> -----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 -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
