RAID1 performs better than a single drive because of split reads (with an assumption that in these test, there was more reading than writing or that the required writing was within buffer size such that reading is the determining factor). The Linux softRAID reads from both drives in a mirrored pair thereby doubling the read performance. While in RAID0, the softRAID reads from both drives, it must spend time calculating which drive has what data to read giving RAID1 a slight advantage (both drives have the same data in RAID1). RAID0 wins on write performance because the write is spit across drives (1/2 in each direction). In RAID1 however, the writes a the same as a single drive as both drives must get the full write (mirror).
--- Sadly most other softRAIDS (promise/adaptec, marvel, intel) are single drive read. Even more sad is that most hardware RAID cards read single drive at a time. Areka seems to be the bright spot in my work tests. They seem to split read/write very effectively. See my previous comment about the 400/450Mb sec Areka/Mtron SSD setup.
[Modified by: Woody on January 21, 2008 11:02 PM]