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diff --git a/sag-0.6.1-www/sag-0.6.1.html/x1650.html b/sag-0.6.1-www/sag-0.6.1.html/x1650.html new file mode 100644 index 0000000..341f578 --- /dev/null +++ b/sag-0.6.1-www/sag-0.6.1.html/x1650.html @@ -0,0 +1,321 @@ +<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN"> +<HTML +><HEAD +><TITLE +>More about shutdowns</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="Boots And Shutdowns" +HREF="c1582.html"><LINK +REL="PREVIOUS" +TITLE="The boot process in closer look" +HREF="x1602.html"><LINK +REL="NEXT" +TITLE="Rebooting" +HREF="x1684.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="x1602.html" +>Prev</A +></TD +><TD +WIDTH="80%" +ALIGN="center" +VALIGN="bottom" +>Chapter 6. Boots And Shutdowns</TD +><TD +WIDTH="10%" +ALIGN="right" +VALIGN="bottom" +><A +HREF="x1684.html" +>Next</A +></TD +></TR +></TABLE +><HR +ALIGN="LEFT" +WIDTH="100%"></DIV +><DIV +CLASS="SECT1" +><H1 +CLASS="SECT1" +><A +NAME="AEN1650" +>More about shutdowns</A +></H1 +><P +>It is important to follow the correct procedures when you shut + down a Linux system. If you fail do so, your filesystems probably + will become trashed and the files probably will become scrambled. + This is because Linux has a disk cache that won't write things + to disk at once, but only at intervals. This greatly improves + performance but also means that if you just turn off the power + at a whim the cache may hold a lot of data and that what is on + the disk may not be a fully working filesystem (because only + some things have been written to the disk).</P +><P +>Another reason against just flipping the power switch is that + in a multi-tasking system there can be lots of things going on + in the background, and shutting the power can be quite + disastrous. By using the proper shutdown sequence, you ensure + that all background processes can save their data.</P +><P +>The command for properly shutting down a Linux system + is <B +CLASS="COMMAND" +>shutdown</B +>. It is usually used in one of + two ways.</P +><P +>If you are running a system where you are the only user, + the usual way of using <B +CLASS="COMMAND" +>shutdown</B +> is to quit + all running programs, log out on all virtual consoles, log + in as root on one of them (or stay logged in as root if you + already are, but you should change to root's home directory or + the root directory, to avoid problems with unmounting), then + give the command <B +CLASS="COMMAND" +>shutdown -h now</B +> (substitute + <TT +CLASS="LITERAL" +>now</TT +> with a plus sign and a number in minutes + if you want a delay, though you usually don't on a single user + system).</P +><P +>Alternatively, if your system has many users, use the command + <B +CLASS="COMMAND" +>shutdown -h +time message</B +>, where <TT +CLASS="LITERAL" +>time</TT +> + is the + time in minutes until the system is halted, and <TT +CLASS="LITERAL" +>message</TT +> + is a short explanation of why the system is shutting down. + +<PRE +CLASS="SCREEN" +><TT +CLASS="PROMPT" +>#</TT +> <TT +CLASS="USERINPUT" +><B +>shutdown -h +10 'We will install a new disk. System should +> be back on-line in three hours.'</B +></TT +> +<TT +CLASS="PROMPT" +>#</TT +></PRE +> + + This will warn everybody that the system will shut down in + ten minutes, and that they'd better get lost or lose data. + The warning is printed to every terminal on which someone is + logged in, including all <B +CLASS="COMMAND" +>xterm</B +>s: + +<PRE +CLASS="SCREEN" +><TT +CLASS="COMPUTEROUTPUT" +>Broadcast message from root (ttyp0) Wed Aug 2 01:03:25 1995... + +We will install a new disk. System should +be back on-line in three hours. +The system is going DOWN for system halt in 10 minutes !!</TT +></PRE +> + + The warning is automatically repeated a few times before the boot, + with shorter and shorter intervals as the time runs out.</P +><P +>When the real shutting down starts after any delays, all + filesystems (except the root one) are unmounted, user processes + (if anybody is still logged in) are killed, daemons are shut down, + all filesystem are unmounted, and generally everything settles + down. When that is done, <B +CLASS="COMMAND" +>init</B +> prints out a + message that you can power down the machine. Then, and only then, + should you move your fingers towards the power switch.</P +><P +>Sometimes, although rarely on any good system, it is + impossible to shut down properly. For instance, if the kernel + panics and crashes and burns and generally misbehaves, it might + be completely impossible to give any new commands, hence shutting + down properly is somewhat difficult, and just about everything + you can do is hope that nothing has been too severely damaged + and turn off the power. If the troubles are a bit less severe + (say, somebody hit your keyboard with an axe), and the kernel + and the <B +CLASS="COMMAND" +>update</B +> program still run normally, + it is probably a good idea to wait a couple of minutes to give + <B +CLASS="COMMAND" +>update</B +> a chance to flush the buffer cache, + and only cut the power after that.</P +><P +>Some people like to shut down using the command + <B +CLASS="COMMAND" +>sync</B +> + + <A +NAME="AEN1678" +HREF="#FTN.AEN1678" +>[1]</A +> + + three times, waiting for the disk I/O to stop, then turn off + the power. If there are no running programs, this is about + equivalent to using <B +CLASS="COMMAND" +>shutdown</B +>. However, it + does not unmount any filesystems and this can lead to problems + with the ext2fs ``clean filesystem'' flag. The triple-sync + method is <I +CLASS="EMPHASIS" +>not recommended</I +>.</P +><P +>(In case you're wondering: the reason for three syncs is + that in the early days of UNIX, when the commands were + typed separately, that usually gave sufficient time for most + disk I/O to be finished.) + </P +></DIV +><H3 +>Notes</H3 +><TABLE +BORDER="0" +CLASS="FOOTNOTES" +WIDTH="100%" +><TR +><TD +ALIGN="LEFT" +VALIGN="TOP" +WIDTH="5%" +><A +NAME="FTN.AEN1678" +HREF="x1650.html#AEN1678" +>[1]</A +></TD +><TD +ALIGN="LEFT" +VALIGN="TOP" +WIDTH="95%" +><P +><B +CLASS="COMMAND" +>sync</B +> flushes the + buffer cache. </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="x1602.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="x1684.html" +>Next</A +></TD +></TR +><TR +><TD +WIDTH="33%" +ALIGN="left" +VALIGN="top" +>The boot process in closer look</TD +><TD +WIDTH="34%" +ALIGN="center" +VALIGN="top" +><A +HREF="c1582.html" +>Up</A +></TD +><TD +WIDTH="33%" +ALIGN="right" +VALIGN="top" +>Rebooting</TD +></TR +></TABLE +></DIV +></BODY +></HTML +>
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