Xen users - Jan 2009 - Best practices for httpd & MySQL under Xen w/DRBD & iSCSI?

Hi All,

I apologize in advance if this strays too far from the etiquette on the 
Xen userlist, however the amount of help, brainpower and experience
I''ve
received from this list with Xen and "peripheral" related issues (i.e.
DRBD) have been worth more than it''s weight on gold and I''m
hoping
someone will be kind enough to give me a "best practices" outline of 
what I should do and to pick their brain! ;)

Basically, I''m requesting comments for the following configuration 
before I build it out.  Basically, I have 4 servers.  Two are PowerEdge 
1750''s (4GB RAM each) with 3x36GB 15K U320 drives in RAID-5 in
identical
configurations and two are home-built ASUS RS120-E5/PA2''s with 2x500GB 
7.2K SATA drives in RAID-1 with Intel Xeon X3220 Quad-Core CPU''s (8GB 
RAM each).  Each of the four servers have two on-board GbE ports 
(Broadcom) and an Intel PRO/1000 (9401) PCIe NIC.  I''m planning to use 
CentOS 5.x x64 on all four servers.

Basically, I would like to use the two PowerEdge 1750''s as iSCSI
targets
for the two ASUS servers hosting Xen VM''s along with DRBD so each 
PowerEdge would mirror each other.  Normally, each ASUS server would run 
a single Xen PV domU instance of CentOS 5.x at any one time; so in this 
case, ASUS Server #1 would run a domU for MySQL and ASUS Server #2 would 
run a domU of Apache/httpd or vice-versa.  In case of a server failure 
or maintenance (i.e. kernel), I would like to be able to migrate both 
VM''s onto a single server then migrate a VM back when a server-node 
comes back up.

So my "high-level" questions so far are:

#1. DRBD on the PowerEdge''s, I would like to use GFS.  However,
I''m not
sure if I should use single-primary mode (HA) or dual-primary mode 
(Load-Balancing).  I guess this would depend if I would like to, in the 
future, run a cluster of MySQL servers and cluster of httpd servers for 
load-balancing?  (i.e. Both ASUS servers would have a VM of Apache/httpd 
and a VM of MySQL on each?)

#2. I was planning on teaming (802.3ad) the two on-board GbE on all four 
servers and putting them onto their own subnet (I have a 24-port Netgear 
L2 managed switch I use).  But then I realized if I did that, I''d be 
putting both DRBD traffic and iSCSI traffic on the same physical wire.  
I need at least one port on the ASUS servers for "external" traffic.  
The two remaining ports can be used for DRBD and iSCSI whether 
separately (own network/VLAN) or teamed together and shared.  No 
"external" traffic is required on the PowerEdge''s so all
three ports can
be used for DRBD & iSCSI with two of the ports being able to be teamed 
(802.3ad).

I hope someone has a similar setup or experience with DRBD & iSCSI in 
getting the best performance out of them with the limited GbE ports I 
have available to me.  I''m ready to delve into a scenario where I can 
"pull" the plug on a physical DRBD-node and a physical App server
(under
Xen), and still be ticking along!

Thanks in advance everyone.  Cheers!

-- 
Daniel Kao
Übermind, Inc.
Seattle, WA, U.S.A.


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