why ODA use SSD for redo-logs

a very clever explanation http://www.pythian.com/news/33245/insiders-guide-to-oda-performance/

Another question during the webinar was how ODA storage differs from Exadata storage and why ODA can’t use storage cache.
If you was the webinar, you already know that ODA’s storage is simple and elegant. 20 SAS disks, 4 SSDs each with two ports, connected to the server nodes by two HBAs and two extenders per node. This is about as direct as shared storage can be. Which accounts in part for the performance we measured. No more misconfigured SAN switches. The catch is that because nothing is shared except the disks themselves, there is no place to locate a shared cache. Due to RAC, unshared cache (for example on the HBAs) can cause corruptions and cannot be used. This means that the storage system can easily get saturated, causing severe performance issues, especially for writes to redo logs. This is part of the reason the redo logs are located on SSD. We suggested additional methods to avoid saturating the disks in the webinar.