On 7/7/2023 12:42 PM, home user wrote:
When I try to verify a back-up, I use "diff -r". The
directory trees
being compared contain about 870 files (mostly binary, like PNG, JPG,
and so on), and take up about 707 megabytes. The trees being compared
are on the hard drive and on a USB-3 stick. When I run the "diff -r"
command, it seems to finish too quickly - it seems like less than a
half of a second. I saw similar results a few weeks ago comparing
about 30 gigabyte trees on the hard drive vs. on a USB-3.1 stick; the
results were practically instantaneous. Is diff actually checking
every bit (or byte), or is it using some "short cut"?
Was this immediately after your backup/copy completed? You may be
comparing against the in-memory disk caches.
You may (simply) flush the in-memory disk caches to force reads from
the external disk with (run as root or sudo):
sync ; echo 3 > /proc/sys/vm/drop_caches
then try your diff again.