bug#51345: dd with conv=fsync sometimes returns when its writes are still cached

[Bob Proulx]

Sworddragon wrote:
> On Knoppix 9.1 with the Linux Kernel 5.10.10-64 x86_64 and GNU Coreutils
> 8.32 I wanted to overwrite my USB Thumb Drive a few times with random data
> via "dd if=/dev/random of=/dev/sdb bs=1M conv=fsync". While it usually
> takes ~2+ minutes to perform this action dd returned once after less than
> 60 seconds which made me a bit curious.

I suggest another try using oflag=direct instead of conv=fsync.

    dd if=/dev/random of=/dev/sdb bs=1M oflag=direct

Also with rates status.

    dd if=/dev/random of=/dev/sdb bs=1M oflag=direct status=progress

Here is the documentation for it.

  ‘oflag=FLAG[,FLAG]...’

     ‘direct’
          Use direct I/O for data, avoiding the buffer cache.  Note that
          the kernel may impose restrictions on read or write buffer
          sizes.  For example, with an ext4 destination file system and
          a Linux-based kernel, using ‘oflag=direct’ will cause writes
          to fail with ‘EINVAL’ if the output buffer size is not a
          multiple of 512.

Bob