From: "Jason Pyeron" <jpyeron@pdinc.us>
To: <cygwin@cygwin.com>
Subject: RE: [cygwin] DD bug fails to wipe last 48 sectors of a disk
Date: Sun, 28 Jun 2020 13:50:16 -0400 [thread overview]
Message-ID: <00f601d64d74$959826d0$c0c87470$@pdinc.us> (raw)
In-Reply-To: <5c9c5b44-c8a6-a075-705e-1761533cc966@t-online.de>
> -----Original Message-----
> From: Christian Franke
> Sent: Sunday, June 28, 2020 10:35 AM
>
> 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.
The reason I have never encountered this is because I use a block size which is the largest practical GCD of the drive size and 512 bytes (typically between 32 MB and 64 MB).
E.g. I have a drive that is 160,041,885,696 bytes, which divides 312,581,808 times evenly into 512. I would use a block size of 39,072,726 bytes, which gives 4,096 blocks to write.
-Jason
next prev parent reply other threads:[~2020-06-28 17:50 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
2020-06-28 17:50 ` Jason Pyeron [this message]
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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