Re: [Xen-users] Network based storage - NBD/AoE/iSCSI other?

[Harald Kubota <hkubota@xxxxxxx>]

Ian P. Christian wrote:
> I'm currently investigating setting up few Xen servers.  I've noticed
> that currently live migration of Xen's require you to use a network
> based storage system.
>   
There is no such requirement. FC is certainly ok, as is anything which 
can be shared and
directly connected (e.g. Firewire drives with a bridge from Oxford) and 
of course
the old way of using (parallel) SCSI drives (which have their own problems).

Using network is likely the cheapest and most convenient one unless you 
happen to have
FC already.

> iSCSI looked like a good bet, but the fact it's using TCP/IP seems like
> a crazy overhead to use when I'm most likely going to be hosting the
> storage on the same layer 2 network as the servers.
>   
The overhead is not that large and it's nice to be using a standard 
which will continue to
exist for a while. Should you need performance, you can replace the 
iSCSI client (the server
with the storage) with a faster box without changing anything at the 
iSCSI initiator side (the Xen
running machine).

> AoE solves that above concern but I'm confused as to what would happen
> should I need to move from SATA storage drives to SCSI for performance
> reasons.
>   
The A in "AoE" only says, that it's using the ATA protocol to move data. 
It does not matter
what storage device you actually use. Can be a USB stick if you have to. 
Or any LVM storage.
Or 15k rpm FC disks.

Both iSCSI and AoE like to have their own storage network, just like any 
SAN has: otherwise
you have contention on the one network you use.

That of course does not matter much if the data you handle is in the 
sub-MB area.

> Someone also advised my against NBD saying that his notes suggest that
> it will not start servicing a second write untill the first has completed.
>
>   
That's how good mirroring is supposed to work.
But if you mirror iSCSI/AoE, you have the same penalty.

Then there's good old NFS you can use.

Harald


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