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diff --git a/sag-0.6.1-www/Invisible/sag-0.6/node103.html b/sag-0.6.1-www/Invisible/sag-0.6/node103.html new file mode 100644 index 0000000..ffd932b --- /dev/null +++ b/sag-0.6.1-www/Invisible/sag-0.6/node103.html @@ -0,0 +1,101 @@ +<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> +<!--Converted with LaTeX2HTML 96.1-h (September 30, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds --> +<HTML> +<HEAD> +<TITLE>Multilevel backups</TITLE> +<META NAME="description" CONTENT="Multilevel backups"> +<META NAME="keywords" CONTENT="sag"> +<META NAME="resource-type" CONTENT="document"> +<META NAME="distribution" CONTENT="global"> +<LINK REL=STYLESHEET HREF="sag.css"> +</HEAD> +<BODY LANG="EN" > + <A NAME="tex2html1613" HREF="node104.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="./next_motif.gif"></A> <A NAME="tex2html1611" HREF="node96.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="./up_motif.gif"></A> <A NAME="tex2html1605" HREF="node102.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="./previous_motif.gif"></A> <A NAME="tex2html1615" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="./contents_motif.gif"></A> <A NAME="tex2html1616" HREF="node114.html"><IMG WIDTH=43 HEIGHT=24 ALIGN=BOTTOM ALT="index" SRC="./index_motif.gif"></A> <BR> +<B> Next:</B> <A NAME="tex2html1614" HREF="node104.html">What to back up</A> +<B>Up:</B> <A NAME="tex2html1612" HREF="node96.html">Backups</A> +<B> Previous:</B> <A NAME="tex2html1606" HREF="node102.html">Restoring files with tar</A> +<BR> <P> +<H1><A NAME="SECTION001150000000000000000">Multilevel backups</A></H1> +<P> + The simple backup method outlined in the previous section + is often quite adequate for personal use or small sites. + For more heavy duty use, multilevel backups are more + appropriate. +<P> + The simple method has two backup levels: full and incremental + backups. This can be generalized to any number of levels. + A full backup would be level 0, and the different levels of + incremental backups levels 1, 2, 3, ...At each incremental + backup level you back up everything that has changed since + the previous backup at the same or a previous level. +<P> + The purpose for doing this is that it allows a longer + <b>backup history</b> cheaply. In the example in the previous + section, the backup history went back to the previous full backup. + This could be extended by having more tapes, but only a week + per new tape, which might be too expensive. A longer backup + history is useful, since deleted or corrupted files are often + not noticed for a long time. Even a version of a file that is + not very up to date is better than no file at all. +<P> + With multiple levels the backup history can be extended + more cheaply. For example, if we buy ten tapes, we could use + tapes 1 and 2 for monthly backups (first Friday each month), + tapes 3 to 6 for weekly backups (other Fridays; note that + there can be five Fridays in one month, so we need four more + tapes), and tapes 7 to 10 for daily backups (Monday to Thursday). + With only four more tapes, we've been able to extend the backup + history from two weeks (after all daily tapes have been + used) to two months. It is true that we can't restore every + version of each file during those two months, but what we can + restore is often good enough. +<P> + Figure <A HREF="node103.html#figbackuphistorytimeline">10.1</A> shows which backup level + is used each day, and which backups can be restored from + at the end of the month. +<P> + <P><A NAME="2983"> </A><A NAME="figbackuphistorytimeline"> </A><IMG WIDTH=592 HEIGHT=96 ALIGN=BOTTOM ALT="figure2979" SRC="img14.gif"><BR> +<STRONG>Figure 10.1:</STRONG> A sample multilevel backup schedule.<BR> +<P> +<P> + Backup levels can also be used to keep filesystem restoration + time to a minimum. If you have many incremental backups with + monotonously growing level numbers, you need to restore all + of them if you need to rebuild the whole filesystem. Instead + you can use level numbers that aren't monotonous, and keep down the + number of backups to restore. +<P> + To minimize the number of tapes needed to restore, you could + use a smaller level for each incremental tape. However, then + the time to make the backups increases (each backup copies + everything since the previous full backup). A better scheme + is suggested by the <tt>dump</tt><A NAME="3081"> </A> manual page and described by the + table <A HREF="node103.html#tabefficientbackuplevels">10.2</A>. Use the following + succession of backup levels: 3, 2, 5, 4, 7, 6, 9, 8, 9...This + keeps both the backup and restore times low. The most you + have to backup is two day's worth of work. The number of tapes + for a restore depends on how long you keep between full backups, + but it is less than in the simple schemes. +<P> +<P><A NAME="2989"> </A><A NAME="tabefficientbackuplevels"> </A><IMG WIDTH=360 HEIGHT=302 ALIGN=BOTTOM ALT="figure2988" SRC="img15.gif"><BR> +<STRONG>Figure 10.2:</STRONG> Efficient backup scheme using many backup levels<BR> +<P> +<P> + A fancy scheme can reduce the amount of labor needed, but it + does mean there are more things to keep track of. You must + decide if it is worth it. +<P> + <tt>dump</tt><A NAME="3083"> </A> has built-in support for backup levels. For + <tt>tar</tt><A NAME="3085"> </A> and <tt>cpio</tt><A NAME="3087"> </A> it must be implemented with shell + scripts. +<P> +<HR><A NAME="tex2html1613" HREF="node104.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="./next_motif.gif"></A> <A NAME="tex2html1611" HREF="node96.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="./up_motif.gif"></A> <A NAME="tex2html1605" HREF="node102.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="./previous_motif.gif"></A> <A NAME="tex2html1615" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="./contents_motif.gif"></A> <A NAME="tex2html1616" HREF="node114.html"><IMG WIDTH=43 HEIGHT=24 ALIGN=BOTTOM ALT="index" SRC="./index_motif.gif"></A> <BR> +<B> Next:</B> <A NAME="tex2html1614" HREF="node104.html">What to back up</A> +<B>Up:</B> <A NAME="tex2html1612" HREF="node96.html">Backups</A> +<B> Previous:</B> <A NAME="tex2html1606" HREF="node102.html">Restoring files with tar</A> +<P><ADDRESS> +<I>Lars Wirzenius <BR> +Sat Nov 15 02:32:11 EET 1997</I> +</ADDRESS> +</BODY> +</HTML> |