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



On 30/4/11 14:27, Corey Kovacs wrote:

This has nothing to do with any network. It's all over the fiber...

True, my bad, I was thinking of DRBD.

Points in time? It's a raid 1, it's relatively instant. It's more
complex to manage a failover in the way you describe if anything.
I didn't mean that. What I meant is with any enterprise storage filer I can walk in and take a point in time snapshot of my entire datacenter - all hundreds of servers - with almost no effort. And restore it. That's a pretty fantastic thing to be able to do before, say, a major upgrade on hundreds of servers. And you manage all of it in one place. Take a situation like if the company decides it needs a third copy of the data. It'd be a fun job to map and configure the third LUN on 500 servers, when on the SAN it'd be a a few minutes to configure. Or if that third copy needs to be async instead, I don't even think you can do that with LVM or software raid. Host-based mirroring is great for many situations, but when it comes to larger environments, I think most companies tend to prefer SAN mirroring.

Well, my $0.02 anyway.

-C

On Sat, Apr 30, 2011 at 11:03 AM, urgrue<urgrue bulbous org>  wrote:
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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