[Linux-cluster] Quorum disk over RAID software device

[Gordan Bobic]

Thomas Meller wrote:
> You're right, I am unclear.
> 
> Some years ago, we tried two versions: storage-based
> mirroring and host-based mirroring. As the processes were
> too complicated in our company we decided to mirror the
> disks host-based. So currently there is a /dev/md0
> (simplified) consisting of sda (in Bern) and sdb (in Zurich),
> and each node has it's own root-fs exclusively.

Since MD RAID cannot be mounted simultaneously from more than one place, 
I can only assume that you have a fail-over rather than an active-active 
solution.

> This cannot work with a shared GFS, as there are several
> machines doing updates on the FS and no central instance
> does always know the current state of the device's contents,
> thus no host-mirroring possible.

I thought you have an MD device doing mirroring...

> You are talking of storage-based mirrors. In case of a
> failure, we would have to direct the storage system to use
> the second mirror as primary and direct our nodes to write
> on sdb instead of sda.

Right - so you are using it as active-passive, then.

> That will involve controlling the storage from our machines
> (our storage people will love the idea) and installing the
> storage-specific software on them.

Or you can have DRBD do the mirroring and fail-over handling for you on 
whatever device(s) you have exposed to the servers.

> If the Hardware in use changes, we need to re-engineer this
> solution and adapt to the new storage manufacturer's
> philosophy, if at all possible.

Well, you'll always need to at least make sure you have a suitable SCSI 
driver available - unless you use something nice and open like iSCSI SANs.

> I still have a third opportunity. I can use Qlocig's driver-based
> multipathing and keep using host-based mirroring instead of
> using dm-multipath, which currently prevents me from setting up
> raid-devices as root-fs.

I'm still not sure how all these relate in your setup. Are you saying 
that you are using the qlogic multi-path driver pointing at two 
different SANs while the SANs themselves are sorting out the synchronous 
real-time mirroring between them?

> Well, that will work, but is somewhat ugly.
> 
> So far, I had only a short glimpse on OSR. I think I will need
> to dive deeper.

It sounds like you'll need to add support for qlogic multi-path 
proprietary stuff to OSR before it'll do exactly what you want, but 
other than that, the idea behind it is to enable you have have a shared 
rootfs on a suitable cluster file system (GFS, OCFS, GlusterFS, etc.). 
It's generally useful when you need a big fat initrd (although there has 
been a significant effort to make the initrd go on a serious diet over 
time) to bootstrap things such as RHCS components, block device drivers 
(e.g. DRBD), or file systems that need a fuller environment to start up 
than a normal initrd (e.g. GlusterFS, things that need glibc, etc.).

Gordan