ZFS doing insane I/O reads

[Ram]

I just deployed zfs on my newer cyrus servers.
These servers get less than 2000 mails per hour and around 400 
concurrent pop/imap connections


I have seen that even if there is no incoming pop or imap connection 
still there is large amount of READ happenning on the zfs partitions.
Is this normal behaviour for an imap server. Iostat shows sometimes upto 
2000 TPS

The reads are infact more than 10x of what writes are. I am afraid I 
will be trashing the  harddisk.
Do I need to tune ZFS specially for cyrus  ?


This is the typical zpool iostat output

zpool iostat 1
pool        alloc   free   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
imap         145G   655G    418     58  18.0M  1.78M
imap         146G   654G    258    118  8.28M   960K
imap         145G   655G    447    146  19.4M  4.37M
imap         145G   655G    413     32  19.4M  1.46M
imap         145G   655G    339      4  14.8M  20.0K
imap         145G   655G    341     40  15.7M   755K
imap         145G   655G    305     10  15.0M  55.9K
imap         145G   655G    328     12  14.8M   136K