After testing zimbra 3.1.4
I had decided to pass to zimbra OSS
Now in zimbra 4 Administrator's guide I find the note in my title

I have 380 accounts, my server has 4 hard-disk in raid5 (hardware)
This is the df-h output
Filesystem Size Used Avail Use% Mounted on
/dev/sda3 201G 2.3G 199G 2% /
tmpfs 506M 0 506M 0% /dev/shm
/dev/sda1 102M 39M 64M 38% /boot
/dev/shm 150M 0 150M 0% /opt/zimbra/amavisd-new-2.3.3/tmp

/dev/shm 150M 24M 127M 16% /opt/zimbra/amavisd-new-2.4.1/tmp

Can you help me in understanding what's the reason for not recommending raid5?
If i use only pop3 for the most part of my account and webaccess only for my special accounts can i hope to reduce the impact in the using of the resources?