Re: [Xen-users] Xen and iSCSI

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Sun, Jan 29, 2006 at 02:00:11PM +0100, Markus Hochholdinger wrote:
> Hi,
> 
> Am Sonntag, 29. Januar 2006 12:38 schrieb Per Andreas Buer:
> > Markus Hochholdinger wrote:
> > > Background is that i am planing virtual servers within xen and they
> > > should get their disks from the network so that i can perform live
> > > migration. Has anyone a setup like this working?
> > Sure. I've got a similar setup with gnbd. gnbd works like a charm. I've
> > heard of people combining gnbd with drbd to get a true HA setup. I've
> > only used mine for test and development.
> 
> 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.
> 
> Advantages:
>  - storage servers can easily upgraded. Because of raid1 you can savely 
> disconnect on storage server and upgrade hard disk space. After resync the 
> raid1 you can do the same with the other storage server.
>  - If you use a kind of lvm on the storage servers you can easily expand the 
> exportet iscsi block devices (the raid1 and the filesystem has also to be 
> expanded).
>  - You can make live migration without configuring the destination Xen host 
> specially (e.g. provide block devices in dom0 to export to domU) because all 
> is done in domU.
>  - If one domU dies or the Xen host you can easily start the domUs on other 
> Xen hosts.
> 
> 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)
> 
> Not sure:
>  - Performance? Can we get full network performance in domU? Ideal is we can 
> use full bandwith of the SANs (e.g. 1GBit/s). And if the SANs can handle this 
> (i will make raid0 with three SATA disks in each storage server).
>  - How is the CPU load on dom0 and domU when using iscsi in domU?
> 
> 

Wouldn't it be better to use iSCSI initiator in dom0, and map luns from two
different storage servers (and SANs).. raid1 them, and create LVM (or CLVM)
on top of raid.. and export LV to each domU.

Then you could more easily upgrade the storage servers and only rebuild the
raid in dom0.. domU's wouldn't see any problems / failures.

-- Pasi 

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