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Groups > comp.os.linux.misc > #90457

Re: Overwriting with random data

From "Carlos E.R." <robin_listas@es.invalid>
Newsgroups comp.os.linux.misc
Subject Re: Overwriting with random data
Date 2026-08-24 20:34 +0200
Organization Tebibyte_Retro_Gaming
Message-ID <vefulmxca.ln2@Telcontar.valinor> (permalink)
References (8 earlier) <5b2dnb6l8sPvPhf3nZ2dnZfqnPGdnZ2d@giganews.com> <6d3rlmxqg4.ln2@Telcontar.valinor> <116fs5p$28rhg$2@dont-email.me> <6r8tlmxeb9.ln2@Telcontar.valinor> <116h5be$2nco1$1@dont-email.me>

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On 2026-08-24 12:10, Nuno Silva wrote:
> On 2026-08-24, Carlos E.R. wrote:
> 
>> On 2026-08-24 00:28, vallor wrote:
>>> At Sun, 23 Aug 2026 13:50:30 +0200, "Carlos E.R." <robin_listas@es.invalid> wrote:
>>
> [snipping a lot to keep this shorter, apologies if I ended up snipping
> too much]
>>
>>>> I wrote a program that writes bytes on big partitions.
>>>>
>>>>
>>>> excerpted:
>>>>
>>>> const
>>>>         sourcefilename = './BigRandom';
>>>>         rawdest: string = '';     // example: '/dev/sda11'
>>>>
>>>>         shuflecount = 1024;
>>>>         arraysize = 1023;
>>>>         ChunkSz=1048576;
>>>> type
>>>>         tCacho= array [1..ChunkSz] of byte;
>>>>
>>>> var
>>>>       Fin, Fout: file of tCacho;
>>>>       gotresult: Word;
>>>>
>>>>       BigData : array [1..shuflecount] of tCacho;
>>>>       OutputIndex : Int64;	// counts MiB written.
>>>>
>>>>
>>>> begin
>>>>
>>>> // writes one MiB
>>>>          {$I-}write(Fout,BigData[RandomIndex]);{$I+}
>>>>          gotresult:= ioresult;
>>>>
>>>>
>>>>
>>>> The purpose of the program is to fast fill a partition or disk with
>>>> nearly true random data. It is as fast as dd could be.
>>>
>>> SHRED(1)                 User Commands                 SHRED(1)
>>>
>>> NAME
>>>          shred  -  overwrite  a  file  to  hide its contents, and
>>>          optionally delete it
>>>
>>> SYNOPSIS
>>>          shred [OPTION]... FILE...
>>>
>>> DESCRIPTION
>>>          Overwrite the specified FILE(s) repeatedly, in order  to
>>>          make  it harder for even very expensive hardware probing
>>>          to recover the data.
>>
>> Not the same thing.
> 
> IIRC shred can do the same, and on block devices too, what's the
> difference here? block sizes?


I mentioned my code because it accesses huge devices in pascal, way 
beyond 4G.

c186282 said:
   Note Lazarus/FPC *says* it supports huge seek numbers
   but DOESN'T ... limited to maybe 4gb.


Some functions work, others don't. This does not work:

const
      rawdest: string = '';     // example: '/dev/sda11'
      ChunkSz=1048576;

type
      tCacho= array [1..ChunkSz] of byte;

var
    Fin, Fout: file of tCacho;


begin

    rawdest:= ParamStr(1);
    writeln('Apparent size of ', rawdest, ' is ', filesize(Fout));


It writes '0' (I am currently erasing an 8TB disk)


Notice that I am not using a file of bytes, but a file of 1 MiB arrays. 
And I am not using seek, but writing contiguously.


Now the shred part.


Using true random data is slow, specially if the disk or partition is 
very big. Depends on how fast can the machine create new random data 
(see man random and urandom). So I do tricks to do it faster. And shred 
does it several times.

The algorithm I use is:

   Fill a 1 GiB file with true random data.
   Read that file into ram.
   repeat
      take 1 MiB of that and write it into the destination device
      advance
      Every 100 advances, replace 1 MiB of the array of random bytes
         with new random data
   until end of device


So, the data is truly random, but used repeatedly (and slowly renovated).

