Re: [Xen-users] Xen and iSCSI

[Alvin Starr <alvin@xxxxxxxxxx>]

Markus Hochholdinger wrote:

> Hi,
>
> Am Sonntag, 29. Januar 2006 22:58 schrieb Alvin Starr:
>  
>
> > Markus Hochholdinger wrote:
> >    
> [..]
>  
>
> > > well, my idea of HA is as follows:
> > > - Two storage servers on individual SANs connected to the Xen hosts. Each
> > > storage server provides block devices per iscsi.
> > > - On domU two iscsi block devices are combined to a raid1. On this raid1
> > > we will have the rootfs.
> > >      
> > We are doing doing this. Well sort of.
> > We are having the dom0 attach to the iscsi devices and then pass them up
> > as hda/hdb. The domU's deal with the raiding of the devices.
> >    
> And the domU makes the raid1?
>
>  
Yes.

>  
>
> > My reason for doing this is that the domU's don't need any access to the
> > network that contains the iscsi devices and has all the iscsi
> > information hidden from them so that they don't know anthing about the
> > arrays.
> >    
> Yes, thats a good point.
>
>
>  
>
> > All my dom0's have access to the all the iscsi devices so that
> > migration is possible.
> >    
> But you have to take care that no two dom0s write to the same iscsi target!
>
>  
I have burned myself there more than a few times.

Currently I use the vmid as a unique handle. I have a simple mapping 
between vmid and iscsi device and I use the vmid to create the MAC 
address of the domU.
My quick hack idea is to ping and see if a domU has taken the MAC 
address and if so then  I will not start the new domain.
It would be really nice if Xen had some global idea of what domains are 
running that would make most of these problems go away.
>  
>
> > I have done the resync thing to upgrade the storage server software. It
> > is a pain but it is do-able.
> >    
> Sounds not that good i hoped.
>
>  
The process is workable and I was able to keep 8 domains running on 3 
systems for my switch over and back.
>  
>
> > The other advantages hold true even if the dom0's attach to the iscsi
> > devices.
> >    
> Yes.
>
>
>  
>
> > > Disadvantages:
> > > - When one storage server dies ALL domU have to rebuild their raid1 when
> > > storage this storage server comes back. High traffic on the SANs.
> > > - Not easy to setup a new domU in this environment (lvm, iscsi, raid1)
> > >      
> > Rebuilding the arrays can suck back a lot of network bandwidth. It is
> > much better to rebuild the raid arrays consecutivly. rebuilding them
> > concurrently really slows things down and causes the drives in the iscsi
> > targets to really go mad seeking.
> >    
> OK.
>
>
>  
>
> > building a new domU is quite easy.
> > I build the raid array in the dom0 and then mount it locally and extract
> > a minimal install onto the array. I then unmount the array and stop the
> > raid. I can then boot the domU.
> > then ..... bob's your uncle.
> >    
> It is easy to handle for the domUs.
>
>  
--
Alvin Starr                   ||   voice: (416)585-9971
Interlink Connectivity        ||   fax:   (416)585-9974
alvin@xxxxxxxxxx              ||




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