Is it necessary to synchronize cached writes to persistent storage with the sync command when wiping a USB, or any device, with /dev/urandom?

This is a common practice, i.e. dd if=/dev/urandom of=/dev/sdx && sync.

But is that really necessary? Is it okay to omit the sync command?

Please note this question is not referring to the conv=sync option of the dd utility, it refers to sync as a stand alone command that can also be used apart from dd. Here is a reference to sync, and here is a reference to conv=sync. Both are from https://www.kernel.org/doc/man-pages/.

Thank you

  • 1
    You can omit sync of course. Keep in mind if there is caching involved, that not all the writes are not necessarily on the medium. sync ensures that before countinuing. – stoney Jun 13 '18 at 12:12
  • Are you sure that you need /dev/urandom? I think it is good enough to wipe a USB drive or memory card with zeros from /dev/zero which is faster. And there are other tools to wipe a HDD or SSD, tools that are more efficient than dd. Ask for details, if you wish. – sudodus Jun 13 '18 at 12:21

sync isn’t used specifically because /dev/urandom is involved; it’s used to ensure that all pending writes have been sent to the device. This is especially important when using removable devices: running sync, and waiting for it to finish, ensures that the device can be safely removed.

When wiping a device, if you omit the sync and disconnect the device, you might not have finished wiping the device...


The sync() call is related to filesystems - not to devices.

If you write to a block device, sync() may cause a buffer flush, but if you use the raw device, as recommended in this case, sync() does not make sense.

WARNING: On Linux, it may be hard to find out the related raw device. Check the Linux documentation on how to get the related raw device.

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