CentOS - Jan 2020 - [semi-OT] C7 Possible bug but I can't determine what tool has the problem

Il 15/01/20 17:51, Jon Pruente ha scritto:
> On Wed, Jan 15, 2020 at 10:18 AM Alessandro Baggi <
> alessandro.baggi at gmail.com> wrote:
>
>> I made several test to see what happens and noticed that removing -S
>> (--sparse) from rsync command problem does not occour.
>> In another test, thinking about a problem on 0ed file, I tried file
>> generation using /dev/urandom then /dev/zero and running rsync -avS the
>> problem is disappeared.
>>
> https://wiki.archlinux.org/index.php/Sparse_file#Creating_sparse_files
>
> In short, rsync is being told to create sparse files with the -S flag, so
> it does.  Could you share what you did with the urandom then zero test you
> mentioned? I'm curious what exact sequence of commands you used.
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Hi Jon,
I wrote in the first mail the script with the current order of command 
that I used. Try to run in a bash script and you will see the result.

If not my sequence is:

dd if=/dev/zero of=src/testfile bs=1M count=100
rsync -avS src/ dest/
du -h dest/testfile
du -b dest/testfile

for urandom:

dd if=/dev/urandom of=src/testfile bs=1M count=100
rsync -avS src/ dest/
du -h dest/testfile
du -b dest/testfile

without --sparse the same as first sequence without -S option.

But why du reports 0M when with -b reports correct bytes and why this 
happens only with zeroed file?




I don't know if in the original post mail script

On Wed, Jan 15, 2020 at 11:38 AM Alessandro Baggi <
alessandro.baggi at gmail.com> wrote:
> Hi Jon,
> I wrote in the first mail the script with the current order of command
> that I used. Try to run in a bash script and you will see the result.
>
> If not my sequence is:
>
> dd if=/dev/zero of=src/testfile bs=1M count=100
> rsync -avS src/ dest/
> du -h dest/testfile
> du -b dest/testfile
>
> for urandom:
>
> dd if=/dev/urandom of=src/testfile bs=1M count=100
> rsync -avS src/ dest/
> du -h dest/testfile
> du -b dest/testfile
>
> without --sparse the same as first sequence without -S option.
>
> But why du reports 0M when with -b reports correct bytes and why this
> happens only with zeroed file?
>
>
Ah, I misunderstood what you meant. I had thought you might have created a
file with urandom first and then overwrote it with zeros. This is behaving
as expected with sparse files. You can create a sparse file with dd by
using seek:
https://www.thegeekdiary.com/how-to-create-sparse-files-in-linux-using-dd-command/

Il 15/01/20 18:54, Jon Pruente ha scritto:
> On Wed, Jan 15, 2020 at 11:38 AM Alessandro Baggi <
> alessandro.baggi at gmail.com> wrote:
>
>> Hi Jon,
>> I wrote in the first mail the script with the current order of command
>> that I used. Try to run in a bash script and you will see the result.
>>
>> If not my sequence is:
>>
>> dd if=/dev/zero of=src/testfile bs=1M count=100
>> rsync -avS src/ dest/
>> du -h dest/testfile
>> du -b dest/testfile
>>
>> for urandom:
>>
>> dd if=/dev/urandom of=src/testfile bs=1M count=100
>> rsync -avS src/ dest/
>> du -h dest/testfile
>> du -b dest/testfile
>>
>> without --sparse the same as first sequence without -S option.
>>
>> But why du reports 0M when with -b reports correct bytes and why this
>> happens only with zeroed file?
>>
>>
> Ah, I misunderstood what you meant. I had thought you might have created a
> file with urandom first and then overwrote it with zeros. This is behaving
> as expected with sparse files. You can create a sparse file with dd by
> using seek:
>
https://www.thegeekdiary.com/how-to-create-sparse-files-in-linux-using-dd-command/
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> https://lists.centos.org/mailman/listinfo/centos
Thank you for the suggestion. I meant -S of rsync to use disk space 
efficiently but this is a (great) misunderstood. Now I read again rsync 
man page (and your link) and this means "treat sparse files efficiently 
to save space on disk".
My question is: is rsync capable to detect sparse files from "regular"
files? If -S is invoked and no sparse files are not in dataset, it 
treats those files as sparse files or "regular" files?

Why I get different behaviour using urandom and /dev/zero? This is 
casual/accidental?

Thank you again for your help.