linux-kernel - Re: Zero-clearing all zero-clearable bytes.

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Message-id: <20081122174406.GQ3186@webber.adilger.int>
Date:	Sat, 22 Nov 2008 11:44:06 -0600
From:	Andreas Dilger <adilger@....com>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Zero-clearing all zero-clearable bytes.

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.

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