Re: [Xen-users] HA Xen on 2 servers!! No NFS, special hardware, DRBD or iSCSI...

[Eric Windisch <lists@xxxxxxxxxx>]

> a little while before I can find the time to try it.
> 3.) Is it reliable?  Should be; AoE is relatively new but very simple.  LVM is 
> well-tested and
> software mirroring is as old as the hills.
>
>   
To confirm, you're talking about doing software raid within the DomU?  
If one of the devices drops out, no big deal?  Interesting.  The biggest 
problem here when one of the devices becomes desyncronized, such as 
after a temporary network failure, you will rewrite the entire secondary 
device.
I have been thinking of testing NBD for a similar setup, instead of 
AoE.  My understanding is that NBD natively supports raid-1 mirroring 
with logic particularly useful for RAID-over-TCP.  That is, during a 
normal active-sync it should work just like regular software raid, but 
when the volumes become desynchronized (such as after a crash), it will 
not rewrite the whole disk from scratch (as is common in raid-1) but 
will only update inconsistent blocks (sort of like rsync).  NBD also 
supports Raid-5, btw.
Client-side RAID-1 of AoE devices will be problematic unless you use a 
driver modified in the above-described fashion.   I'm not (currently) 
aware of a driver that will solve that for you (but I also haven't 
really looked).

--
Eric Windisch

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