I'm well aware of the widespread recommendation that any disk system that is holding a database should not be RAID-5 because of the poor write performance on RAID-5. You can look at BAARF's website to see the arguments over RAID-5 and the problems with it for databases.
I'm building an IDOL indexing server - our first - and I was wondering if people who have experience with IDOL know what the read/write balance is. RAID-5 is faster for read-heavy systems; RAID-10 for write-heavy systems (if more than about 5-10% of your disk accesses are writes, you're better off with RAID-10). I would expect that IDOL does enough writes to justify RAID-10, but I don't actually know and I was hoping someone else here does.
... so the question is: Is an IDOL indexing server likely to perform enough writes to justify RAID-10 over RAID-5?
I do not know about IDOL specifically, but during day to day operations I would not expect an indexing service to generate many write requests unless the content being indexed changes a lot regularly.
The initial indexing of blocks of new content will produce a lot of writing though, as will any re-index operations that are started for any reason. For either of these operations you will notice a significant performance difference between RAID10 and RAID5/6.
The other operation that will produce write is logging, but is is only going to be an issue if the service is configured to record copious amounts of information as the index is accessed and if that is the case then you are likely to have your logs on another volume anyway.
RAID-10 is the preferred option in most circumstances, particularly if you are using large disks.
RAID-5/6 leaves you:
Additionally, RAID-5 is not faster RAID-10 for most read purposes.
Edit: Changed marginally more disk space to significantly more disk space.
I agree with the other posters about the particular details around RAID 5/6/10 and such so I won't add much on those points. But you didn't specify what kind of hardware was being used, so I remind you that using a solid hardware RAID controller should also be kept in mind.
I've done software RAID before and after several failures, I've sworn them off. After using hardware RAID (such as Adaptec and 3Ware), the difference is substantial. Not only do they perform better then software RAID but I have yet to run into a failure with any disks. I'm not saying it will never happen, but hardware while being more costly will definitely help you with performance and availability. Don't neglect the RAID controller!
Many setups include two arrays; one RAID-5 for data and one RAID-1 (or 10) for logs. I've never worked with any database wich required RAID10 for data storage, but maybe others have..