498 lines
22 KiB
XML
498 lines
22 KiB
XML
<?xml version="1.0" encoding="iso-8859-1"?>
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<!DOCTYPE article PUBLIC "-//FreeBSD//DTD DocBook XML V5.0-Based Extension//EN"
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"http://www.FreeBSD.org/XML/share/xml/freebsd50.dtd">
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<!-- Copyright (c) 2001 The FreeBSD Documentation Project
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Redistribution and use in source (SGML DocBook) and 'compiled' forms
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(SGML, HTML, PDF, PostScript, RTF and so forth) with or without
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modification, are permitted provided that the following conditions
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are met:
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1. Redistributions of source code (SGML DocBook) must retain the above
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copyright notice, this list of conditions and the following
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disclaimer as the first lines of this file unmodified.
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2. Redistributions in compiled form (transformed to other DTDs,
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converted to PDF, PostScript, RTF and other formats) must reproduce
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the above copyright notice, this list of conditions and the
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following disclaimer in the documentation and/or other materials
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provided with the distribution.
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THIS DOCUMENTATION IS PROVIDED BY THE FREEBSD DOCUMENTATION PROJECT "AS
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IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
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THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL NIK CLAYTON BE LIABLE FOR ANY
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DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
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STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
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ANY WAY OUT OF THE USE OF THIS DOCUMENTATION, EVEN IF ADVISED OF THE
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POSSIBILITY OF SUCH DAMAGE.
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$FreeBSD$
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-->
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<article xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0"
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xml:lang="en"> <info>
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<title>&os; and Solid State Devices</title>
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<authorgroup>
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<author>
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<personname>
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<firstname>John</firstname>
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<surname>Kozubik</surname>
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</personname>
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<affiliation>
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<address>
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<email>john@kozubik.com</email>
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</address>
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</affiliation>
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</author>
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</authorgroup>
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<copyright>
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<year>2001</year>
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<year>2009</year>
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<holder>The FreeBSD Documentation Project</holder>
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</copyright>
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<legalnotice xml:id="trademarks" role="trademarks">
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&tm-attrib.freebsd;
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&tm-attrib.general;
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</legalnotice>
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&legalnotice;
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<pubdate>$FreeBSD$</pubdate>
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<releaseinfo>$FreeBSD$</releaseinfo>
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<abstract>
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<para>This article covers the use of solid state disk devices in
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&os; to create embedded systems.</para>
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<para>Embedded systems have the advantage of increased stability
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due to the lack of integral moving parts (hard drives).
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Account must be taken, however, for the generally low disk
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space available in the system and the durability of the
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storage medium.</para>
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<para>Specific topics to be covered include the types and
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attributes of solid state media suitable for disk use in &os;,
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kernel options that are of interest in such an environment,
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the <filename>rc.initdiskless</filename> mechanisms that
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automate the initialization of such systems and the need for
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read-only filesystems, and building filesystems from scratch.
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The article will conclude with some general strategies for
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small and read-only &os; environments.</para>
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</abstract>
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</info>
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<sect1 xml:id="intro">
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<title>Solid State Disk Devices</title>
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<para>The scope of this article will be limited to solid state
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disk devices made from flash memory. Flash memory is a solid
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state memory (no moving parts) that is non-volatile (the memory
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maintains data even after all power sources have been
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disconnected). Flash memory can withstand tremendous physical
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shock and is reasonably fast (the flash memory solutions covered
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in this article are slightly slower than a EIDE hard disk for
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write operations, and much faster for read operations). One
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very important aspect of flash memory, the ramifications of
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which will be discussed later in this article, is that each
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sector has a limited rewrite capacity. You can only write,
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erase, and write again to a sector of flash memory a certain
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number of times before the sector becomes permanently unusable.
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Although many flash memory products automatically map bad
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blocks, and although some even distribute write operations
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evenly throughout the unit, the fact remains that there exists a
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limit to the amount of writing that can be done to the device.
