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Date: Thu, 09 Nov 2006 00:56:53 -0800
From: Linda Walsh <cygwin AT tlinx DOT org>
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Subject: cyg mem limit or network failure? Other? dd(dsk->net).blksz>=64M:  Fail
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I was running some network bandwidth tests using "dd" (WinXP2/Cygwin)

I was timing copies of a 300MB file from local disk to a remote
server.  The local computer has enough memory to hold the file
in the memory cache once it is loaded.

I ran through increasing power-of-two block sizes from
512 bytes up to 512MB, where it, theoretically could read
and write the file in one read and write for the whole file.
Essentially (say "s:" is a network drive):

for (i = 512;  i<=512MB;  i = i*2) do {
    dd  bs=$i  if=/tmp/input  of=/s/Video/output.dat
}

Unfortunately, the test /fails/ at or above 64MB (actually at or
above 65,005KB).  The ERROR message is:
 
 dd: writing '/s/Video/output.dat': Resource temporarily unavailable


If output is redirected to a local file (i.e. "of=/tmp/output.dat"),
copying works fine up to my test limit of 5122MB.

Has anyone experience network copy problems on large files? 

Doesn't make much sense to me, but maybe someone here, with more
internals knowledge than I, might have an idea?

Space on the target doesn't seem to be an issue, as the file
is copy-able using smaller (<=63MB) block sizes.

If any output would be useful, let me know.  I'm not presuming,
at this point, it is specific to my machine, but if it is,
I can send more debug info.

Thanks,
-linda



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