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Full Discussion: 'cp' and slack space
Special Forums Hardware Filesystems, Disks and Memory 'cp' and slack space Post 302117958 by sysgate on Thursday 17th of May 2007 10:48:20 AM
Old 05-17-2007
This is a good question, actually, each filesystem has its own rules for slack space. Let's consider UFS or ext3, as they act the same in this meaning :
Quote:
By default, sparse SOURCE files are detected by a crude heuristic and the corresponding DEST file is made sparse as well. That is the behavior selected by --sparse=auto. Specify --sparse=always to create a sparse DEST file whenever the SOURCE file contains a long enough sequence of zero bytes. Use --sparse=never to inhibit creation of sparse files.
where sparse files are a type of computer files that attempt to use file system space more efficiently; so, "cp" by default doesn't use slack space, means , it's optimized for work.
 

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E2IMAGE(8)						      System Manager's Manual							E2IMAGE(8)

NAME
e2image - Save critical ext2 filesystem data to a file SYNOPSIS
e2image [ -r ] device image-file DESCRIPTION
The e2image program will save critical filesystem data on the ext2 filesystem located on device to a file specified by image-file. The image file may be examined by dumpe2fs and debugfs, by using the -i option to those programs. This can be used by an expert in assisting the recovery of catastrophically corrupted filesystems. In the future, e2fsck will be enhanced to be able to use the image file to help recover a badly damaged filesystem. If image-file is -, then the output of e2image will be sent to standard output. The -r option will create a raw image file instead of a normal image file. A raw image file differs from a normal image file in two ways. First, the filesystem metadata is placed in the proper position so that e2fsck, dumpe2fs, debugfs, etc. can be run directly on the raw image file. In order to minimize the amount of disk space consumed by a raw image file, the file is created as a sparse file. (Beware of copying or compressing/decompressing this file with utilities that don't understand how to create sparse files; the file will become as large as the filesystem itself!) Secondly, the raw image file also includes indirect blocks and data blocks, which the current image file does not have, although this may change in the future. It is a very good idea to periodically (at boot time and every week or so) to create image files for all of filesystems on a system, as well as saving the partition layout (which can be generated using the using fdisk -l command). Ideally the image file should be stored on some filesystem other that the filesystem whose data it contains, to ensure that its data is accessible in the case where the filesystem has been badly damaged. To save disk space, e2image creates the image file as a sparse file. Hence, if the image file needs to be copied to another location, it should either be compressed first or copied using the --sparse=always option to GNU version of cp. The size of an ext2 image file depends primarily on the size of the filesystems and how many inodes are in use. For a typical 10 gigabyte filesystem, with 200,000 inodes in use out of 1.2 million inodes, the image file be approximately 35 megabytes; a 4 gigabyte filesystem with 15,000 inodes in use out of 550,000 inodes will result in a 3 megabyte image file. Image files tend to be quite compressible; an image file taking up 32 megabytes of space on disk will generally compress down to 3 or 4 megabytes. AUTHOR
e2image was written by Theodore Ts'o (tytso@mit.edu). AVAILABILITY
e2image is part of the e2fsprogs package and is available from anonymous http://e2fsprogs.sourceforge.net. SEE ALSO
dumpe2fs(8), debugfs(8) E2fsprogs version 1.32 November 2002 E2IMAGE(8)
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