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Competitive units have between 1,000,000 and 10,000,000 writes
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per sector in their specification. This figure varies due to
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the temperature of the environment.</para>
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<para>Specifically, we will be discussing ATA compatible
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compact-flash units, which are quite popular as storage media
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for digital cameras. Of particular interest is the fact that
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they pin out directly to the IDE bus and are compatible with the
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ATA command set. Therefore, with a very simple and low-cost
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adaptor, these devices can be attached directly to an IDE bus in
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a computer. Once implemented in this manner, operating systems
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such as &os; see the device as a normal hard disk (albeit
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small).</para>
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<para>Other solid state disk solutions do exist, but their
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expense, obscurity, and relative unease of use places them
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beyond the scope of this article.</para>
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</sect1>
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<sect1 xml:id="kernel">
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<title>Kernel Options</title>
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<para>A few kernel options are of specific interest to those
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creating an embedded &os; system.</para>
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<para>All embedded &os; systems that use flash memory as system
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disk will be interested in memory disks and memory filesystems.
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Because of the limited number of writes that can be done to
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flash memory, the disk and the filesystems on the disk will most
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likely be mounted read-only. In this environment, filesystems
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such as <filename>/tmp</filename> and <filename>/var</filename>
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are mounted as memory filesystems to allow the system to create
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logs and update counters and temporary files. Memory
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filesystems are a critical component to a successful solid state
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&os; implementation.</para>
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<para>You should make sure the following lines exist in your
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kernel configuration file:</para>
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<programlisting>options MFS # Memory Filesystem
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options MD_ROOT # md device usable as a potential root device
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pseudo-device md # memory disk</programlisting>
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</sect1>
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<sect1 xml:id="ro-fs">
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<title>The <literal>rc</literal> Subsystem and Read-Only
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Filesystems</title>
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<para>The post-boot initialization of an embedded &os; system is
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controlled by <filename>/etc/rc.initdiskless</filename>.</para>
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<para><filename>/etc/rc.d/var</filename> mounts
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<filename>/var</filename> as a memory filesystem, makes a
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configurable list of directories in <filename>/var</filename>
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with the &man.mkdir.1; command, and changes modes on some of
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those directories. In the execution of
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<filename>/etc/rc.d/var</filename>, one other
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<filename>rc.conf</filename> variable comes into play –
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<literal>varsize</literal>. A <filename>/var</filename>
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partition is created by <filename>/etc/rc.d/var</filename> based
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on the value of this variable in
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<filename>rc.conf</filename>:</para>
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<programlisting>varsize=8192</programlisting>
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<para>Remember that this value is in sectors by default.</para>
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<para>The fact that <filename>/var</filename> is a read-write
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filesystem is an important distinction, as the
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<filename>/</filename> partition (and any other partitions you
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may have on your flash media) should be mounted read-only.
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Remember that in <xref linkend="intro"/> we detailed the
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limitations of flash memory - specifically the limited write
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capability. The importance of not mounting filesystems on flash
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media read-write, and the importance of not using a swap file,
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cannot be overstated. A swap file on a busy system can burn
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through a piece of flash media in less than one year. Heavy
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logging or temporary file creation and destruction can do the
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same. Therefore, in addition to removing the
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<literal>swap</literal> entry from your
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<filename>/etc/fstab</filename>, you should also change the
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Options field for each filesystem to <literal>ro</literal> as
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follows:</para>
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<programlisting># Device Mountpoint FStype Options Dump Pass#
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/dev/ad0s1a / ufs ro 1 1</programlisting>
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<para>A few applications in the average system will immediately
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begin to fail as a result of this change. For instance, cron
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will not run properly as a result of missing cron tabs in the
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<filename>/var</filename> created by
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<filename>/etc/rc.d/var</filename>, and syslog and dhcp will
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encounter problems as well as a result of the read-only
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filesystem and missing items in the <filename>/var</filename>
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that <filename>/etc/rc.d/var</filename> has created. These are
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only temporary problems though, and are addressed, along with
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solutions to the execution of other common software packages in
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<xref linkend="strategies"/>.</para>
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<para>An important thing to remember is that a filesystem that was
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mounted read-only with <filename>/etc/fstab</filename> can be
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made read-write at any time by issuing the command:</para>
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<screen>&prompt.root; <userinput>/sbin/mount -uw <replaceable>partition</replaceable></userinput></screen>
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<para>and can be toggled back to read-only with the
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command:</para>
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<screen>&prompt.root; <userinput>/sbin/mount -ur <replaceable>partition</replaceable></userinput></screen>
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</sect1>
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<sect1>
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<title>Building a File System from Scratch</title>
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<para>Because ATA compatible compact-flash cards are seen by &os;
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as normal IDE hard drives, you could theoretically install &os;
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from the network using the kern and mfsroot floppies or from a
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CD.</para>
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<para>However, even a small installation of &os; using normal
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installation procedures can produce a system in size of greater
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than 200 megabytes. Because most people will be using smaller
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flash memory devices (128 megabytes is considered fairly large -
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32 or even 16 megabytes is common) an installation using normal
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mechanisms is not possible—there is simply not enough disk
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space for even the smallest of conventional
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installations.</para>
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<para>The easiest way to overcome this space limitation is to
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install &os; using conventional means to a normal hard disk.
