do not put redo logs on SSD disks

Oracle’s redo log is a frequent source of intensive IO and, by design, Oracle sessions will normally wait for
redo log IO to complete. However, the nature of redo log IO operations is essentially optimal for magnetic
disk but least suitable for solid state disk. Redo log IO consists almost entirely of sequential write operations,
which are the worst case for solid state disk and the best case for magnetic spinning disk. Placing
redo logs on solid state disk is generally not recommended

When IO bottlenecks are limited to specific segments (tables, partitions, or indexes) then relocating those
segments to solid state disk will almost always be effective in optimizing IO to those segments, regardless of
the type of IO to which the segments are subjected

from guy harrison  blog : http://guyharrison.squarespace.com/blog/2011/10/27/using-flash-disk-for-redo-on-exadata.html