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Subject:	Re: [cygwin] DD bug fails to wipe last 48 sectors of a disk
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From:	Christian Franke <Christian DOT Franke AT t-online DOT de>
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Date:	Sun, 28 Jun 2020 16:34:33 +0200
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Andrey Repin wrote:
>
>> dd if=/dev/zero of=/dev/sda iflag=fullblock bs=4M status=progress

The root of the problem is that the Windows WriteFile() function 
apparently does not support truncated writes at EOM. If seek_position + 
write_size > disk_size, then WriteFile() does nothing and returns an error.


> oflag=direct
>
> Although I'm unsure how Cygwin/Windows handles it. But without this flag, the
> write is cached, and the problem may be outside dd, or even Cygwin.

If 'oflag=direct' is used, dd passes O_DIRECT flag to open() call of 
output file. Cygwin's open() function then passes 
FILE_NO_INTERMEDIATE_BUFFERING to NtCreateFile() and the write() 
function calls WriteFile() directly with original address and size.

Without O_DIRECT, Cygwin ensures that address and size passed to 
WriteFile() are both aligned to sector size. All writes are then done 
through a 64KiB internal buffer.

As a consequence, oflag=direct in the above dd command may increase 
speed but would also let the final 4MiB WriteFile() fail. Without 
oflag=direct, only the last 64KiB WriteFile() fails.

To clear the last sectors of the disk, use an appropriate small block 
size. I did this several times with Cygwin 'dd seek=... bs=512 ...' to 
get rid of Intel RST RAID metadata.

-- 
Regards,
Christian

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