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After the installation is complete, pare down the operating
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system to a size that will fit onto your flash media, then tar
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the entire filesystem. The following steps will guide you
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through the process of preparing a piece of flash memory for
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your tarred filesystem. Remember, because a normal installation
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is not being performed, operations such as partitioning,
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labeling, file-system creation, etc. need to be performed by
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hand. In addition to the kern and mfsroot floppy disks, you
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will also need to use the fixit floppy.</para>
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<procedure>
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<step>
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<title>Partitioning Your Flash Media Device</title>
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<para>After booting with the kern and mfsroot floppies, choose
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<literal>custom</literal> from the installation menu. In
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the custom installation menu, choose
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<literal>partition</literal>. In the partition menu, you
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should delete all existing partitions using
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<keycap>d</keycap>. After deleting all existing
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partitions, create a partition using <keycap>c</keycap>
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and accept the default value for the size of the
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partition. When asked for the type of the partition, make
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sure the value is set to <literal>165</literal>. Now write
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this partition table to the disk by pressing
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<keycap>w</keycap> (this is a hidden option on this
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screen). If you are using an ATA compatible compact flash
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card, you should choose the &os; Boot Manager. Now press
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<keycap>q</keycap> to quit the partition menu. You
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will be shown the boot manager menu once more - repeat the
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choice you made earlier.</para>
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</step>
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<step>
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<title>Creating Filesystems on Your Flash Memory
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Device</title>
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<para>Exit the custom installation menu, and from the main
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installation menu choose the <literal>fixit</literal>
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option. After entering the fixit environment, enter the
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following command:</para>
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<screen>&prompt.root; <userinput>disklabel -e /dev/ad0c</userinput></screen>
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<para>At this point you will have entered the vi editor under
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the auspices of the disklabel command. Next, you need to
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add an <literal>a:</literal> line at the end of the file.
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This <literal>a:</literal> line should look like:</para>
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<programlisting>a: <replaceable>123456</replaceable> 0 4.2BSD 0 0</programlisting>
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<para>Where <replaceable>123456</replaceable> is a number that
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is exactly the same as the number in the existing
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<literal>c:</literal> entry for size. Basically you are
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duplicating the existing <literal>c:</literal> line as an
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<literal>a:</literal> line, making sure that fstype is
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<literal>4.2BSD</literal>. Save the file and exit.</para>
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<screen>&prompt.root; <userinput>disklabel -B -r /dev/ad0c</userinput>
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&prompt.root; <userinput>newfs /dev/ad0a</userinput></screen>
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</step>
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<step>
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<title>Placing Your Filesystem on the Flash Media</title>
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<para>Mount the newly prepared flash media:</para>
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<screen>&prompt.root; <userinput>mount /dev/ad0a /flash</userinput></screen>
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<para>Bring this machine up on the network so we may transfer
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our tar file and explode it onto our flash media filesystem.
