Add a short article on how FreeBSD handles leap seconds. Thanks for
contributions from Peter Jeremy, Poul-Henning Kamp, Ian Lepore, Xin LI, and Warner Losh. Requested by Bartek Rutkowski.
This commit is contained in:
Warren Block
parent
45b36e4f1e
commit
349ae4118b
Notes:
svn2git
2020-12-08 03:00:23 +00:00
svn path=/head/; revision=46873
5 changed files with 144 additions and 0 deletions
|
|
@ -17,6 +17,7 @@ SUBDIR+= gjournal-desktop
|
|||
SUBDIR+= hubs
|
||||
SUBDIR+= ipsec-must
|
||||
SUBDIR+= ldap-auth
|
||||
SUBDIR+= leap-seconds
|
||||
SUBDIR+= linux-emulation
|
||||
SUBDIR+= linux-users
|
||||
SUBDIR+= mailing-list-faq
|
||||
|
|
|
|||
19
en_US.ISO8859-1/articles/leap-seconds/Makefile
Normal file
19
en_US.ISO8859-1/articles/leap-seconds/Makefile
Normal file
|
|
@ -0,0 +1,19 @@
|
|||
#
|
||||
# $FreeBSD$
|
||||
#
|
||||
# Article: Leap Seconds
|
||||
|
||||
DOC?= article
|
||||
|
||||
FORMATS?= html
|
||||
WITH_ARTICLE_TOC?= YES
|
||||
|
||||
INSTALL_COMPRESSED?= gz
|
||||
INSTALL_ONLY_COMPRESSED?=
|
||||
|
||||
SRCS= article.xml
|
||||
|
||||
URL_RELPREFIX?= ../../../..
|
||||
DOC_PREFIX?= ${.CURDIR}/../../..
|
||||
|
||||
.include "${DOC_PREFIX}/share/mk/doc.project.mk"
|
||||
118
en_US.ISO8859-1/articles/leap-seconds/article.xml
Normal file
118
en_US.ISO8859-1/articles/leap-seconds/article.xml
Normal file
|
|
@ -0,0 +1,118 @@
|
|||
<?xml version="1.0" encoding="iso-8859-1"?>
|
||||
<!DOCTYPE article PUBLIC "-//FreeBSD//DTD DocBook XML V5.0-Based Extension//EN"
|
||||
"http://www.FreeBSD.org/XML/share/xml/freebsd50.dtd">
|
||||
|
||||
<article xmlns="http://docbook.org/ns/docbook"
|
||||
xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0"
|
||||
xml:id="leapseconds">
|
||||
|
||||
<info>
|
||||
<title>&os; Support for Leap Seconds</title>
|
||||
|
||||
<pubdate>$FreeBSD$</pubdate>
|
||||
</info>
|
||||
|
||||
<sect1 xml:id="leapseconds-definition">
|
||||
<title>Introduction</title>
|
||||
|
||||
<para>A <emphasis>leap second</emphasis> is an ad-hoc one-second
|
||||
correction to synchronize atomic timescales with Earth rotation.
|
||||
This article describes how &os; interacts with leap
|
||||
seconds.</para>
|
||||
|
||||
<para>As of this writing, the next leap second will occur at
|
||||
2015-Jun-30 23:59:60. This particular leap second is unusual,
|
||||
being in the middle of a business day for North and South
|
||||
America and the Asia/Pacific region.</para>
|
||||
|
||||
<para>Leap seconds are announced by <link
|
||||
xlink:href="http://datacenter.iers.org/"><acronym>IERS</acronym></link>
|
||||
on <link
|
||||
xlink:href="http://datacenter.iers.org/web/guest/bulletins/-/somos/5Rgv/product/16">Bulletin
|
||||
C</link>.</para>
|
||||
|
||||
<para>Standard leap second behavior is described in <link
|
||||
xlink:href="https://tools.ietf.org/html/rfc7164#section-3">RFC
|
||||
7164</link>. Also see &man.time2posix.3;.</para>
|
||||
</sect1>
|
||||
|
||||
<sect1 xml:id="leapseconds-posix">
|
||||
<title>Default Leap Second Handling on &os;</title>
|
||||
|
||||
<para>The easiest way to handle leap seconds is with the
|
||||
<acronym>POSIX</acronym> time rules &os; uses by default,
|
||||
combined with <link
|
||||
xlink:href="&url.books.handbook;/network-ntp.html"><acronym>NTP</acronym></link>.
