[Linux-cluster] GNBD & Network Outage
Benjamin Marzinski
bmarzins at redhat.com
Fri Feb 4 17:41:10 UTC 2005
On Fri, Feb 04, 2005 at 10:26:36AM +0000, Nigel Jewell wrote:
> Dear Ben,
Oh yeah, or you could take a look at http://www.drbd.org/
-Ben
> Thank you for your detailed reply. It is always refreshing to get a
> decent response on a mailing list ;) .
>
> >Sure. You see the -c in you export line. Don't put it there. That puts
> >the device in (the very poorly named) uncached mode. This does two
> >things.
> >One: It causes the server to use direct IO to write to the exported
> >device,
> >so your read performance will take a hit. Two: It will time out after
> >a period (default to 10 sec). After gnbd times out, it must be able
> >to fence
> >the server before it will let the requests fail. This is so that you
> >know
> >that the server isn't simply stalled and might write out the requests
> >later
> >(if gnbd failed out, and the requests were rerouted to the backend
> >storage over
> >another gnbd server, if the first server wrote it's requests out
> >later, it
> >could cause data corruption).
> >
> >
>
> My understanding was that the "-c" put the device in cached mode, as
> described here:
>
> http://www.redhat.com/docs/manuals/csgfs/admin-guide/s1-gnbd-commands.html
>
> Or are you saying that by not putting the "-c" put its in uncached mode?
>
> >This means that to run in uncached mode, you need to have a cluster
> >manager and
> >fencing devices, which I'm not certain that you have.
> >
> >
>
> No we don't as we didn't really see the need, given what we want to do.
>
> >I've got some questions about your setup. Will this be part of a
> >clustered
> >filesystem setup? If it will, I see some problems with your mirror. When
> >other nodes (including the gnbd server node A) write to the exported
> >device,
> >these writes will not appear on the local partion of B. So won't your
> >mirror
> >get out of sync? If only B will write to the exported device, (and
> >that's
> >the only way I see this working) you can probably get by with nbd, which
> >simply fails out if it loses connection.
> >
> >
>
> The intention of the setup was to have two hosts both exporting an
> unmounted device, and the alternative device using it as a RAID-1
> device. Then to use heartbeat to mount and unmount the partitions as
> required. For example:
>
> HOST A:
>
> /dev/hda1 (md0, ext3, mounted)
> /dev/hda2 (ext3, unmounted, gnbd_exported as A)
> /dev/gnbd/B (md0, ext3, mounted)
>
> HOST B:
>
> /dev/hda1 (ext3, unmounted, gnbd_exported as B)
> /dev/hda2 (md0, ext3, mounted)
> /dev/gnbd/A (md0, ext3, mounted)
>
> I hope that makes sense.
>
> If so, does what we are trying to achieve sound sensible? Any
> gotchas/advice?
>
> --
> Nige.
>
> PixExcel Limited
> URL: http://www.pixexcel.co.uk
>
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