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One example of how to do this is:</para>
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<screen>&prompt.root; <userinput>ifconfig xl0 192.168.0.10 netmask 255.255.255.0</userinput>
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&prompt.root; <userinput>route add default 192.168.0.1</userinput></screen>
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<para>Now that the machine is on the network, transfer your
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tar file. You may be faced with a bit of a dilemma at this
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point - if your flash memory part is 128 megabytes, for
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instance, and your tar file is larger than 64 megabytes, you
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cannot have your tar file on the flash media at the same
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time as you explode it - you will run out of
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space. One solution to this problem, if you are using FTP,
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is to untar the file while it is transferred over FTP. If
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you perform your transfer in this manner, you will never
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have the tar file and the tar contents on your disk at the
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same time:</para>
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<screen><prompt>ftp></prompt> <userinput>get tarfile.tar "| tar xvf -"</userinput></screen>
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<para>If your tarfile is gzipped, you can accomplish this as
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well:</para>
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<screen><prompt>ftp></prompt> <userinput>get tarfile.tar "| zcat | tar xvf -"</userinput></screen>
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<para>After the contents of your tarred filesystem are on your
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flash memory filesystem, you can unmount the flash memory
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and reboot:</para>
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<screen>&prompt.root; <userinput>cd /</userinput>
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&prompt.root; <userinput>umount /flash</userinput>
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&prompt.root; <userinput>exit</userinput></screen>
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<para>Assuming that you configured your filesystem correctly
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when it was built on the normal hard disk (with your
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filesystems mounted read-only, and with the necessary
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options compiled into the kernel) you should now be
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successfully booting your &os; embedded system.</para>
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</step>
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</procedure>
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</sect1>
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<sect1 xml:id="strategies">
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<title>System Strategies for Small and Read Only
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Environments</title>
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<para>In <xref linkend="ro-fs"/>, it was pointed out that the
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<filename>/var</filename> filesystem constructed by
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<filename>/etc/rc.d/var</filename> and the presence of a
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read-only root filesystem causes problems with many common
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software packages used with &os;. In this article, suggestions
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for successfully running cron, syslog, ports installations, and
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the Apache web server will be provided.</para>
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<sect2>
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<title>Cron</title>
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<para>Upon boot, <filename>/var</filename> gets populated by
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<filename>/etc/rc.d/var</filename> using the list from
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<filename>/etc/mtree/BSD.var.dist</filename>, so the
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<filename>cron</filename>, <filename>cron/tabs</filename>,
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<filename>at</filename>, and a few other standard directories
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get created.</para>
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<para>However, this does not solve the problem of maintaining
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cron tabs across reboots. When the system reboots, the
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<filename>/var</filename> filesystem that is in memory will
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disappear and any cron tabs you may have had in it will also
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disappear. Therefore, one solution would be to create cron
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tabs for the users that need them, mount your
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<filename>/</filename> filesystem as read-write and copy those
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cron tabs to somewhere safe, like
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<filename>/etc/tabs</filename>, then add a line to the end of
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<filename>/etc/rc.initdiskless</filename> that copies those
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crontabs into <filename>/var/cron/tabs</filename> after that
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directory has been created during system initialization. You
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may also need to add a line that changes modes and permissions
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on the directories you create and the files you copy with
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<filename>/etc/rc.initdiskless</filename>.</para>
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</sect2>
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<sect2>
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<title>Syslog</title>
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<para><filename>syslog.conf</filename> specifies the locations
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of certain log files that exist in
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<filename>/var/log</filename>. These files are not created by
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<filename>/etc/rc.d/var</filename> upon system initialization.
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Therefore, somewhere in <filename>/etc/rc.d/var</filename>,
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after the section that creates the directories in
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<filename>/var</filename>, you will need to add something like
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this:</para>
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<screen>&prompt.root; <userinput>touch /var/log/security /var/log/maillog /var/log/cron /var/log/messages</userinput>
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&prompt.root; <userinput>chmod 0644 /var/log/*</userinput></screen>
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</sect2>
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<sect2>
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<title>Ports Installation</title>
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<para>Before discussing the changes necessary to successfully
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use the ports tree, a reminder is necessary regarding the
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read-only nature of your filesystems on the flash media.
