support.
However, calm down!. You still can read your old compressed archives produced by ftape-3.04d and earlier versions with ftape-4.03. Decompression of compressed archives is still supported for compatibility reasons.
If you need to compress your archives get a backup program with support
for compression. Do not use tar -czf /dev/qft0
. A single
wrong byte in the compressed archive will make it almost impossible to
recover your data. Instead, get a decent backup software with support
for compression. Refer to the Ftape Home Page for some examples.
So, what is hard write error recovery
? Here a write error
refers to a write error detected by the FDC during data transfer to the
tape (there might be other errors which aren't recognised by the FDC
...). All ftape versions prior to ftape-4.03 retried several time
when such an error was detected. If the data still couldn't be written
successfully after a given number of retries, such an error was marked
hard
. However, this was the end of story for ftape-2.x and
ftape-3.04d.
After retrying several times the write error simply was ignored!
ftape-4.03 introduces a major change: if there are hard write errors
in the above sense, then the sector(1) is marked as bad in the bad sector map
(2) and is
skipped and ftape-4.03 continues writing behind the bad sector.
However, in order to keep the tape streaming the ftape driver
caches up to three floppy tape segments(3). This means, if
there is a hard write error
then ftape needs to shift the
data around cycling some sectors through its cache.
And this is the reason for dropping the compression feature (or bug). The layout of the compressed data that ftape-3.04d stored on the tape depended on the segment layout of the tape. As floppy tape segments have varying size caused by bad sectors, this introduces several complications when additional bad sectors are detected by ftape.
Of course, keeping the compression support would have been possible, but complicated.
(1) data entity of 1024 bytes
(2) a region of the header segments where media defects are recorded
(3) data entities of 32 sectors, 29 data, 3 ECC (Error Correction Code)
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