From the Arch Linux Wiki: https://wiki.archlinux.org/index.php/USB_flash_installation_media
# dd bs=4M if=/path/to/archlinux.iso of=/dev/sdx status=progress && sync
[…] Do not miss sync to complete before pulling the USB drive.
I would like to know
- What does it do?
- What consequences are there if left out?
Notes
dd command used with optional status=progress:
tar -xzOf archlinux-2016-09-03-dual.iso | dd of=/dev/disk2 bs=4M status=progress && sync
Or using pv for progress
tar -xzOf archlinux-2016-09-03-dual.iso | pv | dd of=/dev/disk2 bs=4M && sync
Answers:
Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.
Method 1
The dd does not bypass the kernel disk caches when it writes to a device, so some part of data may be not written yet to the USB stick upon dd completion. If you unplug your USB stick at that moment, the content on the USB stick would be inconsistent. Thus, your system could even fail to boot from this USB stick.
Sync flushes any still-in-cache data to the device.
Instead of invoking sync you could use fdatasync dd‘s conversion option:
fdatasync
physically write output file data before finishing
In your case, the command would be:
tar -xzOf archlinux-2016-09-03-dual.iso | dd of=/dev/disk2 bs=4M status=progress conv=fdatasync
The conv=fdatasync makes dd effectively call fdatasync() system call at the end of transfer just before dd exits (I checked this with dd‘s sources).
This confirms that dd would not bypass nor flush the caches unless explicitly instructed to do so.
Method 2
From the sync(1) manual page : “sync – Synchronize cached writes to persistent storage”. Basically sync makes sure that all your data is written to the stick from the cache.
All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0