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Since they are read-only, you will need to temporarily mount
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them read-write using the mount syntax shown in <xref
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linkend="ro-fs"/>. You should always remount those
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filesystems read-only when you are done with any maintenance -
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unnecessary writes to the flash media could considerably
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shorten its lifespan.</para>
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<para>To make it possible to enter a ports directory and
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successfully run <command>make</command>
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<buildtarget>install</buildtarget>, we must create a packages
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directory on a non-memory filesystem that will keep track of
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our packages across reboots. Because it is necessary to mount
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your filesystems as read-write for the installation of a
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package anyway, it is sensible to assume that an area on the
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flash media can also be used for package information to be
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written to.</para>
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<para>First, create a package database directory. This is
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normally in <filename>/var/db/pkg</filename>, but we cannot
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place it there as it will disappear every time the system is
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booted.</para>
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<screen>&prompt.root; <userinput>mkdir /etc/pkg</userinput></screen>
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<para>Now, add a line to <filename>/etc/rc.d/var</filename> that
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links the <filename>/etc/pkg</filename> directory to
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<filename>/var/db/pkg</filename>. An example:</para>
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<screen>&prompt.root; <userinput>ln -s /etc/pkg /var/db/pkg</userinput></screen>
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<para>Now, any time that you mount your filesystems as
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read-write and install a package, the <command>make</command>
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<buildtarget>install</buildtarget> will work, and package
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information will be written successfully to
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<filename>/etc/pkg</filename> (because the filesystem will, at
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that time, be mounted read-write) which will always be
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available to the operating system as
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<filename>/var/db/pkg</filename>.</para>
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</sect2>
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<sect2>
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<title>Apache Web Server</title>
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<note>
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<para>The steps in this section are only necessary if Apache
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is set up to write its pid or log information outside of
|
|
<filename>/var</filename>. By default, Apache keeps its pid
|
|
file in <filename>/var/run/httpd.pid</filename> and its log
|
|
files in <filename>/var/log</filename>.</para>
|
|
</note>
|
|
|
|
<para>It is now assumed that Apache keeps its log files in a
|
|
directory
|
|
<filename><replaceable>apache_log_dir</replaceable></filename>
|
|
outside of <filename>/var</filename>. When this directory
|
|
lives on a read-only filesystem, Apache will not be able to
|
|
save any log files, and may have problems working. If so, it
|
|
is necessary to add a new directory to the list of directories
|
|
in <filename>/etc/rc.d/var</filename> to create in
|
|
<filename>/var</filename>, and to link
|
|
<filename><replaceable>apache_log_dir</replaceable></filename>
|
|
to <filename>/var/log/apache</filename>. It is also necessary
|
|
to set permissions and ownership on this new directory.</para>
|
|
|
|
<para>First, add the directory <literal>log/apache</literal> to
|
|
the list of directories to be created in
|
|
<filename>/etc/rc.d/var</filename>.</para>
|
|
|
|
<para>Second, add these commands to
|
|
<filename>/etc/rc.d/var</filename> after the directory
|
|
creation section:</para>
|
|
|
|
<screen>&prompt.root; <userinput>chmod 0774 /var/log/apache</userinput>
|
|
&prompt.root; <userinput>chown nobody:nobody /var/log/apache</userinput></screen>
|
|
|
|
<para>Finally, remove the existing
|
|
<filename><replaceable>apache_log_dir</replaceable></filename>
|
|
directory, and replace it with a link:</para>
|
|
|
|
<screen>&prompt.root; <userinput>rm -rf <replaceable>apache_log_dir</replaceable></userinput>
|
|
&prompt.root; <userinput>ln -s /var/log/apache <replaceable>apache_log_dir</replaceable></userinput></screen>
|
|
</sect2>
|
|
</sect1>
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|
</article>
|