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

[urgrue]

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 at 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 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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>>>>>
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