Re: mdadm usage

From	Andy Smith <andy@strugglers.net>
Newsgroups	linux.debian.user
Subject	Re: mdadm usage
Date	2020-12-30 15:00 +0100
Message-ID	<BrKb8-47p-25@gated-at.bofh.it> (permalink)
References	(2 earlier) <BriHU-2Cq-7@gated-at.bofh.it> <BrtWG-262-7@gated-at.bofh.it> <Bry0i-4LA-7@gated-at.bofh.it> <Brytj-5as-1@gated-at.bofh.it> <BrJRL-40i-13@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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Hello,

On Wed, Dec 30, 2020 at 02:38:46PM +0100, Jesper Dybdal wrote:
> I would hope that the most recently modified half of the array would be the
> one to overwrite the least recently modified one, so that a temporary
> absence of one disk which later comes back unmodified, will not destroy
> data.

Consider what happens if you take an mdadm RAID-1 member and put it
in another machine, mount it and then start writing to it. The
events count of the device will increase with each write.

If you then take that device and put it back in the original
machine, and it has a higher event count than the device in the
machine already, on next assemble mdadm will overwrite the device
that has the lower event count.

You can see the event count with:

# mdadm --examine /dev/sda1 # or whatever the member device is

So yes in one way your idea that the most recently modified half is
the one chosen could be said to be correct, if by "most recently
modified" you actually mean "most number of events".

As you were thinking, it is pretty safe to do if you never write to
the device you take out.

