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



Yes, cmirror seems like what you are looking for (along with GNBD/iSCSI).

I am testing cmirror right now, it adds the ability to:
1) have a mirrored device in a cluster environment
2) do pvmoves on clustered volumes

Many of the pieces you need are in the form of patches currently, but we're working on pulling them together... maybe a month to get the kernel changes in + user space tools cleaned up.

If anyone is interested in helping me test this beast, I'd be thankful. I've recently uncovered a data corruption issue (I think) in cluster mirrors when simultaneously doing looping lvs's, pvmove, and dd's from different machines. You need only a few machines and you can use gnbd for shared storage. (Actually, gnbd is the _best_ device to use for this testing, as you can forcibly remove a single device while it is in use. This obviously makes gnbd ideal for single machine mirror testing, as well.)

 brassow

On Jul 27, 2005, at 3:10 PM, Fury wrote:

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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