On Thursday 14 May 2020 17:30:53 Nathan Stratton Treadway wrote: > On Thu, May 14, 2020 at 09:14:17 +0200, Stefan G. Weichinger wrote: > > Interesting, how can a "dirty" drive trigger this behavior? > > > > I'd expect failures all along and not after ~200 or 300 GB written. > > > > I don't see any interrupted writing or so (until that End Of Tape). > > (We switched to disk-drive vtapes a long time ago so when I was last > looking into the details of backup-tape-drive behavior it was probably > for pre-LTO technology, but I would assume that for this discussion > LTO is similar....) > > For "modern" error-correcting tape drives, when the computer sends > data out to the tape drive to be written to tape, the drive actually > then uses the read head to immedately read back in the data it just > wrote. If that read fails, the drive will automatically/transparently > try the write again... repeating the process until it is able to > achieve a successful confirmation read of that block of data. > > Normally this just happens once in a while, when there's a bad spot on > the tape or some fluke of writing makes the data unreadable, and one > doesn't even notice it's happening. > > However, if the drive head is dirty or the tape media in general is > wearing out, then what happens is that many many many of the data > blocks either will be written badly or will fail to read back in > [depending on what exactly is dirty or failing], and the drive will > have to re-write data multiple times before a succesful write/read > cycle. > > When that happens, then lots of the linear space on the tape is used > by all the repeated writes -- thus making the tape appear to have a > lower capacity than you would expect -- and also all that re-writing > means the data throughput from the server's point of view is much > reduced. > > (Note that in this scenario the drive just keeps retrying to write a > block up data until it succeeds... or until it hits the end of the > tape. So that's why you don't get "interrupted writing" in the sense > of having mid-tape write errors returned by the tape device the > computer. [But it is "interrupted" in the sense that a block takes > much longer to write than it should so the computer has to wait a long > time before it can sent the next block of data down to the drive.]) > > Hope that makes sense. > > Nathan It makes perfect sense Nathan, what makes no sense is the drive continuing to try forever, continueing to wear out its heads when what it should if the 2nd or maybe the third write fails, it should at the very minimum start a blinking "clean me" led and abort the run. That at the very least would make me change the makers name on a frame rail someplace, its simply not excusable.
You buy these things looking for dependability, and I've never yet had a tape drive last over 1000 tape moving hours. I have now removed it from service, but I've a 1t drive in a drawer I will likely use for my next linux install, still has the 25 re-allocated sectors it had the first time I asked it many years ago. That report caused me to update its firmware. And that spinning rust drive has over 70,000 spinning hours on it now, retired because it was getting too small. And people wonder why I use vtapes. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene>