-- 
Cheers, Carlos.
ES🇪🇸, EU🇪🇺;

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Thread

/dev/tcp Eli the Bearded <*@eli.users.panix.com> - 2026-08-19 21:20 +0000
  Re: /dev/tcp Richard Kettlewell <invalid@invalid.invalid> - 2026-08-19 23:53 +0100
  Re: /dev/tcp Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-08-19 23:42 +0000
    Re: /dev/tcp Eli the Bearded <*@eli.users.panix.com> - 2026-08-20 00:59 +0000
      Re: /dev/tcp Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-08-20 03:59 +0000
        Re: /dev/tcp Nuno Silva <nunojsilva@invalid.invalid> - 2026-08-20 10:10 +0100
        Re: /dev/tcp Eli the Bearded <*@eli.users.panix.com> - 2026-08-20 18:15 +0000
          Re: /dev/tcp Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-08-20 23:30 +0000
            Re: /dev/tcp Nuno Silva <nunojsilva@invalid.invalid> - 2026-08-21 01:18 +0100
            Re: /dev/tcp Marc Haber <mh+usenetspam2616@zugschl.us> - 2026-08-21 09:52 +0200
              Re: /dev/tcp Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-08-21 08:12 +0000
                Re: /dev/tcp Marc Haber <mh+usenetspam2616@zugschl.us> - 2026-08-21 11:16 +0200
                Re: /dev/tcp Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-08-21 22:57 +0000
                Re: /dev/tcp Marc Haber <mh+usenetspam2616@zugschl.us> - 2026-08-22 06:17 +0200
              Re: /dev/tcp c186282 <c186282@nnada.net> - 2026-08-21 21:50 -0400
              Re: /dev/tcp Anssi Saari <anssi.saari@usenet.mail.kapsi.fi> - 2026-08-24 17:42 +0300
                Re: /dev/tcp Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-08-24 23:53 +0000
            Re: /dev/tcp vallor <vallor@vallor.earth> - 2026-08-23 00:56 +0000
              Re: /dev/tcp Rich <rich@example.invalid> - 2026-08-23 04:01 +0000
                Re: /dev/tcp c186282 <c186282@nnada.net> - 2026-08-23 03:37 -0400
                Re: /dev/tcp "Carlos E.R." <robin_listas@es.invalid> - 2026-08-23 13:50 +0200
                Re: /dev/tcp vallor <vallor@vallor.earth> - 2026-08-23 22:28 +0000
                Re: /dev/tcp "Carlos E.R." <robin_listas@es.invalid> - 2026-08-24 09:35 +0200
                Overwriting with random data (was: Re: /dev/tcp) Nuno Silva <nunojsilva@invalid.invalid> - 2026-08-24 11:10 +0100
                Re: Overwriting with random data "Carlos E.R." <robin_listas@es.invalid> - 2026-08-24 20:34 +0200
                Re: Overwriting with random data Richard Kettlewell <invalid@invalid.invalid> - 2026-08-24 20:57 +0100
                Re: Overwriting with random data "Carlos E.R." <robin_listas@es.invalid> - 2026-08-24 22:26 +0200
                Re: /dev/tcp c186282 <c186282@nnada.net> - 2026-08-24 00:31 -0400
                Re: /dev/tcp "Carlos E.R." <robin_listas@es.invalid> - 2026-08-24 09:45 +0200
                Re: /dev/tcp The Natural Philosopher <tnp@invalid.invalid> - 2026-08-23 12:52 +0100
                Re: /dev/tcp The Natural Philosopher <tnp@invalid.invalid> - 2026-08-23 12:50 +0100
  Re: /dev/tcp Rich <rich@example.invalid> - 2026-08-20 13:50 +0000
    Re: /dev/tcp c186282 <c186282@nnada.net> - 2026-08-20 13:02 -0400
      Re: /dev/tcp Rich <rich@example.invalid> - 2026-08-20 18:26 +0000
      Re: /dev/tcp The Natural Philosopher <tnp@invalid.invalid> - 2026-08-21 13:54 +0100
        Re: /dev/tcp c186282 <c186282@nnada.net> - 2026-08-21 22:57 -0400
          Re: /dev/tcp "Carlos E.R." <robin_listas@es.invalid> - 2026-08-22 21:47 +0200