Cheers,
Andy

-- 
https://bitfolk.com/ -- No-nonsense VPS hosting

Thread

mdadm usage "Thomas A. Anderson" <thomas.anderson@little-beak.com> - 2020-12-29 08:10 +0100
  Re: mdadm usage Reco <recoverym4n@enotuniq.net> - 2020-12-29 08:30 +0100
    Re: mdadm usage deloptes <deloptes@gmail.com> - 2020-12-29 09:40 +0100
  Re: mdadm usage Felix Miata <mrmazda@earthlink.net> - 2020-12-29 08:50 +0100
    Re: mdadm usage basti <mailinglist@unix-solution.de> - 2020-12-29 09:40 +0100
      Re: mdadm usage "Thomas A. Anderson" <thomas.anderson@little-beak.com> - 2020-12-29 21:40 +0100
        Re: mdadm usage Linux-Fan <Ma_Sys.ma@web.de> - 2020-12-30 00:00 +0100
        Re: mdadm usage deloptes <deloptes@gmail.com> - 2020-12-30 02:00 +0100
          Re: mdadm usage Michael Stone <mstone@debian.org> - 2020-12-30 02:30 +0100
            Re: mdadm usage deloptes <deloptes@gmail.com> - 2020-12-30 09:20 +0100
            Re: mdadm usage Jesper Dybdal <jd-debian-user@dybdal.dk> - 2020-12-30 14:40 +0100
              Re: mdadm usage Andy Smith <andy@strugglers.net> - 2020-12-30 15:00 +0100
                Re: mdadm usage Jesper Dybdal <jd-debian-user@dybdal.dk> - 2021-01-02 14:40 +0100
              mdadm usage Nicholas Geovanis <nickgeovanis@gmail.com> - 2020-12-30 15:10 +0100
                Re: mdadm usage Nicholas Geovanis <nickgeovanis@gmail.com> - 2020-12-30 15:10 +0100
                Re: mdadm usage deloptes <deloptes@gmail.com> - 2020-12-30 21:50 +0100
                Re: mdadm usage Michael Stone <mstone@debian.org> - 2020-12-30 22:40 +0100
                Re: mdadm usage deloptes <deloptes@gmail.com> - 2020-12-30 23:20 +0100
                Re: mdadm usage Andy Smith <andy@strugglers.net> - 2020-12-31 00:30 +0100
                Re: mdadm usage rhkramer@gmail.com - 2020-12-31 13:30 +0100
                Re: mdadm usage Michael Stone <mstone@debian.org> - 2020-12-31 15:20 +0100
                Re: mdadm usage Andy Smith <andy@strugglers.net> - 2020-12-31 15:30 +0100
                Re: mdadm usage Reco <recoverym4n@enotuniq.net> - 2020-12-31 15:50 +0100
                Re: mdadm usage Andy Smith <andy@strugglers.net> - 2020-12-31 16:10 +0100
                Re: mdadm usage Reco <recoverym4n@enotuniq.net> - 2020-12-31 16:20 +0100
                Re: mdadm usage Andy Smith <andy@strugglers.net> - 2020-12-31 16:40 +0100
                Re: mdadm usage deloptes <deloptes@gmail.com> - 2021-01-01 23:10 +0100
                Re: mdadm usage Andrei POPESCU <andreimpopescu@gmail.com> - 2021-01-02 09:40 +0100
                Re: mdadm usage deloptes <deloptes@gmail.com> - 2021-01-02 10:10 +0100
                Re: mdadm usage Andrei POPESCU <andreimpopescu@gmail.com> - 2021-01-02 11:30 +0100
                Re: mdadm usage Linux-Fan <Ma_Sys.ma@web.de> - 2021-01-02 14:50 +0100
                Re: mdadm usage deloptes <deloptes@gmail.com> - 2021-01-02 16:40 +0100
                Re: mdadm usage Linux-Fan <Ma_Sys.ma@web.de> - 2021-01-02 18:10 +0100
                Re: mdadm usage The Wanderer <wanderer@fastmail.fm> - 2021-01-02 10:40 +0100
                Re: mdadm usage Andrei POPESCU <andreimpopescu@gmail.com> - 2021-01-02 12:00 +0100
                Re: mdadm usage The Wanderer <wanderer@fastmail.fm> - 2021-01-02 12:30 +0100
                Re: mdadm usage Andrei POPESCU <andreimpopescu@gmail.com> - 2021-01-02 12:50 +0100
                Re: mdadm usage Andrei POPESCU <andreimpopescu@gmail.com> - 2021-01-02 13:10 +0100
                Re: mdadm usage The Wanderer <wanderer@fastmail.fm> - 2021-01-02 14:20 +0100
                Re: mdadm usage The Wanderer <wanderer@fastmail.fm> - 2021-01-02 14:10 +0100
                Re: mdadm usage deloptes <deloptes@gmail.com> - 2021-01-02 12:50 +0100
          Re: mdadm usage "Thomas A. Anderson" <thomas.anderson@little-beak.com> - 2020-12-30 11:50 +0100
            Re: mdadm usage "Alexander V. Makartsev" <avbetev@gmail.com> - 2020-12-30 13:30 +0100
            Re: mdadm usage Dan Ritter <dsr@randomstring.org> - 2020-12-30 13:30 +0100
            Re: mdadm usage Andy Smith <andy@strugglers.net> - 2020-12-30 15:00 +0100
            Re: mdadm usage deloptes <deloptes@gmail.com> - 2020-12-30 22:10 +0100
              Re: mdadm usage Nicholas Geovanis <nickgeovanis@gmail.com> - 2020-12-31 00:00 +0100
        Re: mdadm usage Andrei POPESCU <andreimpopescu@gmail.com> - 2020-12-30 09:10 +0100
      Re: mdadm usage Dan Ritter <dsr@randomstring.org> - 2020-12-30 04:00 +0100
  Re: mdadm usage Andy Smith <andy@strugglers.net> - 2020-12-29 14:20 +0100
    Re: mdadm usage mick crane <mick.crane@gmail.com> - 2020-12-29 16:40 +0100
      Re: mdadm usage "Alexander V. Makartsev" <avbetev@gmail.com> - 2020-12-29 20:40 +0100
      Re: mdadm usage Andy Smith <andy@strugglers.net> - 2020-12-30 14:40 +0100
        Re: mdadm usage The Wanderer <wanderer@fastmail.fm> - 2020-12-30 14:50 +0100
        Re: mdadm usage mick crane <mick.crane@gmail.com> - 2020-12-30 18:00 +0100
          Re: mdadm usage Andrei POPESCU <andreimpopescu@gmail.com> - 2020-12-30 19:40 +0100

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