GNU bug report logs - #68216
An instance of DD receiving data from another instance with bs=1M reports the wrong record count.

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: "jared <at> johnstone.net.au" <jared <at> johnstone.net.au>
To: bug-coreutils <at> gnu.org
Subject: An instance of DD receiving data from another instance with bs=1M
 reports the wrong record count.
Date: Tue, 2 Jan 2024 21:45:36 +1100

[Message part 1 (text/plain, inline)]

Hi all,

It seems an instance of `dd`  with only an output file doesn't report 
the count of records accurately despite bs= being described as 
influencing both ibs= and obs=

dd'ing from an unstable remote over ssh today I had it continue from the 
last received block between crashes in preparation for a P2V. The remote 
had bs=1M set with its in=device and the local instance had bs=1M too 
with the destination of= device but upon referring to the receiving 
end's stable count of records I noticed it ignored my bs=1M setting. 
This were worked around using obs=1M to make the counter show a value I 
could reliable skip=/seek= with to resume.

I have an example here:

   /$ dd if=/dev/zero bs=1M count=1 | dd bs=1M of=/dev/null
   1+0 records in
   1+0 records out
   1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.0010021 s, 1.0 GB/s
   *0+16 records in
   0+16 records out*
   1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.000771722 s, 1.4 GB/s/

This is being experienced on Archlinux with kernel '6.6.4' and package 
'core/coreutils 9.4-2'

dd's man page for this distribution shows:

   /bs=BYTES
                  read and write up to BYTES bytes at a time (default:
   512); overrides ibs and obs/

So I feel it not overriding obs= when invoked with only of= and bs= 
arguments could be a bug.

Running a single dd with /if=source of=dest bs=1M/ works as intended.

While in my case this isn't a fatal bug working with virtual block 
devices it could be problematic for use cases which require a proper 
treatment of block sizes such as tape devices.

Best regards,
Jared

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