            The hammer is best (Was: /dev/tcp) gazelle@shell.xmission.com (Kenny McCormack) - 2026-08-22 23:50 +0000
              Re: The hammer is best (Was: /dev/tcp) c186282 <c186282@nnada.net> - 2026-08-23 04:28 -0400
              Re: The hammer is best Richard Kettlewell <invalid@invalid.invalid> - 2026-08-23 10:14 +0100
                Re: The hammer is best "Carlos E.R." <robin_listas@es.invalid> - 2026-08-23 13:55 +0200
                Re: The hammer is best The Natural Philosopher <tnp@invalid.invalid> - 2026-08-23 13:16 +0100
                Re: The hammer is best Marc Haber <mh+usenetspam2616@zugschl.us> - 2026-08-23 21:20 +0200
                Re: The hammer is best c186282 <c186282@nnada.net> - 2026-08-24 02:18 -0400
                Re: The hammer is best The Natural Philosopher <tnp@invalid.invalid> - 2026-08-24 11:26 +0100
                Re: The hammer is best Joed Oakes <lost@noway.home.invalid> - 2026-08-23 19:42 -0400
                Re: The hammer is best Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-08-23 23:56 +0000
                Re: The hammer is best c186282 <c186282@nnada.net> - 2026-08-24 02:25 -0400
                Re: The hammer is best rbowman <bowman@montana.com> - 2026-08-24 01:14 +0000
                Re: The hammer is best "Carlos E.R." <robin_listas@es.invalid> - 2026-08-24 09:50 +0200
                Re: The hammer is best c186282 <c186282@nnada.net> - 2026-08-24 00:53 -0400
                Re: The hammer is best rbowman <bowman@montana.com> - 2026-08-24 06:42 +0000
                Re: The hammer is best c186282 <c186282@nnada.net> - 2026-08-24 23:34 -0400
                Re: The hammer is best rbowman <bowman@montana.com> - 2026-08-25 04:08 +0000
                Re: The hammer is best The Natural Philosopher <tnp@invalid.invalid> - 2026-08-24 11:28 +0100
                Re: The hammer is best c186282 <c186282@nnada.net> - 2026-08-23 22:16 -0400
      Re: /dev/tcp Geoff Clare <geoff@clare.See-My-Signature.invalid> - 2026-08-21 13:41 +0100
        Re: /dev/tcp Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-08-21 22:58 +0000
        Re: /dev/tcp c186282 <c186282@nnada.net> - 2026-08-21 22:28 -0400
          ksh (was: /dev/tcp) Geoff Clare <geoff@clare.See-My-Signature.invalid> - 2026-08-24 13:56 +0100
            Bash vs. ksh - Does it matter? And the 'vi' mode thing... (Was: ksh (was: /dev/tcp)) gazelle@shell.xmission.com (Kenny McCormack) - 2026-08-24 14:57 +0000
            Re: ksh (was: /dev/tcp) Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-08-24 21:08 +0000
              Re: ksh (was: /dev/tcp) gazelle@shell.xmission.com (Kenny McCormack) - 2026-08-24 21:28 +0000
              Re: ksh (was: /dev/tcp) rbowman <bowman@montana.com> - 2026-08-25 01:11 +0000
    Re: /dev/tcp Eli the Bearded <*@eli.users.panix.com> - 2026-08-20 18:22 +0000
      Re: /dev/tcp The Natural Philosopher <tnp@invalid.invalid> - 2026-08-21 13:57 +0100
  Re: /dev/tcp Lars Poulsen <lars@beagle-ears.com> - 2026-08-23 06:37 -0700
    Re: /dev/tcp Richard Kettlewell <invalid@invalid.invalid> - 2026-08-23 14:49 +0100
    Re: /dev/tcp The Natural Philosopher <tnp@invalid.invalid> - 2026-08-23 16:13 +0100
    Re: /dev/tcp Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-08-23 22:31 +0000
      Re: /dev/tcp Richard Kettlewell <invalid@invalid.invalid> - 2026-08-24 19:08 +0100

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