On 11/27/25 5:30 PM, Samuel Sieb wrote:
On 11/27/25 4:25 PM, home user via users wrote:
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.
So you're trying to compare the blu-ray to the copy you made from it? I
would say that the diff is working and everything is the same. Is there
some reason you're expecting it to be different?
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.
Why would you want to clear the cache?
To be sure that the diff really is comparing the actual source of the cp
and the actual destination of the cp. In the past, it made a huge
difference.
When I backed up 30+ GB of data to a USB stick 7 months ago a year ago,
etc., I wanted to know that what was on the stick matched what was on
the hard drive. Without clearing the cache, the diff took about half a
minute. After clearing the cache, the diff took about 6 minutes. It
matters!
Given the problems I've been having with devices and/or removable media,
I think it's wise to do the diff, and to make sure what's on the hard
drive is being compared to what's on the blu-ray, not in cache.
--
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