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

[Chris de Vidal <chris@xxxxxxxxxx>]

--- Eric Windisch <lists@xxxxxxxxxx> wrote:
> > 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?

Correct.


> 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.

Bummer, you are correct.


> 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.

Yeah it uses the fast sync patch -- but that's not specific to NBD, if I 
recall; that is, one
could still use AoE and client-side software RAID with the fast sync patch and 
have the same
benefit.  Downside is this is Linux-only.

If you don't mind patching your DomU kernel (I prefer not to) with the fast 
sync patch, this might
actually work.


> 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).

Yeah, looks like I can't have my cake and eat it, too.  Either I use a 
technology like DRBD on
Dom0 and do regular (non-live) migrations or use an external SAN or (perhaps) 
patch my DomU kernel.

CD

TenThousandDollarOffer.com

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