Your message dated Wed, 30 Jul 2014 23:19:29 +0000
with message-id <[email protected]>
and subject line Bug#263116: fixed in util-linux 2.25-5
has caused the Debian Bug report #263116,
regarding util-linux: hwclock documentation misleading
to be marked as done.
This means that you claim that the problem has been dealt with.
If this is not the case it is now your responsibility to reopen the
Bug report if necessary, and/or fix the problem forthwith.
(NB: If you are a system administrator and have no idea what this
message is talking about, this may indicate a serious mail system
misconfiguration somewhere. Please contact [email protected]
immediately.)
--
263116: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=263116
Debian Bug Tracking System
Contact [email protected] with problems
--- Begin Message ---
Package: util-linux
Version: 2.12-3_i386.
The manual page explains it OK but the Debian initialization file will
interfere with following what's in the man page. The documentation in
/usr/share/doc/util-linux/README.Debian.hwclock.gz is erroneous (as are
the comments in /etc/init.d/hwclock.sh). Sorry to be so blunt but my
previous report (either as a bug or directly to the maintainer) on this
topic was not satisfactorily dealt with.
README.Debian.hwclock.gz says:
>
> A Debian installation will, by default, call hwclock --hctosys during system
> startup and hwclock --systohc during system shutdown.
>
> To set the date/time of the system, just use the standard UNIX date facilities
> (such as date) or any of the advanced timekeeping utilities (ntp, ntpdate,
> chrony). Other methods of setting the clock (such as hwclock) are likely to
> cause trouble, do not use them.
What about someone that just reads the man page? It's bad to have
a contradiction between the man page and the README doc.
> Please note that because the shutdown scripts call hwclock --systohc, you
> cannot set the clock using hwclock only, as your adjustment will be lost on
> the next reboot. THIS MEANS YOU MUST *NOT* FOLLOW THE PROCEDURES IN THE
> hwclock(8) MAN PAGE TO SET THE CLOCK DATE/TIME USING A REBOOT UNLESS YOU
> EDIT THE SHUTDOWN SCRIPTS.
>
>
> The full story:
It's by no means the full story.
>
> A Linux system actually has two clocks:
>
> - The System Clock, kept by the kernel. This is the clock that Linux
> uses for day-to-day activities, and this is also the clock you set
> using date.
>
> - The Hardware Clock, also called RTC, which is used as a backup to
> keep time while the computer is turned off, or in APM suspended
> state. This is the clock you set using hwclock --set.
>
> (you can get more information about these two clocks in the hwclock(8)
> man page).
>
> hwclock is used to copy time between these two clocks. For the Debian
> standard install, the system clock is initialized with the value of
> the hardware clock during startup, and the value of the system clock
> is copied back to the hardware clock during system shutdown/reboot.
>
> So, in a Debian default install, you can keep the illusion that
> there's a single clock. Unless you use a program that modifies the
> hardware clock directly and does not set the system clock as well,
> that is.
>
>
> Issues with hwclock --adjust:
>
> hwclock has a facility to try to correct for systematic drift in the
> hardware clock, accessed by hwclock --adjust. This facility is
> *dangerous* because it has a severe drawback: it assumes that no
> program other than hwclock --systohc will ever be used to change the
> hardware clock.
No. Even if all you ever use is --systohc, the hwclock --adjust will
create huge errors in your system (and hw) time. Here's why. At each
shutdown a calibration is done since --systohc is run. If it's been over
a day since --systohc was last run, /etc/adjtime is modified. This
modification assumes that the systime is correct but it isn't.
For example: You set the hw and sys clocks exactly on day 0 and the
drift factor in /etc/adjtime is 0, but it should be 3 since the hwclock
loses 3 sec/day. Then you shut down your PC and on day 2 use your PC
again for a short time. The hw clock will then be 6 sec slow and so
will the sys clock. At shutdown the calibration routine will note that
the sys and hw clocks are about the same and thus establish a very small
drift factor, say 0.1 when it should be 3. Then you continue like this,
using your PC every day or two without using any commands to set time.
By day 30 (the end of the month) the hwclock time is almost 90 sec slow
so then you set the sys time manually to the correct value. Now the 90
sec took about a month to accumulate but since a previous calibration
has been made about a day ago, the hwclock program erroneously thinks
that this 90 sec slowness happened in one day and sets the drift factor
to 90 which will overcompensate for the 3 sec/day slowness by 87
sec/day! If then you repeat this whole process over again, your clock
will eventually be many hours in error.
The drift factor, instead of converging to a correct value, is in fact
rapidly diverging. Very bad.
>
> This assumption is often false, as many common utilities such as ntp,
> chrony, as well as your computer's System Setup BIOS program, and any
> other OS you have in your machine will change the clock.
Well, just don't use them then.
>
> Also, if hwclock --adjust is used, one must make sure the drift file
> (/etc/adjtime) is deleted every time the system clock is set to a very
> different value (even if you're using hwclock itself to do it!), or
> the drift computation might become invalid and cause the hardware
> clock to be incorrectly set the next time hwclock --adjust is used.
