336 lines
6.1 KiB
HTML
336 lines
6.1 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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>Using a swap space</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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REL="UP"
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TITLE="Memory Management"
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HREF="memory-management.html"><LINK
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REL="PREVIOUS"
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TITLE="Creating a swap space"
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HREF="swap-space.html"><LINK
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REL="NEXT"
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TITLE="Sharing swap spaces with other operating systems"
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HREF="sharing-swap.html"></HEAD
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><BODY
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CLASS="SECT1"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Linux System Administrators Guide: </TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="swap-space.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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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 6. Memory Management</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="sharing-swap.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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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="USING-SWAP"
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></A
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>6.3. Using a swap space</H1
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><P
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>An initialized swap space is taken into use with
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<B
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CLASS="COMMAND"
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>swapon</B
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>. This command tells the kernel that
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the swap space can be used. The path to the swap space is given
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as the argument, so to start swapping on a temporary swap file
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one might use the following command.
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<TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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><TT
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CLASS="PROMPT"
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>$</TT
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> <TT
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CLASS="USERINPUT"
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><B
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>swapon /extra-swap</B
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></TT
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>
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<TT
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CLASS="PROMPT"
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>$</TT
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></PRE
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></FONT
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></TD
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></TR
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></TABLE
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>
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Swap spaces can be used automatically by listing them in
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the <TT
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CLASS="FILENAME"
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>/etc/fstab</TT
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> file.
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<TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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>/dev/hda8 none swap sw 0 0
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/swapfile none swap sw 0 0</PRE
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></FONT
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></TD
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></TR
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></TABLE
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>
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The startup scripts will run the command <B
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CLASS="COMMAND"
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>swapon
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-a</B
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>, which will start swapping on all the swap
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spaces listed in <B
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CLASS="COMMAND"
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>/etc/fstab</B
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>. Therefore,
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the <B
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CLASS="COMMAND"
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>swapon</B
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> command is usually used only when
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extra swap is needed.</P
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><P
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>You can monitor the use of swap spaces with
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<B
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CLASS="COMMAND"
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>free</B
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>. It will tell the total amount of swap
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space used.
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<TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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><TT
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CLASS="PROMPT"
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>$</TT
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> <TT
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CLASS="USERINPUT"
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><B
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>free</B
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></TT
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>
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<TT
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CLASS="COMPUTEROUTPUT"
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> total used free shared
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buffers
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Mem: 15152 14896 256 12404 2528
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-/+ buffers: 12368 2784
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Swap: 32452 6684 25768</TT
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>
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<TT
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CLASS="PROMPT"
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>$</TT
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></PRE
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></FONT
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></TD
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></TR
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></TABLE
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>
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The first line of output (<TT
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CLASS="LITERAL"
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>Mem:</TT
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>) shows the
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physical memory. The total column does not show the physical
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memory used by the kernel, which is usually about a megabyte.
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The used column shows the amount of memory used (the second
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line does not count buffers). The free column shows completely
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unused memory. The shared column shows the amount of memory
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shared by several processes; the more, the merrier. The buffers
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column shows the current size of the disk buffer cache.</P
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><P
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>That last line (<TT
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CLASS="LITERAL"
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>Swap:</TT
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>) shows similar
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information for the swap spaces. If this line is all zeroes,
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your swap space is not activated.</P
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><P
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>The same information is available via
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<B
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CLASS="COMMAND"
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>top</B
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>, or using the proc filesystem in file
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<TT
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CLASS="FILENAME"
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>/proc/meminfo</TT
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>. It is currently difficult
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to get information on the use of a specific swap space.</P
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><P
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>A swap space can be removed from use with
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<B
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CLASS="COMMAND"
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>swapoff</B
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>. It is usually not necessary to do it,
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except for temporary swap spaces. Any pages in use in the swap
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space are swapped in first; if there is not sufficient physical
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memory to hold them, they will then be swapped out (to some other
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swap space). If there is not enough virtual memory to hold all
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of the pages Linux will start to thrash; after a long while it
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should recover, but meanwhile the system is unusable. You should
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check (e.g., with <B
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CLASS="COMMAND"
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>free</B
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>) that there is enough
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free memory before removing a swap space from use.</P
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><P
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>All the swap spaces that are used automatically
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with <B
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CLASS="COMMAND"
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>swapon -a</B
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> can be removed from use
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with <B
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CLASS="COMMAND"
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>swapoff -a</B
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>; it looks at the file
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<TT
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CLASS="FILENAME"
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>/etc/fstab</TT
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> to find what to remove.
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Any manually used swap spaces will remain in use.</P
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><P
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>Sometimes a lot of swap space can be in use even though
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there is a lot of free physical memory. This can happen for
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instance if at one point there is need to swap, but later a big
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process that occupied much of the physical memory terminates
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and frees the memory. The swapped-out data is not automatically
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swapped in until it is needed, so the physical memory may remain
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free for a long time. There is no need to worry about this,
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but it can be comforting to know what is happening. </P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
|
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
|
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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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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><A
|
|
HREF="swap-space.html"
|
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ACCESSKEY="P"
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>Prev</A
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></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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HREF="index.html"
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ACCESSKEY="H"
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>Home</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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><A
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HREF="sharing-swap.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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><TR
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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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>Creating a swap space</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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HREF="memory-management.html"
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ACCESSKEY="U"
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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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>Sharing swap spaces with other operating systems</TD
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></TR
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></TABLE
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></DIV
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></BODY
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></HTML
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> |