198 lines
3.7 KiB
HTML
198 lines
3.7 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>Compressed backups</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Linux System Administrators Guide"
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HREF="index.html"><LINK
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TITLE="Backups"
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HREF="backups-intro.html"><LINK
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TITLE="What to back up"
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HREF="what-to-backup.html"><LINK
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TITLE="Task Automation --To Be Added"
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>Linux System Administrators Guide: </TH
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>Prev</A
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 12. Backups</TD
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><TD
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WIDTH="10%"
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VALIGN="bottom"
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="COMPRESSED-BACKUPS"
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></A
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>12.7. Compressed backups</H1
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><P
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> Backups take a lot of space, which can cost quite
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a lot of money. To reduce the space needed, the backups
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can be compressed. There are several ways of doing this.
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Some programs have support for for compression built in; for
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example, the <TT
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CLASS="OPTION"
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>--gzip</TT
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> (<TT
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CLASS="OPTION"
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>-z</TT
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>)
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option for GNU <B
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CLASS="COMMAND"
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>tar</B
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> pipes the whole backup
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through the <B
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CLASS="COMMAND"
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>gzip</B
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> compression program, before
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writing it to the backup medium. </P
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><P
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> Unfortunately, compressed backups can cause trouble.
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Due to the nature of how compression works, if a single bit is
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wrong, all the rest of the compressed data will be unusable.
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Some backup programs have some built in error correction, but no
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method can handle a large number of errors. This means that if
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the backup is compressed the way GNU <B
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CLASS="COMMAND"
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>tar</B
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> does
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it, with the whole output compressed as a unit, a single error
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makes all the rest of the backup lost. Backups must be reliable,
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and this method of compression is not a good idea. </P
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><P
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> An alternative way is to compress each file separately.
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This still means that the one file is lost, but all other files
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are unharmed. The lost file would have been corrupted anyway,
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so this situation is not much worse than not using compression
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at all. The <B
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CLASS="COMMAND"
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>afio</B
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> program (a variant of
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<B
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CLASS="COMMAND"
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>cpio</B
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>) can do this. </P
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><P
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> Compression takes some time, which may make the backup program
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unable to write data fast enough for a tape drive.
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This can be avoided by buffering the output (either internally, if
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the backup program if smart enough, or by using another program),
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but even that might not work well enough. This should only be
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a problem on slow computers. </P
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></DIV
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VALIGN="top"
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><A
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HREF="what-to-backup.html"
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ACCESSKEY="P"
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>Prev</A
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>What to back up</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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>Up</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>Task Automation --To Be Added</TD
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></TR
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></TABLE
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></DIV
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></HTML
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> |