On 7/7/23 6:21 PM, home user wrote:
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.
Results:
-----
bash.16[~]: time diff -r .incbak_20230707/ /run/media/weilian.eng//USB\ DISK/.incbak_20230707/
real 0m0.222s
user 0m0.050s
sys 0m0.172s
bash.17[~]: su -
Password:
-bash.1[~]: sync; echo 3 > /proc/sys/vm/drop_caches
-bash.2[~]: exit
logout
bash.18[~]: time diff -r .incbak_20230707/ /run/media/weilian.eng//USB\ DISK/.incbak_20230707/
real 0m27.385s
user 0m0.156s
sys 0m1.324s
bash.19[~]:
-----
This confirms Ron's theory.
Thank-you, Ron!
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