sync dirs, mark in by content, compare very slow in same hdd

[jgustavo]

hello people (sorry my english)

syncronize directories, mark in compare by content, compare very slow in same hdd

I use this function very much.
after recording dvd, for example, or of copying dozens of GB among different partitions, hdd and/or LAN.
it always works very quick, unless it compares among 2 folders in the same physical hdd.
be in the same partition or in different partition.

if I copy on the net lan or on different hdd, it copies at 40MB/s min, and it compares at 40MB/s min (aprox)
but if I copy on the same hdd, it copies at 20MB/s min, but it compares at 1MB/s max
that which is unbearable when you have to compare, let us say 20GB

what I make is to copy on the net, then to compare, and then copy from the net again, in the other folder of the same hdd (generally different partitions) and compare again, because it is much quicker.

where I am wandering in the configuration?
or is an error?

Thank you from already

[sqa_wizard]

First of all you may know, that comparing files located at the same HD is very slow in general.

Comparing two files located at the same HD means: the same HD controller has to read two files.
This cannot be done at the same time, but just in sequence :
- find and position the head at the first file
- read a part of the first file
- find and position the head on the second file
- read a part of the second file
- compare both parts
- find and position the head at the next part of the first file
... and so on.

Comparing a local file with a network located file means: both files can be read simultaneously, because each HD controller can read his own one file only.
- find and position the head at the first file
- read a part of the first file
- while both files are read, they are compared.

The data is read in one uninterupted stream. No need the reposition the HD reading head.

Due to alternating reads at local HD compare, the speed is at least 3 times slower.

Another slow down is often related to a local virus scanner, which scans the data while it is read.
You may disable yours temporarily, just to see if it there is any difference...

[jgustavo]

thank you to respond.

what you say is correct. but...

when I copy on the same hdd, that is more or less the same thing that to compare, I obtain near 50% of the speed that copying to another hdd...
on the other hand comparing on the same hdd obtains 1-3% of the speed that comparing to another hdd.

that is to say:
to copy on the same hdd: 20MB/s
to compare on the same hdd: 1MB/s maximum
and to copy on the same hdd should be slower than to compare on the same hdd.
I suppose that there is some problem in the cache handling or something similar.

thank you again

[ghisler(Author)]

I tried using a larger buffer in the compare tool for exactly this reason, but got so much negative feedback that I had to change it back to a small buffer.

[jgustavo]

thank you to respond.
was there some version that used the biggest buffer to compare?
or some form of increasing that buffer?
or something that I can make?
I only need to know if the files have the same content. I don't need to compare lines neither any other thing.

thank you again

[WatchUer]

I also have encountered this problem some times ago, the speed of comparing large files in the same HDD is unbearable. During comparing, the Seagate 7200.10 making a crazy noise, I think it really hurt the HDD, and I'll never do that again.

Thank god I have two 7200.10, when I want to compar large files in the same HDD, I'll copy one to the other HDD and then compare.