|
||||
When &man.ntpd.8; is running and the time is synchronized with
|
||||
upstream <acronym>NTP</acronym> servers that handle leap seconds
|
||||
correctly, the leap second will cause the system time to
|
||||
automatically repeat the last second of the day. No other
|
||||
adjustments are necessary.</para>
|
||||
|
||||
<para>If the upstream <acronym>NTP</acronym> servers do not handle
|
||||
leap seconds correctly, &man.ntpd.8; will step the time by one
|
||||
second after the errant upstream server has noticed and stepped
|
||||
itself.</para>
|
||||
|
||||
<para>If <acronym>NTP</acronym> is not being used, manual
|
||||
adjustment of the system clock will be required after the leap
|
||||
second has passed.</para>
|
||||
</sect1>
|
||||
|
||||
<sect1 xml:id="leapseconds-cautions">
|
||||
<title>Cautions</title>
|
||||
|
||||
<para>Leap seconds are inserted at the same instant all over the
|
||||
world: <acronym>UTC</acronym> midnight. In Japan that is
|
||||
mid-morning, in the Pacific mid-day, in the Americas late
|
||||
afternoon, and in Europe at night.</para>
|
||||
|
||||
<para>We believe and expect that &os;, if provided correct and
|
||||
stable <acronym>NTP</acronym> service, will work as designed
|
||||
during this leap second, as it did during the previous
|
||||
ones.</para>
|
||||
|
||||
<para>However, we caution that practically no applications have
|
||||
ever asked the kernel about leap seconds. Our experience is
|
||||
that, as designed, leap seconds are essentially a replay of the
|
||||
second before the leap second, and this is a surprise to most
|
||||
application programmers.</para>
|
||||
|
||||
<para>Other operating systems and other computers may or may not
|
||||
handle the leap-second the same way as &os;, and systems without
|
||||
correct and stable <acronym>NTP</acronym> service will not know
|
||||
anything about leap seconds at all.</para>
|
||||
|
||||
<para>It is not unheard of for computers to crash because of
|
||||
leap seconds, and experience has shown that a large fraction
|
||||
of all public <acronym>NTP</acronym> servers might handle and
|
||||
announce the leap second incorrectly.</para>
|
||||
|
||||
<para>Please try to make sure nothing horrible happens because of
|
||||
the leap second.</para>
|
||||
</sect1>
|
||||
|
||||
<sect1 xml:id="leapseconds-testing">
|
||||
<title>Testing</title>
|
||||
|
||||
<para>It is possible to test whether a leap second will be used.
|
||||
Due to the nature of <acronym>NTP</acronym>, the test might work
|
||||
up to 24 hours before the leap second. Some major reference
|
||||
clock sources only announce leap seconds one hour ahead of the
|
||||
event. Query the <acronym>NTP</acronym> daemon:</para>
|
||||
|
||||
<screen>&prompt.user; <userinput>ntpq -c 'rv 0 leap'</userinput></screen>
|
||||
|
||||
<para>Output that includes <literal>leap_add_sec</literal>
|
||||
indicates proper support of the leap second. Before the 24 hour
|
||||
period before the leap second, or after the leap second has
|
||||
passed, <literal>leap_none</literal> will be reported.</para>
|
||||
</sect1>
|
||||
|
||||
<sect1 xml:id="leapseconds-conclusion">
|
||||
<title>Conclusion</title>
|
||||
|
||||
<para>We hope this discussion of leap seconds on &os; has been of
|
||||
use. Now let us never speak of it again.</para>
|
||||
</sect1>
|
||||
</article>
|
||||
|
|
@ -170,6 +170,10 @@
|
|||
A practical guide about setting up an LDAP server on &os; and
|
||||
how to use it for authenticating users.</p>
|
||||
|
||||
<p><a href="&url.articles;/leap-seconds/index.html">&os; Support
|
||||
for Leap Seconds</a> (leap-seconds)<br/>
|
||||
A short description of how leap seconds are handled on &os;.</p>
|
||||
|
||||
<p><a href="&url.articles;/linux-emulation/index.html">Linux
|
||||
emulation in &os;</a> (linux-emulation)<br/>
|
||||
A technical description about the internals of the Linux
|
||||
|
|
|
|||
|
|
@ -75,6 +75,8 @@
|
|||
<!ENTITY url.articles.ipsec-must.en "&url.doc.langbase.en;/articles/ipsec-must">
|
||||
<!ENTITY url.articles.ldap-auth "&url.doc.langbase;/articles/ldap-auth">
|
||||
<!ENTITY url.articles.ldap-auth.en "&url.doc.langbase.en;/articles/ldap-auth">
|
||||
<!ENTITY url.articles.leap-seconds "&url.doc.langbase;/articles/leap-seconds">
|
||||
<!ENTITY url.articles.leap-seconds.en "&url.doc.langbase.en;/articles/leap-seconds">
|
||||
<!ENTITY url.articles.linux-emulation "&url.doc.langbase;/articles/linux-emulation">
|
||||
<!ENTITY url.articles.linux-emulation.en "&url.doc.langbase.en;/articles/linux-emulation">
|
||||
<!ENTITY url.articles.linux-users "&url.doc.langbase;/articles/linux-users">
|
||||
|
|
|
|||
Loading…
Reference in a new issue