In RAID what is better: fewer larger drives or more smaller drives?

When having the choice, go with more smaller drives instead a few large drives. This has different reasons:

• You can stripe reads and writes over more drives, leading to a speed increase
• Rebuilding smaller drives takes much less time.
• Very fast (15K SAS) drives will not be available with larger capacities
• Random reads could be handled by more heads, leading to a speed increase for applications requiring many small random reads.

Beware though that depending on your total capacity, a RAID5 could be considered unsafe, as encountering an URE while reconstructing a failed drive might become very likely.

Also, consider that using only two drives either gives you no redundancy or no speed increase, depending on your selected RAID type.

IMHO this question can not be answered correctly with the information provided. Prior to evaluating which RAID to implement one needs to have a specificition of the use. A RAID 5 implementation will satisify some requirements and fail to satisify others, as will other RAID implementations.

Layout all your requirements, performance, read write ratios, budgetg, etc and then decide or ask which implemention will suit your needs.

There are many WHITE papers on the web which describe the best use of each raid type.

I support SvenW's answer, but there are some drawbacks to consider about using smaller drives.

• The number of disk slots may be constrained so using smaller drives may limit total capacity and future upgrade options.
• More drives use more power and create more heat, vibration and noise, potentially reducing overall reliability.

Also, with the advent of SSDs, there is a school of thought that you can combine the capacity benefits of large disks with the performance benefits of solid-state disk, reducing the need for small high-performance disks. See my answer about cache devices in ZFS.