On 11/27/25 4:22 PM, Samuel Sieb wrote:
On 11/27/25 2:26 PM, home user via users wrote:
While working on my back-up recovery, I wanted to compare 2 large
directories:
* one on the hard drive, and
* one on a blu-ray (/run/media/root/[directory]
It took a mere 37 seconds. The directory is about 6.7 GB (not Gb). I
do not believe that the diff could have done that comparison anywhere
near that fast.
I've seen this before.
What do I need to do to be sure that diff really checks every bit?
Reading the other thread, I think this description is a bit misleading.
You're not diffing off the blu-ray drive itself, you made a copy to the
hard drive. Depending on how much RAM you have, a lot of that copy
could be still in RAM cache. Also, I assume you have an SSD or NVME
drive and depending on exactly what that is, it could be extremely fast.
I have an NVME in this computer and I just did a speed test and got
2GB/s read speed. That might be a reasonable time. Did you get a
reasonable output?
root@road-runner:~/backup_202504_disc# time diff -r .
/run/media/root/F40_20250409
real 0m37.147s
user 0m1.160s
sys 0m3.161s
root@road-runner:~/backup_202504_disc#
I clearly heard the blu-ray drive spin during the compare. By the way,
that desktop is 12+ years old. It has 16 GB of memory, I don't recall
the type.
I recall having a thread a few years about this. Unfortunately,
searching the list gives me a 4 or 5 year gap. I was searching for
"diff" and "cache". Someone (I don't recall who) suggested a command,
run as root, to clear the cache. I have forgotten the command.
--
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