>
> hwclock currently does not perform any sort of sanity checks in the
> values it uses to compute the drift file, and will corrupt your clock
> time by potentially very large amounts if anything goes wrong.
>
> Don't use the hwclock --adjust facility, refer to alternate (and much
> safer) programs such as ntp or chrony if you need precision
> timekeeping.
It is safe if you remove the running of --systohc at shutdown and then
use only --set to change the hc time. --systohc should only run at
shutdown if you don't use --adjust. And without using --adjust you will
need to make time corrections much more often. So --adjust can save a
lot of bother keeping the clock running correctly, provided the
documentation is clear.
Also, in the script it echoes "Saving the System Clock time to the
Hardware Clock..." when in fact this may not happen due to the
environment variables: HWCLOCKACCESS or HWCLOCKSET
--- End Message ---
--- Begin Message ---
Source: util-linux
Source-Version: 2.25-5
We believe that the bug you reported is fixed in the latest version of
util-linux, which is due to be installed in the Debian FTP archive.
A summary of the changes between this version and the previous one is
attached.
Thank you for reporting the bug, which will now be closed. If you
have further comments please address them to [email protected],
and the maintainer will reopen the bug report if appropriate.
Debian distribution maintenance software
pp.
Andreas Henriksson <[email protected]> (supplier of updated util-linux package)
(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing [email protected])
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
Format: 1.8
Date: Thu, 31 Jul 2014 00:55:15 +0200
Source: util-linux
Binary: util-linux util-linux-locales mount bsdutils fdisk-udeb cfdisk-udeb
libblkid1 libblkid1-udeb libblkid-dev libmount1 libmount-dev libsmartcols1
libsmartcols1-udeb libsmartcols-dev libuuid1 uuid-runtime libuuid1-udeb
uuid-dev util-linux-udeb
Architecture: source amd64 all
Version: 2.25-5
Distribution: experimental
Urgency: medium
Maintainer: Debian util-linux Maintainers <[email protected]>
Changed-By: Andreas Henriksson <[email protected]>
Description:
bsdutils - basic utilities from 4.4BSD-Lite
cfdisk-udeb - Manually partition a hard drive (cfdisk) (udeb)
fdisk-udeb - Manually partition a hard drive (fdisk) (udeb)
libblkid-dev - block device id library - headers and static libraries
libblkid1 - block device id library
libblkid1-udeb - stripped down block device id library, for debian-installer
(udeb)
libmount-dev - device mounting library - headers and static libraries
libmount1 - device mounting library
libsmartcols-dev - smart column output alignment library - headers and static
librar
libsmartcols1 - smart column output alignment library
libsmartcols1-udeb - stripped down smart column output aligment library, for
debian-in (udeb)
libuuid1 - Universally Unique ID library
libuuid1-udeb - stripped down universally unique id library, for
debian-installer (udeb)
mount - Tools for mounting and manipulating filesystems
util-linux - Miscellaneous system utilities
util-linux-locales - Locales files for util-linux
util-linux-udeb - stripped down miscellaneous system utilities, for
debian-installe (udeb)
uuid-dev - universally unique id library - headers and static libraries
uuid-runtime - runtime components for the Universally Unique ID library
Closes: 263116 734544
Changes:
util-linux (2.25-5) experimental; urgency=medium
.
The "big maintainer-script cleanup" release
.
* Drop debian/uuid-runtime.prerm (and related lintian override)
- dh_installinit will automatically start and stop services as needed.
* Drop debian/libuuid1.postinst (user/group addition)
* uuid-runtime: improved user/group handling (Closes: #734544)
- pre-depend on new libuuid1 to make sure no old user handling is present
- add code to rename existing libuuid user/group to uuidd
and set nologin shell and new home directory.
- switch to adduser instead of opencoding it since uuid-runtime
is Priority: optional (as opposed to libuuid1 which is required)
and adduser --system should just do the right thing.
- change user/group addition to add uuidd instead of libuuid.
- stop making uuidd setuid, not needed and we don't want anyone to
be able to kill the daemon (via uuidd -k) for example.
* Drop d/p/Use-libuuid-user-group-in-sysvinit-script-systemd-un.patch
* util-linux: drop obsolete hwclock handling from maint-scripts
* util-linux: drop obsolete update-mime calls
* util-linux: drop obsolete 2.17 upgrade warning
* util-linux: drop obsolete /etc/default/rcS -> /etc/adjtime migration
* Reindent/cleanup all maintainer scripts
* Drop outdated debian/README.Debian.hwclock (Closes: #263116)
* Drop unused debian/rejected-upstream
* Drop outdated debian/uuid-dev.README.Debian
* Drop diffutils build-dependency
* Drop debian/*.dirs
* Attempt to avoid dumb term problem in "more: regexp" test
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