Date: Wed, 28 Mar 2007 18:59:34 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: Ivan Voras <ivoras@fer.hr> Cc: freebsd-current@freebsd.org Subject: Re: NFS write() calls lead to read() calls? Message-ID: <20070328185815.I1185@fledge.watson.org> In-Reply-To: <eue3ku$mq2$1@sea.gmane.org> References: <7ad7ddd90703280238r5dd3f30ftc1641926ecdf44a8@mail.gmail.com> <eudfat$g2m$1@sea.gmane.org> <7ad7ddd90703280611p5c0ca4e1y600315551391a813@mail.gmail.com> <eue3ku$mq2$1@sea.gmane.org>
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On Wed, 28 Mar 2007, Ivan Voras wrote:
> Ulrich Spoerlein wrote:
>
>> What kind of C program or script did you have in mind? My C-foo is very
>> weak ...
>
> If you have python installed, this is a simple way:
Also good is dd between the file system and /dev/null. Something worth
remembering is that some tools (cp(1) in particular) use memory-mapped I/O,
which may behave differently than raw I/O operations.
Robert N M Watson
Computer Laboratory
University of Cambridge
>
> ----
> print "writing"
> f = file("a_file", "w")
> for x in xrange(1024): # write 1024 MB
> f.write(' '*1024*1024)
> f.close()
>
> raw_input("press enter to rewrite")
>
> print "rewriting"
> f = file("a_file", "r+")
> for x in xrange(1024): # write 1024 MB again
> f.write(' '*1024*1024)
> f.close()
> ----
>
> save it to a file, run the file with python interpreter. If you don't observe
> the weird read-before-write effect, then it's not a problem in FreeBSD / NFS.
>
>
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