RE: [Xen-users] easy high availability storage

[Nathan Eisenberg <nathan@xxxxxxxxxxxxxxxx>]

> If double write is what you need, my first suggestion already does
> that:
> - san exports its storage as iscsi
> - server/PC imports iscsi share from both san
> - setup raid1 on server using those two iscsi imports
> 
> no need to modify xen code as linux md can already do it.
> 
> --
> Fajar

The problem I've encountered with doing HA storage in the domU is that you are 
now doubling the required throughput of the dom0's iSCSI interface, which is 
already easy to bottleneck if it's only GigE.  125MB/s may seem like a lot of 
throughput, but if you start by cutting that in half for RAID traffic, and then 
share that remaining bandwidth across 5 domU's, you're going to fill it up 
pretty quick.

The other problem is that if you ever take a node down, and you have 5 domUs 
configured in this way, when that node comes back up, all of your domUs will 
attempt to resync the mirror simultaneously.  I'd bet that this could actually 
lead to catastrophic kernel panics, as the domUs can become IO starved.

-Nathan


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