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Re: backup archive format saved to disk

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On 12/05/06 19:33, Douglas Tutty wrote:
> On Tue, Dec 05, 2006 at 06:57:38PM -0600, Ron Johnson wrote:
>  > 
>>> You could implement your own FEC. A very simple form of FEC is simply
>> Yes, but *why*?  Tape storage systems have been using ECC for decades.
>> There's a whole lot of "Linux people" who's knowledge of computer
>> history seems to have started in 1991, and thus all the many lessons
>> learned in 30 years of computing are lost.
> Hi Ron,
> I'm hoping someone who can remember computer history prior to 1991 can
> give some perspective.
> It think (__please__ correct me if I'm wrong) that the tape systems had
> the ECC as part of the hardware.  Write a plain datastream to the drive
> and the drive did the ECC part transparent to the user.  Read the data
> and a bad block gets fixed by the hardware ECC.
> I'm told that modern hard drives also do ECC but I can't find out how
> that is implemented.  I'm told that if a block starts to fail (whatever
> that means) then the data is transferred to a new unallocated block,
> transparent to the rest of the computer.  Only if the drive runs out of
> unallocated blocks does it give errors.
> The question is, if a block is sucessfully written now, if the drive is
> not used for 5 years then a read is attempted, is the drive able to
> retreive that data using ECC (as a tape drive could)?

Mike is correct, disk drive blocks do have ECC.

Remember, though, that drives are delicate mechanisms, and so the
problem I see is the lubricating oil possibly thickening, and thus
the drive not spinning up properly.  Hopefully the bad spin-up would
not cause the r/w head to gouge the platter.  Otherwise, the data
could still be retrieved, easily, for a price, from a data recovery

> In the absence of an all-in-one archive format, I'll use tar (which can
> detect errors just not fix them) to take care of names, owners,
> permissions, etc.  Then that tar needs to be made ECC and compressed.
> If I want to throw in a monkey, I'll consider encryption.

Remember what "tar" means: Tape ARchive.  It's designed as a
container file.

OTOH, if you're backing up a hard disk, you could do file-by-file
backups, compressing the big, compressible files, and leaving alone
the not-so-compressible files.  Thus, if a sector goes blooey,
you've still got most of your data.

> Yes tape drives do that.  Its probably why they cost so much.  Hard

And lower production volumes.

> drives are much cheaper and are supposed to be able to hang on to their
> data (Seagate gives a 5 year warranty).  But having seagate give me a
> new drive when I can't get my data off after 4 years is cold comfort.  
> The other problem with tapes is their fragility.  Drop a DLT and I'm
> told that its toast.  Put that tape in the drive and I'm told it can
> damage the drive.

We've used DLT drives for years, and never had that problem.

>                   A laptop drive in a ruggedized enclosure is much more
> robust and has a wider environmental range.

Drop a HDD and you've got worse problems.

> Perhaps what I'm looking for doesn't exist.  If it doesn't, I'll start
> work on it.
> As far as computer history prior to 1991, I could never get the hang of
> C.  I'll stick with fortran77.

Give me VAX COBOL.  But then, I've always been on the DP side.

- --
Ron Johnson, Jr.
Jefferson LA  USA

Is "common sense" really valid?
For example, it is "common sense" to white-power racists that
whites are superior to blacks, and that those with brown skins
are mud people.
However, that "common sense" is obviously wrong.
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