> Would you mind to explain me better why you do suggest me to use the
> sas raid for wal please?
SSDs are known to shine when they have to deal with random access pattern
rather than sequential, on the other hand 10/15K rpm SAS disk is known to be
better for sequential io workloads (in general "rotating" disk use to be
better at sequential rather than random access)
Having said that it seems that SSDs are catching up, see:
http://www.anandtech.com/show/6935/seagate-600-ssd-review/5
Andrea