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"Paul Rubin" wrote in message
... Malcolm Weir writes: Still, what you've experienced shows the benefit of writing lots of file marks. Classic Unix (and Unix-like) things (tar, etc.) want to write *one* tape file, and if part of that file is corrupted, then it's all lost. But if you write a filemark every (say) 100MB, then the chances of being able to seek past the bad spot improve dramatically. I've heard that if you write two consecutive EOF marks on a DDS or 8mm tape, that's interpreted as end-of-tape and there's absolutely no way to read past it. It's not like the old days of 9 track tape. One more example of half-truth. Filemarks are logical entities on tape, just like data blocks are. Reading past two filemarks is just as well possible as reading past any other data entity on tape. You can start your tape write with 500 filemarks, then a single block of data. You'll always be able to get that single block back. The story of not being able to read past two filemarks is based on the fact that it normally marks the end of a write session. In many data streams, the end of *logical* data is marked by writing two consecutive filemarks. It serves as an append point for later sessions. Tape, being sequential media, also writes a *physical* end-of-data marker at that point, and usually it's followed by an erase gap. Depending on the implementation, the erase gap is a single track, or can be a number of tracks wide. It invalidates any tracks behind the end of data marker, by partially or completely erasing them. Reading past such an area is normally prevented in firmware, as anything behind the EOD marker is not considered part of the current tape sequence (remember, sequential data). However, most drives can be persuaded to skip such an erase area to the next valid filemark, and can continue to read from there on. Data recovery centers use this feature to get 'overwritten' data back. I've written software to even automate the recovery process on EXB-8200 through 8505XL, it's very easy to do. On most helical scan drives you can do this. On most linear drives you can do this too (except DLT7000, as it destroys multiple tracks with a single write). Rob |
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