From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andreas Dilger <adilger@sun.com>
Subject: Re: Zero-clearing all zero-clearable bytes.
Date: Sat, 22 Nov 2008 11:44:06 -0600
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Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
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On Nov 22, 2008  12:36 +0900, Tetsuo Handa wrote:
> Compressing whole image files includes compressing deleted/unused bytes within
> a block. This means that non-zero bytes in deleted/unused blocks affect
> compression ratio.
> 
>     static char buffer[4096];
>     memset(buffer, 0, sizeof(buffer));
>     snprintf(buffer, sizeof(buffer) - 1, "%s/XXXXXX", argc > 1 ? argv[1] : "");
>     if ((fd = mkstemp(buffer)) != EOF) {
>         unlink(buffer);
>         memset(buffer, 255, sizeof(buffer));
>         while (write(fd, buffer, sizeof(buffer)) > 0);

Why would you fill the buffer with 0xff instead of 0?
In fact no such program is needed, just "dd if=/dev/zero of=/{fs}/tmp"
and then delete the file.


Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.