Re: [Xen-users] Re: Best way to store domU's. NFS? NBD?

[Markus Hochholdinger <Markus@xxxxxxxxxxxxxxxxx>]

Hello,

Am 26.03.2010 um 16:53 Uhr schrieb Wiebe Cazemier <halfgaar@xxxxxxx>:
> On Friday 26 March 2010 16:36, Markus Hochholdinger wrote:
> >> iSCSI is nearly ubiquitous among commercial SAN products these days.
> >
> > and it's also possible to create a completly open source SAN with a linux
> > host. Just use lvm2 with iscsi enterprise target.
> >
> > I've done this and i use the storage hosts also as xen hosts. I've all
> > the nice features, live migration, storage redundancy with software raid1
> > inside the domUs, backup scripts for the whole volume group, can scale
> > from 2 to n hosts, ...
> I will look into that.
> A question though: "software raid1 inside the domU's"? Why inside the
> domU's? I would have the storage on a RAID+LVM, but not inside the domU's.

inside the domU you will never have a split brain situation for the storage! 
And you can grow the filesystem online without the need to reboot.


> Can you elaborate a bit more about your storage hosts? I was planning to
> have two hosts: a primary host with a failover one. The failover can take
> over by bringing up a virtual network interface with the same IP. Their
> data would be kept up-to-date by DRBD.

There you go with your problems: primary host, failover, bring up network... A 
lot of things that can fail! DRBD needs heartbeat, HA-Cluster, ... These are 
all problems i don't have!


> But, I haven't figured out how I am going to let the xen hosts access the
> data.

Yeah, you see! You could do drbd (raid1 outside domU). Big problem is if you 
mirror the whole volume group, but mirroring each logical volume is also a 
lot of effort. So you should mirror each logical volume and access this 
directly on each host. So you're limited to 2 hosts. Additionally you could 
export the drbd devices over iscsi, but there, again, you get into split 
brain situations...


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