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Date: Wed, 25 Jun 2014 20:12:37 +0700
From: Eugene Grosbein <eugen@grosbein.net>
To: Mike Tancsa <mike@sentex.net>
Subject: Re: nfs slowdown (RELENG_8)
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On Tue, Jun 24, 2014 at 04:14:05PM -0400, Mike Tancsa wrote:

> But if I do
> # cat /dev/zero > /nfsbackup/test
> 
> Its down to 25Mb/s
> 
> Now, if this were consistent across all my boxes, I would not be too 
> interested.
> 
> Why is cat with a redirect so much slower, and why slower just on some 
> boxes and not others ?!?!

cat(1) tries to determine optimal I/O block size by evaluating
st_blksize of its stdout. Try to run:

ktrace  cat /dev/zero > /nfsbackup/test

And then use kdump and see what block size does cat(1) use
while writing to stdout?

Eugene Grosbein