475 lines
22 KiB
XML
475 lines
22 KiB
XML
<?xml version="1.0" encoding="iso-8859-1" standalone="no"?>
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<!DOCTYPE article PUBLIC "-//FreeBSD//DTD DocBook XML V4.2-Based Extension//EN"
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"../../../share/xml/freebsd42.dtd" [
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<!ENTITY % entities PUBLIC "-//FreeBSD//ENTITIES DocBook FreeBSD Entity Set//EN" "../../share/xml/entities.ent">
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%entities;
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]>
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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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1. Redistributions of source code (SGML DocBook) must retain the above
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2. Redistributions in compiled form (transformed to other DTDs,
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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 lang='en'>
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<articleinfo>
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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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<firstname>John</firstname>
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<surname>Kozubik</surname>
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<affiliation>
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<address><email>john@kozubik.com</email></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 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 &os;
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to create embedded systems.</para>
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<para>Embedded systems have the advantage of increased stability due to
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the lack of integral moving parts (hard drives). Account must be
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taken, however, for the generally low disk space available in the
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system and the durability of the storage medium.</para>
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<para>Specific topics to be covered include the types and attributes of
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solid state media suitable for disk use in &os;, kernel options
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that are of interest in such an environment, the
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<filename>rc.initdiskless</filename> mechanisms that automate the
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initialization of such systems and the need for read-only filesystems,
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and building filesystems from scratch. The article will conclude
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with some general strategies for small and read-only &os;
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environments.</para>
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</abstract>
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</articleinfo>
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<sect1 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 disk
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devices made from flash memory. Flash memory is a solid state memory
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(no moving parts) that is non-volatile (the memory maintains data even
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after all power sources have been disconnected). Flash memory can
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withstand tremendous physical shock and is reasonably fast (the flash
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memory solutions covered in this article are slightly slower than a EIDE
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hard disk for write operations, and much faster for read operations).
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One very important aspect of flash memory, the ramifications of which
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will be discussed later in this article, is that each sector has a
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limited rewrite capacity. You can only write, erase, and write again to
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a sector of flash memory a certain number of times before the sector
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becomes permanently unusable. Although many flash memory products
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automatically map bad blocks, and although some even distribute write
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operations evenly throughout the unit, the fact remains that there
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exists a 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 per
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sector in their specification. This figure varies due to the
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temperature of the environment.</para>
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<para>Specifically, we will be discussing ATA compatible compact-flash
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units, which are quite popular as storage media for digital
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cameras. Of particular interest is the fact that they pin out directly
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to the IDE bus and are compatible with the ATA command set. Therefore,
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with a very simple and low-cost adaptor, these devices can be attached
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directly to an IDE bus in a computer. Once implemented in this manner,
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operating systems such as &os; see the device as a normal hard disk
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(albeit small).</para>
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<para>Other solid state disk solutions do exist, but their expense,
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obscurity, and relative unease of use places them beyond the scope of
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this article.</para>
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</sect1>
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<sect1 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 creating
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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. Because
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of the limited number of writes that can be done to flash memory, the
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disk and the filesystems on the disk will most likely be mounted
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read-only. In this environment, filesystems such as
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<filename>/tmp</filename> and <filename>/var</filename> are mounted as
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memory filesystems to allow the system to create logs and update
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counters and temporary files. Memory filesystems are a critical
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component to a successful solid state &os; implementation.</para>
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<para>You should make sure the following lines exist in your kernel
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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 id="ro-fs">
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<title>The <literal>rc</literal> Subsystem and Read-Only 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 <filename>/var</filename>
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as a memory filesystem, makes a configurable list of directories in
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<filename>/var</filename> with the &man.mkdir.1; command, and
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changes modes on some of 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>. The <filename>/etc/rc.d/var</filename>
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file creates a <filename>/var</filename> partition based on the value of
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this variable in <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>
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is a read-write filesystem is an important
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distinction, as the <filename>/</filename> partition (and any other
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partitions you may have on your flash media) should be mounted
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read-only. Remember that in <xref linkend="intro"/> we detailed the
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limitations of flash memory - specifically the limited write capability.
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The importance of not mounting filesystems on flash media read-write,
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and the importance of not using a swap file, cannot be overstated. A
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swap file on a busy system can burn through a piece of flash media in
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less than one year. Heavy logging or temporary file creation and
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destruction can do the 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> file, you should also change the Options
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field for each filesystem to <literal>ro</literal> as 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 begin to
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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 filesystem and
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missing items in the <filename>/var</filename> that
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<filename>/etc/rc.d/var</filename> has created. These are only
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temporary problems though, and are addressed, along with solutions to
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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 mounted
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read-only with <filename>/etc/fstab</filename> can be made read-write at
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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 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; as
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normal IDE hard drives, you could
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theoretically install &os; from the network using the kern and
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mfsroot floppies or from a 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 than 200
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megabytes. Because most people will be using smaller flash memory
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devices (128 megabytes is considered fairly large - 32 or even 16
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megabytes is common) an installation using normal mechanisms is not
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possible—there is simply not enough disk space for even the
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smallest of conventional installations.</para>
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<para>The easiest way to overcome this space limitation is to install
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&os; using conventional means to a normal hard disk. After the
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installation is complete, pare down the operating system to a size that
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will fit onto your flash media, then tar the entire filesystem. The
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following steps will guide you through the process of preparing a piece
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of flash memory for your tarred filesystem. Remember, because a normal
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installation is not being performed, operations such as partitioning,
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labeling, file-system creation, etc. need to be performed by hand. In
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addition to the kern and mfsroot floppy disks, you will also need to use
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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 the custom
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installation menu, choose <literal>partition</literal>. In the
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partition menu, you should delete all existing partitions using the
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<keycap>d</keycap> key. After deleting all existing partitions,
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create a partition using the <keycap>c</keycap> key and accept the
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default value for the size of the partition. When asked for the
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type of the partition, make sure the value is set to
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<literal>165</literal>. Now write this partition table to the disk
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by pressing the <keycap>w</keycap> key (this is a hidden option on
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this screen). If you are using an ATA compatible compact
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flash card, you should choose the &os; Boot Manager. Now press
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the <keycap>q</keycap> key to quit the partition menu. You will be
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shown the boot manager menu once more - repeat the choice you made
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earlier.</para>
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</step>
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<step>
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<title>Creating filesystems on your flash memory 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> option. After
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entering the fixit environment, enter the 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 the
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auspices of the disklabel command. Next, you need to add
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an <literal>a:</literal> line at the end of the file. This
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<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 is
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exactly the same as the number in the existing <literal>c:</literal>
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entry for size. Basically you are duplicating the existing
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<literal>c:</literal> line as an <literal>a:</literal> line, making
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sure that fstype is <literal>4.2BSD</literal>. Save the file and
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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 our tar
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file and explode it onto our flash media filesystem. One example of
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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 tar file.
