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FreeBSD and Solid State Devices

<email>john@kozubik.com</email>
<personname> <firstname>John</firstname> <surname>Kozubik</surname> </personname> <affiliation> <_:address-1/> </affiliation>
<year>2001</year> <year>2009</year> <holder>The FreeBSD Documentation Project</holder>
FreeBSD is a registered trademark of the FreeBSD Foundation.
Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this document, and the FreeBSD Project was aware of the trademark claim, the designations have been followed by the <quote>™</quote> or the <quote>®</quote> symbol.
Copyright
Redistribution and use in source (XML DocBook) and 'compiled' forms (XML, HTML, PDF, PostScript, RTF and so forth) with or without modification, are permitted provided that the following conditions are met:
Redistributions of source code (XML DocBook) must retain the above copyright notice, this list of conditions and the following disclaimer as the first lines of this file unmodified.
Redistributions in compiled form (transformed to other DTDs, converted to PDF, PostScript, RTF and other formats) must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
THIS DOCUMENTATION IS PROVIDED BY THE FREEBSD DOCUMENTATION PROJECT "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FREEBSD DOCUMENTATION PROJECT BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS DOCUMENTATION, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
$FreeBSD: head/en_US.ISO8859-1/articles/solid-state/article.xml 44923 2014-05-23 17:36:54Z bcr $
This article covers the use of solid state disk devices in FreeBSD to create embedded systems.
Embedded systems have the advantage of increased stability due to the lack of integral moving parts (hard drives). Account must be taken, however, for the generally low disk space available in the system and the durability of the storage medium.
Specific topics to be covered include the types and attributes of solid state media suitable for disk use in FreeBSD, kernel options that are of interest in such an environment, the <filename>rc.initdiskless</filename> mechanisms that automate the initialization of such systems and the need for read-only filesystems, and building filesystems from scratch. The article will conclude with some general strategies for small and read-only FreeBSD environments.
Solid State Disk Devices
The scope of this article will be limited to solid state disk devices made from flash memory. Flash memory is a solid state memory (no moving parts) that is non-volatile (the memory maintains data even after all power sources have been disconnected). Flash memory can withstand tremendous physical shock and is reasonably fast (the flash memory solutions covered in this article are slightly slower than a EIDE hard disk for write operations, and much faster for read operations). One very important aspect of flash memory, the ramifications of which will be discussed later in this article, is that each sector has a limited rewrite capacity. You can only write, erase, and write again to a sector of flash memory a certain number of times before the sector becomes permanently unusable. Although many flash memory products automatically map bad blocks, and although some even distribute write operations evenly throughout the unit, the fact remains that there exists a limit to the amount of writing that can be done to the device. Competitive units have between 1,000,000 and 10,000,000 writes per sector in their specification. This figure varies due to the temperature of the environment.
Specifically, we will be discussing ATA compatible compact-flash units, which are quite popular as storage media for digital cameras. Of particular interest is the fact that they pin out directly to the IDE bus and are compatible with the ATA command set. Therefore, with a very simple and low-cost adaptor, these devices can be attached directly to an IDE bus in a computer. Once implemented in this manner, operating systems such as FreeBSD see the device as a normal hard disk (albeit small).
Other solid state disk solutions do exist, but their expense, obscurity, and relative unease of use places them beyond the scope of this article.
Kernel Options
A few kernel options are of specific interest to those creating an embedded FreeBSD system.
All embedded FreeBSD systems that use flash memory as system disk will be interested in memory disks and memory filesystems. Because of the limited number of writes that can be done to flash memory, the disk and the filesystems on the disk will most likely be mounted read-only. In this environment, filesystems such as <filename>/tmp</filename> and <filename>/var</filename> are mounted as memory filesystems to allow the system to create logs and update counters and temporary files. Memory filesystems are a critical component to a successful solid state FreeBSD implementation.
You should make sure the following lines exist in your kernel configuration file:
ComponentTranslation
This translation Translated FreeBSD Doc/articles_solid-state Redistribution and use in source (XML DocBook) and 'compiled' forms (XML, HTML, PDF, PostScript, RTF and so forth) with or without modification, are permitted provided that the following conditions are met:
The following strings have the same context and source.
Translated FreeBSD Doc/books_faq Redistribution and use in source (XML DocBook) and 'compiled' forms (XML, HTML, PDF, PostScript, RTF and so forth) with or without modification, are permitted provided that the following conditions are met:
Translated FreeBSD Doc/books_arch-handbook Redistribution and use in source (XML DocBook) and 'compiled' forms (XML, HTML, PDF, PostScript, RTF and so forth) with or without modification, are permitted provided that the following conditions are met:
Translated FreeBSD Doc/books_handbook Redistribution and use in source (XML DocBook) and 'compiled' forms (XML, HTML, PDF, PostScript, RTF and so forth) with or without modification, are permitted provided that the following conditions are met:
Translated FreeBSD Doc/books_developers-handbook Redistribution and use in source (XML DocBook) and 'compiled' forms (XML, HTML, PDF, PostScript, RTF and so forth) with or without modification, are permitted provided that the following conditions are met:
Translated FreeBSD Doc/books_fdp-primer Redistribution and use in source (XML DocBook) and 'compiled' forms (XML, HTML, PDF, PostScript, RTF and so forth) with or without modification, are permitted provided that the following conditions are met:
Translated FreeBSD Doc/books_porters-handbook Redistribution and use in source (XML DocBook) and 'compiled' forms (XML, HTML, PDF, PostScript, RTF and so forth) with or without modification, are permitted provided that the following conditions are met:

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(itstool) path: legalnotice/para

Flags
read-only
Source string location
article.translate.xml:72
String age
a year ago
Source string age
a year ago
Translation file
articles/solid-state.pot, string 9