[CentOS] Question about iostat output

Hello,

We are planning to moving most of our servers to ESX but before buying  
our SAN, we want to do some I/O stats to see if iSCSI is enough or if  
we have to go with FC. So I found a plugin for Nagios that can log I/O  
stats with iostat. So far it's fine with single disk/one partition  
servers, but on our Oracle Database 10g server, we have two drives in  
RAID 1 (/dev/sda) and 4 other drives in RAID 10 (/dev/sdb). When I  
query iostat, I get :

[root at golgoth ~]# iostat
Linux 2.6.9-55.ELsmp (golgoth.acaiq-ctb.lan) 	09/21/2009

avg-cpu:  %user   %nice    %sys %iowait   %idle
            1.83    0.00    3.04    0.79   94.34

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda               8.44        12.25        32.40  489731534 1295151278
sda1              0.00         0.00         0.00      21360       7478
sda2              6.60        80.90        44.41 3234128950 1775185432
sda3              0.05         0.20         0.23    7852212    9358056
sda4              0.00         0.00         0.00          2          0
sda5             41.16        38.59        95.20 1542695170 3805566528
sdb               2.84        72.63        28.91 2903208392 1155519494
sdb1              3.67        77.14        23.90 3083762026  955474128
sdb2              1.62       102.92         5.00 4114412086  200045262

So why sda have a lower TPS and bytes read/write than one of it's  
partitions (sda5, which is the root partition)? I guess I would have  
to collect the stats for each partitions to get what the total?