On 11/27/25 5:50 PM, Samuel Sieb wrote:
On 11/27/25 4:43 PM, home user via users wrote:
On 11/27/25 5:30 PM, Samuel Sieb wrote:
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!
For writing, sure, that's a reasonable test for verification. It would
have been easier to just unmount and mount it again. That would only
clear the cache for the USB drive instead of everything.
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.
You're *reading* from the blu-ray. Unless you suspect RAM issues, the
cache is good. There's no benefit to forcing a re-read from the blu-ray.
Thank-you, Samuel.
Over the past few days, I've had multiple occurrences of inserting a
disc, and then
* the system knows I did that, but cannot get anything off it; and
* the system sees the first 2K only, and seems to think it's all zeros;
Only 3x did the disc get auto mounted and I could access what was on it.
A failed manual mount advises checking dmesg. I've lost that output by
now, but I do recall something about a bad superblock. Regardless,
something (I don't know what or where) goes wrong between the disc and
something/somewhere in the system proper. If the disc were the problem,
what I did could not have succeeded once, let alone 3x.
I hope you understand my being somewhat uneasy with the read, and
wanting a 99+% confident check. On the other hand, I'm now realizing
that a second disc read is just as vulnerable, given all the failed
attempts that I did experience. If a read can be bad, so can a re-read.
I don't suspect RAM issues; I'm uneasy with whatever is between the
disc and RAM or hard drive. I am almost certain that something
somewhere has gotten "glitchy". So a diff that actually re-reads the
disc would be a poor guarantee.
By the way, is the cache in memory or on the hard drive? I assume it's
a different cache from what's on the CPU.
--
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