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List:       hpux-admin
Subject:    [HPADM] Summary Time Change
From:       "Rogan, Julian" <jqr18000 () glaxowellcome ! co ! uk>
Date:       2001-01-31 11:08:25
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I think I have recieved a definitive reply from Bill Hassel and I will also
include some other. Many thanks.

Julian



Hi,


> If we are running backups while the server's time skips FORWARD due to
time
> zone settings  ($TZ) what effect will this have on the backup?


  The HP-UX time of day NEVER skips.  HP-UX keeps time as UTC (Universal
  Coordinated Time or Zulu or Greenwich Mean Time).  The timezone variable
  $TZ will modify the time  of day returned by system calls and the date
  command to represent what the time would be in that particular time
  zone.  It will also modify the date command's time value when changing
  the date.

  For a database, it is critically incumbent on the DBA to account for
  proper time of day relationships.  Many database programs allow for
  time/date to be represented as Zulu time, thus avoiding all issues
  with Daylight Savings, where the client and/or middleware software
  performs an intelligent translation for displays and reports.  If you
  want to start an interesting discussion with your DBA(s), ask the
  question about what affect timezone changes will have and how the
  database handles time relationships between records and indexes.

  Then ask the DBA(s) what happens if some root operator changes the
  time of day either forward or backward...you might then want to remove
  the time-setting capability of the date command with a wrapper.  (If
  you do, be sure to check /sbin/init.d for any references to date so
  that bootup will not be affected.  The solution for database time
  issues is to use NTP and point the server(s) clock to your firewall or
  open Internet NTP time source.  Be sure to get the latest NTP patch
  that adds the -B option to ntpdate (and also see the 11.0 date command
  option -a).  This allows slewing the HP-UX clock slowly (50ms per
  second either direction) until the time is accurate.  This prevents
  timestamps from experiencing step changes in time.

  For a backup, all the timestamps are kept based on the filesystem
  and whatever backup software you are using.  Simple tools like
  tar and cpio just pick up the inode information and store it with
  each file, while commercial quality tools like Omniback will keep
  details about the actual time of day when each file was saved.

  When files are restored, some backup programs have the option to
  retain creation/modification dates (as saved from the inodes on
  disk), or to timestamp the file with the current time.  But in
  no case (simple tools or commercial backup software) will the
  time of day when a file was backed up, be used to change data
  inside the file or to change the restored file's timestamps.
  Such information might be used to determine relationships between
  two files on a single backup (for instance, which file was copied 
  first) but otherwise, not too useful.

--
Bill Hassell
Remote Engineering Services
HP Atlanta Response Center
email:  blh@atl.hp.com


=======================================================

I'm not an expert, but I'd say it will have no effect at all. The time zone
setting
is a convenience for the user - The system tracks time internally as UTC,
and
uses the TZ to adjust the timestamps you view to local time. I'd think the
information recorded on the backup is the same way. If you had changed the
actualy system clock, that would be a different matter, but if you just
altered
the TZ, I think you're OK. 

I may be wrong. It will be interesting to see what other folks say...

Cheers,
Tom Leber


There should be no effect at all because time stamps on files are kept in
seconds since Jan 1, 1970 UTC.  The various system calls which convert this
for display use the current TZ to interpret this unweildy number for the
benefit of the human viewer.

					Cheers, Tom Myers
--


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