Since I work with the largest SQL Server implementations in the world, it is essential that RAID 5 is removed from the picture, even for read mostly databases. I am a huge proponent of the "kill RAID 5 for databases" movement.

RAID 10 is still the best general recommendation for databases, with RAID 1 being a good recommendation for some data warehouses ( see the Fast Track Implementation Guide). Those DBAs implementing RAID 1 tend to be more advanced and know their application well enough to balance the disk IO over multiple database files and multiple LUNS themselves.

One thing that most people miss is that many of the newer disk controllers/HBAs will do dual reads off the mirror. So even read mostly databases will get a big benefit from implementing RAID 10. Check your cards to see if they have this capability, even in the SANs. An interesting side note is that the lower cost SANs were among the first to include these type of cards.

As far as SSDs go, they are the great equalizer in the random vs sequential IO discussion and really have nothing to do with RAID 5 vs RAID 1/10. You still pay the write penalty with RAID 5. Having said this, you can probably get some relief if you are having IO waits in a RAID 5 implementation with SSD just because it is faster. But at some point if you grow big enough you will end up killing RAID 5 anyway.

And the discussion about disk caching helping RAID 5, what you are really saying is that it is helping up to a point. In the large systems I work with, write caching is practically worthless and is generally turned off because the volume of writes will overrun the cache and the system ends up waiting to get its turn to write to the cache. This holds true for any RAID implementation.

In reality, SSD + write caching will help in the small to mid size RAID 5 implementations. But if you know you are going to experience significant growth over the next few months/years, go ahead and start moving to a mirrored RAID solution.

I would like to close with the statement that those using RAID 5 don't get the right to complain their DBMS doesn't scale.

Kevin Cox - SQL CAT