Xen users - Aug 2007 - Suggestions on XEN configuration with local and remote iSCSI

Dear All,

I''m totally new to DRBD + XEN over iSCSI but I need to configure this 
scenario in order to buy the right hardware.

Our department will provide us with an iSCSI target which will be 
replicated in its own to a secondary iSCSI SAN (probably they will use 
DRBD). This target will be used, in our ideas, to replicate our own 
iSCSI san, that we are going to buy.

We will probably build our iSCSI SAN with OpenFiler (impressions about 
this?) on a 2U server (i.e. PE2950, DL320, ...), 500GB sata disks and 
two dedicated NICs for the SAN. We will have also a L3 switch so we can 
partition network traffic for the SAN, as well.

We will buy also two dual socket servers with 8 or 16 GB ram (already 
looking in the list for dual core vs quad core). We plan less than 16 
VMs where in the worst case only one will be Windows 2003 server with 
1GB and all the other linux web development domains with 256-1GB ram. I 
want live migration for them.

My questions:

1) the remote iSCSI target will be available through a gigabit link... 
while my own will be on a dedicated gigabit network with the servers. Is 
it possible to configure the two SAN  with DRBD for fault tolerance and 
have also the "two switch benefit" to avoid a single point of failure
on
switches? (all the server will be connected to the Dept. network for 
internet broadband)

2) are there known issues and limitations on having DRBD with two 
devices in such a network configuration?

3) the two server will be configured in an Active/Active HA. They will 
see the DRBD through a LVM (I think, I''m new on that) but... can I tell
them (or probably better to DRBD) to use the remote iSCSI target only in 
case of emergency?

4) is it better to put the "choose the available iSCSI logic" on my 
servers instead of providing it with DRBD and write script my own to 
check availability and re-configure XEN VMs or downtime / errors here 
may be tremendous?

5) in this non-symmetric scenario, it is probably more convenient to 
have VMs stored on my SAN and use the remote one only for backup 
purposes, loosing DRBD reliability/HA for the sake of performance?

Thank you very much,

	Ivan

-- 
http://www.bio.dist.unige.it
voice: +39 010 353 2789
fax:   +39 010 353 2948

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