Re: [Xen-users] domUs with iSCSI disks... connect in dom0 or domU?

[Bart Coninckx <bart.coninckx@xxxxxxxxxx>]

On 10/16/11 23:14, xenusers wrote:
> Hi List,
>
> I don't know if this is something that is a simple matter of opinion or
> if there are strong reasons to take one route or the other.  I have dom0
> nodes with dedicated bond interfaces that connect to a storage
> traffic-only VLAN.  Currently I have a few domUs running that have
> large-ish volumes on the iSCSI SAN, and to present the volumes I'm
> connecting the dom0s to the storage, then in the domU config I give it
> the iSCSI volumes as
> 'phy:/dev/disk/by-path/ip-10.10.10.5-iqn....:volume1' and so on.
>
> I'm wondering if instead it's considered better to simply have the domU
> make those iSCSI connections, by giving it a second interface that
> connects to a bridge with access to the iSCSI VLAN?  I set one up that
> way instead, and it works as expected though it seems to perform less
> well in some admittedly simple tests using dd with 'oflag=direct' set.
>   The dom0 can write at about 110MB/s sustained, but if the domU makes
> the iSCSI connection instead it's doing well to manage 60MB/s.
>
> So I'm looking to the wisdom of the list, is it crazy to set things up
> one way or the other, or are they both an OK approach?  It seems like
> having the dom0 handle the iSCSI connection is a big win
> performance-wise, but perhaps that comes at a big negative cost?  Having
> the domU make the connection simplifies the Pacemaker configuration, but
> with an apparent loss in disk throughput.
Do you use HVM or PV DomUs? I wonder if this performance drop should 
hebben with PV.
Anyway, I definitely use the iSCSI connections in Dom0. As far as 
complexity is concerned: I combine iSCSI with multipath, allowing me to 
change the SCSI disks' names to the names of the DomUs that actually use 
them. Complexity for the bigger part gone. It also provides you with the 
benefit of redundant paths towards your iSCSI.

HTH,

B.


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