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>10. Appendix</H1
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="formating"
></A
>10.1. Formating Partitions</H2
><P
> At the shell prompt, I begin making the file systems on my
partitions. Continuing with the example in (see <A
HREF="fdisk_partitioning.html#mixed"
>Section 5.3</A
>),
this is:
<TABLE
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><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
> # mke2fs /dev/sda1
</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
> I need to do this for each of my partitions, but not for <TT
CLASS="filename"
>/dev/sda4</TT
> (my extended partition).
Linux supports types of file systems other than ext2. You can find out
what kinds your kernel supports by looking in:
<TT
CLASS="filename"
>/usr/src/linux/include/linux/fs.h</TT
>
</P
><P
> The most common file systems can be made with programs in <TT
CLASS="filename"
>/sbin</TT
> that start with "mk" like
<B
CLASS="command"
>mkfs.msdos</B
> and <B
CLASS="command"
>mke2fs</B
>.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="swap"
></A
>10.2. Activating Swap Space</H2
><P
>
To set up a swap partition:
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><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
> # mkswap -f /dev/hda5
</PRE
></FONT
></TD
></TR
></TABLE
>
To activate the swap area:
<TABLE
BORDER="0"
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><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
> # swapon /dev/hda5
</PRE
></FONT
></TD
></TR
></TABLE
>
Normally, the swap area is activated by the initialization scripts at
boot time.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="mounting"
></A
>10.3. Mounting Partitions</H2
><P
>
Mounting a partition means attaching it to the linux file system. To
mount a linux partition:
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><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
> # mount -t ext2 /dev/sda1 /opt
</PRE
></FONT
></TD
></TR
></TABLE
>
<P
></P
><DIV
CLASS="variablelist"
><DL
><DT
>-t ext2</DT
><DD
><P
> File system type. Other types you are likely to use are:
<P
></P
><UL
><LI
><P
>ext3 (journaling sile system based on ext2)</P
></LI
><LI
><P
>msdos (DOS)</P
></LI
><LI
><P
>hfs (mac)</P
></LI
><LI
><P
>iso9660 (CDROM)</P
></LI
><LI
><P
>nfs (network file system)</P
></LI
></UL
>
</P
></DD
><DT
>/dev/sda1</DT
><DD
><P
> Device name. Other device names you are likely to use:
<P
></P
><UL
><LI
><P
><TT
CLASS="filename"
>/dev/hdb2</TT
> (second partition in second IDE drive)</P
></LI
><LI
><P
><TT
CLASS="filename"
>/dev/fd0</TT
> (floppy drive A)</P
></LI
><LI
><P
><TT
CLASS="filename"
>/dev/cdrom</TT
> (CDROM)</P
></LI
></UL
>
</P
></DD
><DT
>/opt</DT
><DD
><P
> mount point. This is where you want to "see" your partition.
When you type <TT
CLASS="userinput"
><B
>ls /opt</B
></TT
>, you can see what
is in <TT
CLASS="filename"
>/dev/sda1</TT
>. If there
are already some directories and/or files under <TT
CLASS="filename"
>/opt</TT
>, they will be invisible after
this mount command.
</P
></DD
></DL
></DIV
>
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="fragmentation"
></A
>10.4. Some facts about file systems and fragmentation</H2
><P
> Disk space is administered by the operating system in units of
blocks and fragments of blocks. In ext2, fragments and blocks
have to be of the same size, so we can limit our discussion to
blocks.
</P
><P
> Files come in any size. They don't end on block boundaries. So
with every file a part of the last block of every file is
wasted. Assuming that file sizes are random, there is
approximately a half block of waste for each file on your disk.
Tanenbaum calls this "internal fragmentation" in his book
"Operating Systems".
</P
><P
> You can guess the number of files on your disk by the number of
allocated inodes on a disk. On my disk
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
> # df -i
Filesystem Inodes IUsed IFree %IUsed Mounted on
/dev/hda3 64256 12234 52022 19% /
/dev/hda5 96000 43058 52942 45% /var
</PRE
></FONT
></TD
></TR
></TABLE
>
there are about 12000 files on <TT
CLASS="filename"
>/</TT
> and about 44000 files on <TT
CLASS="filename"
>/var</TT
>. At a block size of 1 KB, about
6+22 = 28 MB of disk space are lost in the tail blocks of files. Had
I chosen a block size of 4 KB, I had lost 4 times this space.
</P
><P
> Data transfer is faster for large contiguous chunks of data, though.
That's why ext2 tries to preallocate space in units of 8 contigous
blocks for growing files. Unused preallocation is released when the
file is closed, so no space is wasted.
</P
><P
> Noncontiguous placement of blocks in a file is bad for performance,
since files are often accessed in a sequential manner. It forces the
operating system to split a disk access and the disk to move the
head. This is called "external fragmentation" or simply
"fragmentation" and is a common problem with MS-DOS file systems. In
conjunction with the abysmal buffer cache used by MS-DOS, the
effects of file fragmentation on performance are very noticeable.
DOS users are accustomed to defragging their disks every few weeks
and some have even developed some ritualistic beliefs regarding
defragmentation.
</P
><P
> None of these habits should be carried over to Linux and ext2. Linux
native file systems do not need defragmentation under normal use and
this includes any condition with at least 5% of free space on a
disk. There is a defragmentation tool for ext2 called defrag, but
users are cautioned against casual use. A power outage during such
an operation can trash your file system. Since you need to back up
your data anyway, simply writing back from your copy will do the
job.
</P
><P
> The MS-DOS file system is also known to lose large amounts of disk
space due to internal fragmentation. For partitions larger than 256
MB, DOS block sizes grow so large that they are no longer useful
(This has been corrected to some extent with FAT32). Ext2 does not
force you to choose large blocks for large file systems, except for
very large file systems in the 0.5 TB range (that's terabytes with 1
TB equaling 1024 GB) and above, where small block sizes become
inefficient. So unlike DOS there is no need to split up large disks
into multiple partitions to keep block size down.
</P
><P
> Use a 1Kb block size if you have many small files. For large
partitions, 4Kb blocks are fine.
</P
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