From: Christian Franke <Christian.Franke@t-online.de>
To: cygwin@cygwin.com
Subject: Re: [cygwin] DD bug fails to wipe last 48 sectors of a disk
Date: Sun, 28 Jun 2020 16:34:33 +0200 [thread overview]
Message-ID: <5c9c5b44-c8a6-a075-705e-1761533cc966@t-online.de> (raw)
In-Reply-To: <1814912576.20200624132126@yandex.ru>
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
next prev parent reply other threads:[~2020-06-28 14:34 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-19 18:19 Hashim Aziz
[not found] ` <1d1801d64677$bea56050$3bf020f0$@pdinc.us>
2020-06-20 17:31 ` [cygwin] " Hashim Aziz
2020-06-20 20:26 ` Eliot Moss
2020-06-23 15:28 ` Hashim Aziz
2020-06-23 15:29 ` Eliot Moss
2020-06-23 16:06 ` Nicholas Clark
2020-06-23 19:46 ` Hashim Aziz
2020-06-24 10:21 ` Andrey Repin
2020-06-25 12:08 ` Hashim Aziz
2020-06-25 13:25 ` Brian Inglis
2020-06-28 14:34 ` Christian Franke [this message]
2020-06-28 17:50 ` Jason Pyeron
2020-06-28 20:28 ` Brian Inglis
2020-06-23 19:33 ` Brian Inglis
2020-12-29 0:46 ` Hashim Aziz
2020-12-29 2:41 ` Jason Pyeron
2020-12-29 17:54 ` Brian Inglis
2020-12-30 1:35 ` Jason Pyeron
[not found] ` <DB7PR01MB5193B13815933B913D11DDC2D5D20@DB7PR01MB5193.eurprd01.prod.exchangelabs.com>
2021-01-04 21:08 ` Jason Pyeron
2020-08-04 22:42 ` Hashim Aziz
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