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RAID flakiness according to HOWTOs?

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  • Nick Rothwell
    I m planning to buy a Slug plus a couple of large disks and use it as a RAID1-based server. I ve read through the various HOWTOs and they seem to make sense.
    Message 1 of 5 , Sep 23, 2007
      I'm planning to buy a Slug plus a couple of large disks and use it as
      a RAID1-based server.

      I've read through the various HOWTOs and they seem to make sense.
      (I've got RAID1 running on a handful of Linux boxes here, so I'm
      familiar with the procedures.) But I'm somewhat dismayed by the
      statements at the end of

      http://www.nslu2-linux.org/wiki/HowTo/Raid1onUnslung6

      which claim that the array needs to be manually reconstructed after a
      power loss, and that one might need to replace a disk if it was doing
      I/O when the power went out. I thought the whole point of RAIDn and
      journalled filesystems was to provide some robustness? What's all
      this about losing disks and/or having to reconstruct everything?

      (I take on board the long resync times if a massive partition isn't
      closed down cleanly, but if the system boots from a small partition,
      I don't see why I should have to worry about this stuff - it should
      just work. No?)

      -- Nick.


      Nick Rothwell / Cassiel.com Limited
      www.cassiel.com
      www.myspace.com/cassieldotcom
      www.loadbang.net
    • Mike (mwester)
      ... Not recommended. The bandwidth available to the drives is very low, and performance will be dismal. You re much better off using the drives mounted as
      Message 2 of 5 , Sep 23, 2007
        > I'm planning to buy a Slug plus a couple of large disks and use it as
        > a RAID1-based server.

        Not recommended. The bandwidth available to the drives is very low, and
        performance will be dismal. You're much better off using the drives mounted
        as separate partitions with an rsync solution to copy one to the other.

        > I've read through the various HOWTOs and they seem to make sense.
        > (I've got RAID1 running on a handful of Linux boxes here, so I'm
        > familiar with the procedures.) But I'm somewhat dismayed by the
        > statements at the end of
        >
        > http://www.nslu2-linux.org/wiki/HowTo/Raid1onUnslung6
        >
        > which claim that the array needs to be manually reconstructed after a
        > power loss, and that one might need to replace a disk if it was doing
        > I/O when the power went out. I thought the whole point of RAIDn and
        > journalled filesystems was to provide some robustness? What's all
        > this about losing disks and/or having to reconstruct everything?

        RAID on Unslung is something that one does only so that one can say one has
        done it. Your points are completely valid, RAID on Unslung fails to meet
        the intent of using RAID in the first place. The fundamental problem is
        that Unslung is nothing much more than "opened up" Linksys firmware.
        Therefore, it must live with all the restrictions along with all the neat
        features. One of those restrictions is that the Linksys firmware lacks any
        sort of proper shutdown mechanism; it's really little more than a controlled
        crash. No matter what you do, you cannot get around the fact that the root
        filesystem is still mounted and active at the time the "halt" command simply
        turns off the power to the device. This leaves you with a filesystem that
        needs to be synced at each boot.

        There was some work done to see if it was possible to implement the normal
        startup/shutdown mechanism for Unslung, but that was abandoned when it
        became clear that adding such a mechanism was basically turning Unslung into
        SlugOS with a 2.4 kernel -- hardly desirable.

        Please note that the above applies to Unslung, not to the NSLU2 itself. If
        you run SlugOS or Debian/ARM instead of Unslung, well, then you have a real,
        full, Linux implementation. RAID will work as you expect, with the only
        stipulation being that it will be slow. It will be reliable, and robust,
        however (at least as robust as USB-based storage can be, I guess).

        > (I take on board the long resync times if a massive partition isn't
        > closed down cleanly, but if the system boots from a small partition,
        > I don't see why I should have to worry about this stuff - it should
        > just work. No?)

        No. :) It's Unslung - it's not a proper Linux system, it's
        Linksys-compatible instead! You want SlugOS or Debian. Or, if performance
        with RAID is important to you, then you'll want something other than
        USB-attached drives.

