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

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Date:	Sat, 22 Nov 2008 12:36:56 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	linux-fsdevel@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org
Subject: Zero-clearing all zero-clearable bytes.

Hello.

I often create a Live CD.
When creating a Live CD, loopback mounted ext3 image files are used.
These image files are then compressed so that the ISO image will fit within
a CD-R's capacity (i.e. 700MB).

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.

I'm using the below program to improve compression ratio.

----------
#define _FILE_OFFSET_BITS 64
#define _LARGEFILE_SOURCE
#define _LARGEFILE64_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
    int fd;
    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);
        sync();
        close(fd);
    }
    return 0;
}
----------

But this program cannot clear some bytes within a block for file data (e.g.
offset from 1 to 511 of a data block used by a file with only 1 byte data)
and blocks for directory entries.

Any suggestions?

Regards.
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