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Does the data ever get deleted from DASD or Tape?

 
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Anuj Dhawan

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Joined: 22 Apr 2006
Posts: 6258
Location: Mumbai, India

PostPosted: Thu Feb 04, 2010 9:10 pm    Post subject: Does the data ever get deleted from DASD or Tape?
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Hi,

I just put some thoughts here which were boggling in my mind - as they say why should only I suffer...icon_razz.gif so I put them here - well not actually that's not the matter - I just want to know if I'm lost in some un-necessary thoughts, please have a look on the follwoing points icon_smile.gif

1. "I need to delete a tape?" -- does the question in double-quotes make a sense? By terminology, a tape-file (a file on a tape) cannot be deleted, right? Then why do we have such threads every now and then...?

2. I think data is nothing but just a "good" combination of bits, a pattern of bits which actually constitutes the information.

Coming from electronics engineering back-ground, point 1 makes me think, if in Mainframes' world, I issue a Delete by whatever means, I believe physically nothing happens. Only the pointers to the data are removed, am I correct?

I just read about allocating datasets using ABSTR and then reallocated the dataset using ABSTR- guess what, hell - data was there.

And other possibility which I think can happen but did not try yet, in case I've no possibility to allocate the dataset without a close being performed, then after reallocating the dataset use the DFDSS print command and I believe I'll see the data, right?

Thanks again for stopping by...icon_smile.gif
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Robert Sample

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Joined: 06 Jun 2008
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Location: Bellevue, IA

PostPosted: Thu Feb 04, 2010 9:24 pm    Post subject:
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Anuj:

When someone says "I need to delete a tape" they are usually saying one of two things: they don't know to expire the tape in the tape management system so it gets reused, or there is sensitive data on the tape and they want the data overwritten (or erased) when they delete the tape file(s). Depending on whether the tape needs to be usable after the deletion, the solution may be as simple as pulling the tape from the library and running a good strong magnet over it for a while. There are commercial products that overwrite data (tape or disk) multiple times to make retrieval of the data nearly impossible (although I've heard the U. S. government's NSA is very good at data recovery).

For a disk data set, a delete usually means the catalog entry is removed and the VTOC entry deleted as well. The actual data set is not usually touched (again, there are commercial products to overwrite the data set). I have had, in the past, occasions where I allocated a disk data set and opened it for input without writing to it first. This used to access the data that was in that spot before, leading to S001 errors as well as various messages about record and block size incompatibilities.
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Anuj Dhawan

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Joined: 22 Apr 2006
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Location: Mumbai, India

PostPosted: Fri Feb 05, 2010 9:03 pm    Post subject:
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Quote:
This used to access the data that was in that spot before, leading to S001 errors as well as various messages about record and block size incompatibilities.
icon_smile.gif Go stand there in the corner Robert...icon_smile.gif.

Robert - You say,
Quote:
there are commercial products to overwrite the data set
if we talk about the geometry of the DASD, actuators will write data on the disk "randomly", right? They can choose any address on the disk and start writing - so they might choose the address "used by other data earlier" unless it's protected - so do we really need the commercial products?

I scratch my head, I'm afraid if I took wrong interpretaion of your comments... icon_redface.gif
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Robert Sample

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PostPosted: Fri Feb 05, 2010 9:28 pm    Post subject:
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Quote:
if we talk about the geometry of the DASD, actuators will write data on the disk "randomly", right? They can choose any address on the disk and start writing - so they might choose the address "used by other data earlier" unless it's protected - so do we really need the commercial products?

I scratch my head, I'm afraid if I took wrong interpretaion of your comments
If you have really sensitive data, or are really paranoid, the commercial products might be recommended. Why? Because disk drives wobble as they are spinning. The wobble means two successive writes to a track aren't likely to exactly overlap -- leaving the old data exposed on the edge. Also, once written the magnetic field will, over time, affect the substrate to some degree.

My understanding is that really advanced processing of the disk can read the edges of the tracks, as well as read the substrate-recorded magnetic fields, to recover data that would normally be considered completely gone.

The commercial products overwrite the track 20, 50, 100 times (or more) so the wobble should be nullified by the multiple overwrites.
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Anuj Dhawan

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PostPosted: Mon Feb 08, 2010 2:30 pm    Post subject:
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Thanks Robert - this helps.

No I'm not paranoid, instead just curious...icon_smile.gif.

I wish I could see what happens in(at) the tape library when I submit a job which requires a tape-unit...
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Robert Sample

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PostPosted: Mon Feb 08, 2010 5:47 pm    Post subject:
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Some of the libraries have windows so you can watch the robot arm go grab the tape and put it in the drive, others do not. If you've got one without a window, there's not a lot to see ...
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Anuj Dhawan

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PostPosted: Mon Feb 08, 2010 6:56 pm    Post subject:
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I see, thanks for that inforamtion Robert...icon_smile.gif
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