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diff --git a/sag-0.6.1-www/sag-0.6.1.html/x2417.html b/sag-0.6.1-www/sag-0.6.1.html/x2417.html new file mode 100644 index 0000000..5d01dd0 --- /dev/null +++ b/sag-0.6.1-www/sag-0.6.1.html/x2417.html @@ -0,0 +1,220 @@ +<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN"> +<HTML +><HEAD +><TITLE +>Compressed backups</TITLE +><META +NAME="GENERATOR" +CONTENT="Modular DocBook HTML Stylesheet"><LINK +REL="HOME" +TITLE="The Linux System Administrators' Guide" +HREF="book1.html"><LINK +REL="UP" +TITLE="Backups" +HREF="c2187.html"><LINK +REL="PREVIOUS" +TITLE="What to back up" +HREF="x2405.html"><LINK +REL="NEXT" +TITLE="Keeping Time" +HREF="c2432.html"></HEAD +><BODY +BGCOLOR="#FFFFFF" +TEXT="#000000" +><DIV +CLASS="NAVHEADER" +><TABLE +WIDTH="100%" +BORDER="0" +CELLPADDING="0" +CELLSPACING="0" +><TR +><TH +COLSPAN="3" +ALIGN="center" +>The Linux System Administrators' Guide</TH +></TR +><TR +><TD +WIDTH="10%" +ALIGN="left" +VALIGN="bottom" +><A +HREF="x2405.html" +>Prev</A +></TD +><TD +WIDTH="80%" +ALIGN="center" +VALIGN="bottom" +>Chapter 10. Backups</TD +><TD +WIDTH="10%" +ALIGN="right" +VALIGN="bottom" +><A +HREF="c2432.html" +>Next</A +></TD +></TR +></TABLE +><HR +ALIGN="LEFT" +WIDTH="100%"></DIV +><DIV +CLASS="SECT1" +><H1 +CLASS="SECT1" +><A +NAME="AEN2417" +>Compressed backups</A +></H1 +><P +> Backups take a lot of space, which can cost quite + a lot of money. To reduce the space needed, the backups + can be compressed. There are several ways of doing this. + Some programs have support for for compression built in; for + example, the <SPAN +CLASS="OPTION" +>--gzip</SPAN +> (<SPAN +CLASS="OPTION" +>-z</SPAN +>) + option for GNU <B +CLASS="COMMAND" +>tar</B +> pipes the whole backup + through the <B +CLASS="COMMAND" +>gzip</B +> compression program, before + writing it to the backup medium. </P +><P +> Unfortunately, compressed backups can cause trouble. + Due to the nature of how compression works, if a single bit is + wrong, all the rest of the compressed data will be unusable. + Some backup programs have some built in error correction, but no + method can handle a large number of errors. This means that if + the backup is compressed the way GNU <B +CLASS="COMMAND" +>tar</B +> does + it, with the whole output compressed as a unit, a single error + makes all the rest of the backup lost. Backups must be reliable, + and this method of compression is not a good idea. </P +><P +> An alternative way is to compress each file separately. + This still means that the one file is lost, but all other files + are unharmed. The lost file would have been corrupted anyway, + so this situation is not much worse than not using compression + at all. The <B +CLASS="COMMAND" +>afio</B +> program (a variant of + <B +CLASS="COMMAND" +>cpio</B +>) can do this. </P +><P +> Compression takes some time, which may make the backup program + unable to write data fast enough for a tape drive. + + <A +NAME="AEN2430" +HREF="#FTN.AEN2430" +>[1]</A +> + + This can be avoided by buffering the output (either internally, if + the backup program if smart enough, or by using another program), + but even that might not work well enough. This should only be + a problem on slow computers. </P +></DIV +><H3 +>Notes</H3 +><TABLE +BORDER="0" +CLASS="FOOTNOTES" +WIDTH="100%" +><TR +><TD +ALIGN="LEFT" +VALIGN="TOP" +WIDTH="5%" +><A +NAME="FTN.AEN2430" +HREF="x2417.html#AEN2430" +>[1]</A +></TD +><TD +ALIGN="LEFT" +VALIGN="TOP" +WIDTH="95%" +><P +>If a tape drive doesn't data fast enough, + it has to stop; this makes backups even slower, and can + be bad for the tape and the drive.</P +></TD +></TR +></TABLE +><DIV +CLASS="NAVFOOTER" +><HR +ALIGN="LEFT" +WIDTH="100%"><TABLE +WIDTH="100%" +BORDER="0" +CELLPADDING="0" +CELLSPACING="0" +><TR +><TD +WIDTH="33%" +ALIGN="left" +VALIGN="top" +><A +HREF="x2405.html" +>Prev</A +></TD +><TD +WIDTH="34%" +ALIGN="center" +VALIGN="top" +><A +HREF="book1.html" +>Home</A +></TD +><TD +WIDTH="33%" +ALIGN="right" +VALIGN="top" +><A +HREF="c2432.html" +>Next</A +></TD +></TR +><TR +><TD +WIDTH="33%" +ALIGN="left" +VALIGN="top" +>What to back up</TD +><TD +WIDTH="34%" +ALIGN="center" +VALIGN="top" +><A +HREF="c2187.html" +>Up</A +></TD +><TD +WIDTH="33%" +ALIGN="right" +VALIGN="top" +>Keeping Time</TD +></TR +></TABLE +></DIV +></BODY +></HTML +>
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