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You may be faced with a bit of a dilemma at this point - if your
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flash memory part is 128 megabytes, for instance, and your tar file
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is larger than 64 megabytes, you cannot have your tar file on the
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flash media at the same 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, is to
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untar the file while it is transferred over FTP. If you perform
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your transfer in this manner, you will never have the tar file and
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the tar contents on your disk at the 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 flash
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memory filesystem, you can unmount the flash memory and
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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 when it
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was built on the normal hard disk (with your filesystems mounted
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read-only, and with the necessary options compiled into the kernel)
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you should now be successfully booting your &os; embedded
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system.</para>
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</step>
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</procedure>
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</sect1>
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<sect1 id="strategies">
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<title>System Strategies for Small and Read Only 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 read-only
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root filesystem causes problems with many common software packages used
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with &os;. In this article, suggestions for successfully running
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cron, syslog, ports installations, and the Apache web server will be
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provided.</para>
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<sect2>
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<title>cron</title>
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<para>Upon boot, <filename class="directory">/var</filename> gets
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populated by <filename>/etc/rc.d/var</filename> using the list from
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<filename>/etc/mtree/BSD.var.dist</filename>, so the <filename
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class="directory">cron</filename>, <filename
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class="directory">cron/tabs</filename>, <filename
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class="directory">at</filename>, and a few other standard
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directories get created.</para>
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<para>However, this does not solve the problem of maintaining cron
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tabs across reboots. When the system reboots, the
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<filename>/var</filename> filesystem that is in memory will disappear
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and any cron tabs you may have had in it will also disappear.
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Therefore, one solution would be to create cron tabs for the users
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that need them, mount your <filename>/</filename> filesystem as
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read-write and copy those 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 crontabs into
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<filename>/var/cron/tabs</filename> after that directory has been
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created during system initialization. You may also need to add a line
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that changes modes and permissions on the directories you create and
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the files you copy with <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 of
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certain log files that exist in <filename>/var/log</filename>. These
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files are not created by <filename>/etc/rc.d/var</filename> upon
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system initialization. Therefore, somewhere in
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<filename>/etc/rc.d/var</filename>, after the section that creates
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the directories in <filename>/var</filename>, you will need to add
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something like 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 use the
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ports tree, a reminder is necessary regarding the read-only nature of
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your filesystems on the flash media. Since they are read-only, you
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will need to temporarily mount them read-write using the mount syntax
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shown in <xref 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 shorten its
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lifespan.</para>
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<para>To make it possible to enter a ports directory and successfully
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run <command>make</command> <maketarget>install</maketarget>,
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we must create a packages directory on
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a non-memory filesystem that will keep track of our packages across
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reboots. Because it is necessary to mount your filesystems as
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read-write for the installation of a package anyway, it is sensible to
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assume that an area on the flash media can also be used for package
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information to be written to.</para>
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<para>First, create a package database directory. This is normally in
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<filename>/var/db/pkg</filename>, but we cannot place it there as it
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will disappear every time the system is 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 read-write and
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install a package, the <command>make</command> <maketarget>install</maketarget> will work,
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and package information
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will be written successfully to <filename>/etc/pkg</filename> (because
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the filesystem will, at that time, be mounted read-write) which will
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always be 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 is
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set up to write its pid or log information outside of
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<filename class="directory">/var</filename>. By default,
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Apache keeps its pid file in <filename
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class="directory">/var/run/httpd.pid</filename> and its log
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files in <filename class="directory">/var/log</filename>.</para>
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</note>
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<para>It is now assumed that Apache keeps its log files in a
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directory <filename
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class="directory"><replaceable>apache_log_dir</replaceable></filename>
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outside of <filename class="directory">/var</filename>.
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When this directory lives on a read-only filesystem, Apache will
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not be able to save any log files, and may have problems working.
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|
If so, it is necessary to add a new directory to the
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|
list of directories in <filename>/etc/rc.d/var</filename> to
|
|
create in <filename>/var</filename>, and to link
|
|
<filename class="directory"><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>
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|
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<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>
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|
|
<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>
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|
|
|
<para>Finally, remove the existing
|
|
<filename class="directory"><replaceable>apache_log_dir</replaceable></filename>
|
|
directory, and replace it with a link:</para>
|
|
|
|
<screen>&prompt.root; <userinput>rm -rf <filename class="directory"><replaceable>apache_log_dir</replaceable></filename></userinput>
|
|
&prompt.root; <userinput>ln -s /var/log/apache <filename class="directory"><replaceable>apache_log_dir</replaceable></filename></userinput></screen>
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</sect2>
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</sect1>
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</article>
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