        Mike (mwester)
      • Nick Rothwell
        ... Ah: solid advice. We like that... ... I would already be expecting pretty dismal performance from the Slug anyway (around 3Mb/sec?). If adding the RAID
        Message 3 of 5 , Sep 24, 2007
          > > I'm planning to buy a Slug plus a couple of large disks and use
          > it as
          > > a RAID1-based server.
          >
          > Not recommended.
          Ah: solid advice. We like that...
          > The bandwidth available to the drives is very low, and
          > performance will be dismal. You're much better off using the drives
          > mounted
          > as separate partitions with an rsync solution to copy one to the
          > other.
          I would already be expecting pretty dismal performance from the Slug
          anyway (around 3Mb/sec?). If adding the RAID layer makes it even
          worse then, yes, perhaps rsyncing might be a better idea.
          > RAID on Unslung is something that one does only so that one can say
          > one has
          > done it. Your points are completely valid, RAID on Unslung fails to
          > meet
          > the intent of using RAID in the first place.
          A colleague of mine is running a Slug + Unslung with a 3-disk RAID5
          setup, which is why I was lead to assume that it made reasonable
          sense. Certainly, the Wiki suggests that lots of people are happily
          running Unslung with RAID (perhaps with more confidence than is
          warranted!).
          > No. :) It's Unslung - it's not a proper Linux system, it's
          > Linksys-compatible instead! You want SlugOS or Debian.
          I've had a read through the installation notes for Debian, and it
          looks a lot cleaner and more familiar than the Unslung setup, so
          that's probably the route to go.

          As for USB performance: well, I suppose I could wait for a Debian
          port to the NAS200 and use SATA disks, but that could take a while,
          and from what I read, that box is even slower than a Slug...
          Alternatively, I could go slightly up-market and build another Mini-
          ITX system; I have a handful of them here, but they're just a little
          bit too noisy.

          Thanks for the reply.

          -- N.


          Nick Rothwell / Cassiel.com Limited
          www.cassiel.com
          www.myspace.com/cassieldotcom
          www.loadbang.net
        • Sean Gibbins
          ... Hi Nick, As much as I love my Slug I hanker after the Epia 5000 based Debian server that I sold on. Yes it was a bit noisy but I wish now that I had
          Message 4 of 5 , Sep 24, 2007
            Nick Rothwell wrote:
            >
            >
            > As for USB performance: well, I suppose I could wait for a Debian
            > port to the NAS200 and use SATA disks, but that could take a while,
            > and from what I read, that box is even slower than a Slug...
            > Alternatively, I could go slightly up-market and build another Mini-
            > ITX system; I have a handful of them here, but they're just a little
            > bit too noisy.
            >













            Hi Nick,

            As much as I love my Slug I hanker after the Epia 5000 based Debian
            'server' that I sold on. Yes it was a bit noisy but I wish now that I
            had simply invested in a laptop PSU to replace the fan-cooled PSU that
            generated the little bit of noise that it put out.

            Anyway, for a compromise, what about one of these:
            http://www.cyrius.com/debian/iop/n2100/index.html

            The Thecus N2100 is quite a bit beefier than a Slug, readily accepts
            more memory, will run Debian and is not as noisy as a regular mini-itx
            machine. The only down side that I can see is the price - around £230
            sans-HDD - since I suspect with a bit of shopping around I could put
            together a silent mini-itx box. Still, if I come into a bit of cash I
            might still be tempted.

            Sean
          • Marcel Nijenhof
            ... who tells you that those who write the wiki are the average users? Probably the average unslung user has added a few packages to unslung and that s it. --
            Message 5 of 5 , Sep 24, 2007
              On Mon, 2007-09-24 at 20:34 +0100, Nick Rothwell wrote:

              > Certainly, the Wiki suggests that lots of people are happily
              > running Unslung with RAID (perhaps with more confidence than is
              > warranted!).

              who tells you that those who write the wiki are the average users?

              Probably the average unslung user has added a few packages to unslung
              and that's it.

              --
              marceln
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