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

[urgrue]

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 at 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 at redhat.com
>>> [mailto:linux-cluster-bounces at redhat.com] On Behalf Of urgrue
>>> Sent: zaterdag 30 april 2011 11:01
>>> To: linux-cluster at 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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