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Re: [Linux-cluster] How do you HA your storage?



Yes, these work, but then I'm having each server handle the job of mirroring their own disks, which has some disadvantages. Network usage instead of fiber, more complex management of points-in-time compared to a nice big fat centralized SAN, etc. In my experience most companies favor SAN-level replication. The challenge is just getting Linux to recover gracefully when the SAN fails over. Worst case you can just reboot, but, that's not very HA.


On 30/4/11 13:23, Corey Kovacs wrote:
What you seem to be describing is the mirror target for device mapper.

Another alternative would be to setup a software raid using multipath'd luns.

SANVOL1            SANVOL2
    |                           |
    \                          /
     \                       /
       \                   /
     MPATH1    MPATH2
          \             /
        RAID 1 DEV
                |
              PV
                |
               VG
                |
               LV

That might work

-C


On Sat, Apr 30, 2011 at 10:08 AM, urgrue<urgrue bulbous org>  wrote:
But, how do you get dm-multipath to consider two different LUNs to be in
fact two paths to the same device?
I mean, normally multipath has two paths to one device.
When we're talking about san-level mirroring, we've got two paths to two
different devices (which just happen to contain identical data).

On 30/4/11 11:47, Kit Gerrits wrote:
With dual-controller arrays, dm-multipath  keeps checking if the current
device is still responding and switches to a different path if it is not.
(for examply, by reading sector 0)

With SAN failover, you may need to tell the secondary SAN LUN to go into
read-write mode.
Unfortunately, I am not familiar with tying this into RHEL.
(also, sector 0 will already be readable on the secundary LUN, but not
writable)

Maybe there is a write test, which tries to write to both SANs
The one which allows write access will become the active LUN.

If you can switch your SANs inside 30 seconds, you might even be able to
salvage/execute pending write operations.


Regards,

Kit

-----Original Message-----
From: linux-cluster-bounces redhat com
[mailto:linux-cluster-bounces redhat com] On Behalf Of urgrue
Sent: zaterdag 30 april 2011 11:01
To: linux-cluster redhat com
Subject: [Linux-cluster] How do you HA your storage?

I'm struggling to find the best way to deal with SAN failover.
By this I mean the common scenario where you have SAN-based mirroring.
It's pretty easy with host-based mirroring (md, DRBD, LVM, etc) but how
can
you minimize the impact and manual effort to recover from losing a LUN,
and
needing to somehow get your system to realize the data is now on a
different
LUN (the now-active mirror)?
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