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Re: [linux-lvm] Mirroring a Drive for load-balancing AND failover



I am now looking at iSCSI.  The problem is exactly what Matt
describes.  Recovery isn't that big of a deal, when the other server
comes up I can carefully set one of the devices as failed, remove it,
add it again, and get them to sync.  after a sync is complete on both
servers, I can mount the drive on the server that went down and resume
services.

Maybe iSCSI will report things diferently to md (raid).  I will try.

I'm looking forward to cmirror, I hear it will be ready soon.  I'm not
completely sure it's what I need, maybe someone involved will see this
and chime in.

-Derek

On 7/27/05, Matthew Gillen <me mattgillen net> wrote:
> AJ Lewis wrote:
> > On Wed, Jul 27, 2005 at 09:06:01AM -0400, Matthew Gillen wrote:
> >
> >>Fury wrote:
> >>
> >>>I've racked my brain on this one, so hopefully someone will be of some help.
> >>>
> >>>I'm trying to set up two servers which share a drive and do not have a
> >>>Single Point of Failure.  They are on a local network with each other.
> >>> The best solution would be to have /dev/sda1 on one server mirrored
> >>>with /dev/sda1 on the second server.
> >>>...
> >>>A second solution was to use GFS/GNBD.  I can export each drive to the
> >>>other server, and do RAID 1 (on both servers) between the local
> >>>/dev/sda1 and the remote gnbd device.  I then format the raid device
> >>>with GFS so both servers can mount it.
> >>>
> >>>Surprisingly, this last system works.  Both systems can mount the
> >>>drive and read-write to it.  However, if either server in this
> >>>configuration drops dead, the other server cannot deal with the dead
> >>>gnbd device, and the raid device and mount point are no longer usable.
> >>> I'm sure there are numerous other problems with this setup, also.
> >>>
> >>>So I'm looking for ideas.  With two servers, how can I mirror a drive
> >>>in real-time, and allow for failover?
> >>
> >>You might want to use something more like iSCSI + RAID:
> >>http://linux-iscsi.sourceforge.net/
> >
> >
> > How is that different than GNBD + RAID?  The issue isn't the network
> > transport, it's recovery of a RAID on two nodes simultaneously.
> I don't think he was even worried about recovery, although you're right
> and that's another problem.  I read that he couldn't access anything
> after a failure of one server, which is what I was addressing.
> 
> Honestly, I don't know how GNBD works.  But if it makes makes the remote
> volume look local and doesn't report problems in a way that RAID
> understands (or at all), I can see how things would hang (just like a
> client system would hang if an NFS server for a mounted filesystem went
> down).  I imagine (but I don't know from personal experience) that iSCSI
> (with the ConnFailTimeout=x sec) would report a failed write and RAID
> knows how to handle that.
> 
> But, like I said, I don't know for sure about any of this, since I
> haven't tried it.  However, the page:
> http://linas.org/linux/raid.html
> mentions iSCSI, so it appears that some people have gotten it to work.
> --Matt
> 
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