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Is running `sync` necessary after writing a disk image?

Jérôme Pouiller

Common way to write an image to disk looks like:

dd if=file.img of=/dev/device

After this command, is it necessary to run sync?

sync(2) explains it only flushes filesystem caches. Since dd command is not related to any filesystem, I think it is not necessary to run sync. However, block layer is complex and in doubt, most people prefers to run sync.

Does anyone has a proof that it is useful or useless?

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Answer

TL;DR: Run blockdev --flushbufs /dev/device after dd.

I tried to follow the different paths in kernel. Here is what I understood:

  • ioctl(block_dev, BLKFLSBUF, 0) call blkdev_flushbuf(). Considering its name, it should flush caches associated with device (or I think you can consider there is bug in device driver). I think it should also responsible to flush hardware caches if they exist. Notice e2fsprogs use BLKFLSBUF.
  • fdatasync() (and fsync()) will call blkdev_fsync(). It looks like blkdev_flushbuf() but it only impact range of data written by current process (It use filemap_write_and_wait_range() while BLKFLSBUF use filemap_write_and_wait).
  • Closing a block device calls blkdev_close() that do not flush buffers.
  • sync() will call sync_fs(). It will flush filesystem caches and call fsync() on underlying block device.
  • Command sync /dev/device will call fsync() on /dev/device. However, I think it is useless since dd didn’t touch to any filesystem.

So my conclusions is that call to sync has no (direct) impact on block device. However, passing fdatasync (or fsync) to dd is the only way to guarantee that data are correctly written on media.

If have you run dd but you missed to pass fdatasync, running sync /dev/device is not sufficient. You have have to run dd with fdatasync on whole device. Alternatively, you can call BLKFLSBUF to flush whole device. Unfortunately, there is no standard command for that.

EDIT

You can issue a BLKFLSBUF with blockdev --flushbufs /dev/device.