Now that the train of hail storms have shifted south, I can attention to this.
On 7/7/23 1:26 PM, Ron Flory wrote:
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.
What? Is "diff" cheating?!
Test #1
I did a back-up yesterday morning.
I've shut down at least 3 times since then. So surely the cache has been
cleared.
So here is a "diff -r":
-----
bash.1[~]: time diff -r .incbak_20230706/ /run/media/weilian.eng//USB\
DISK/.incbak_20230706/
real 0m19.238s
user 0m0.138s
sys 0m1.177s
bash.2[~]:
-----
19+ seconds. So you may be correct.
Next I'll try a new back-up, run the diff, clear the cache, and do another
diff. I'll report back in several minutes.
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