[DRBD-user] (off topic) alternative to drbd

[John Lauro]

> not quite true:
>  stor1 fails -> VM keeps using stor2 -> power outage (all machines are
> down) -> stor1 boots faster and VM starts using it ... oops

That is assuming the vm can come up automatically with only one storage. 
The hypervisor by default should not allow the vm to power on without manual 
intervention due to the missing disk.  However, that may depend on the 
hypervisor...


> - If you have a large data-set you risk for data loss, because of the
> extended rebuild time

With the standard linux md raid, rebuilds are done in the background (and 
rarely trigger a full sync) so things may be slower during rebuild but don't 
see the chance for data-set risk.  However if you have too many VMs 
configured such a way it would probably really thrash your storage servers 
and/or network if they were all rebuilding at the same time (but min/max 
sync rate it's tunable)...  Although the entire disk isn't rebuilt, one 
sector generally means a whole area (ie: 64mb more or less depending on size 
of disk) to be synced, and so for an active system that could mean a lot of 
to resync after being ot of sync for a short period of time.


> > At this point I am not sure I would recommend/use it over drbd or any of
> > the various cluster filesystems, etc.  just that it did test out well
> > enough that I am at least considering it, given that most of my servers
> I
> > don't need the redundant network storage (maybe 3%) beyond what's built
> > into the boxes, as the majority of our servers are active/active with
> > redundant failover loadbalancers in front of them, or active/passive
> with
> > sync'd configs, or are simply not critical 24/7/365.
>
> It has some use cases, but generally not recommended. I would probably use
> such setup for load-balanced (mostly) read-only data or a small partition
> with clustered (session cache) fs for a high load web cloud or really
> (really) large storage pool with small chunks on each server combined in
> LVM after the software raid
>
> For 2 nodes always use DRBD