SAS vs Near-line SAS vs SATA

This has been covered here... See the related links on the right pane of this question.

Right now, the market conditions are such that you should try to use SAS disks everywhere you can.

• Enterprise SAS disks are your fastest and most resilient rotating media available at 10,000 and 15,000 RPM. Performance-optimized
• Nearline or Midline SAS are usually mechanically-equivalent to 7,200 RPM SATA disks, but feature a SAS interface and offer the benefits of the SAS protocol. They are available in higher capacities than enterprise SAS disks. They have a slight price premium over the same sized SATA drives. Capacity-optimized
• Solid-state disks (SSDs) are a higher tier of storage and shouldn't be compared directly with rotating media. Their main characteristic is higher random read and write performance, but the details are beyond the scope of this question.

Also see:

SAS or SATA for 3 TB drives?

How can a single disk in a hardware SATA RAID-10 array bring the entire array to a screeching halt?

"Near-line" is a marketing term for "7.2K RPM drives not designed for 24/7/365 continual usage". Using them in such a role will result in an increased failure rate compared to drives designed to be used flat out for years at a time.

SAS vs SATA, in many cases there are little meaningful differences between the two bus specs, but SAS was designed for massive scale and sophisticated signaling where SATA was not. If all you're looking for is a pile of disks, the difference probably won't matter. There are different on disk cache-handling protocols though, which can cause SAS to yield some single percentage-point increases in efficiency when used at high utilizations.

That said, the market seems to have settled on "7.2K RPM is SATA, 10K and 15K RPM are SAS" as another differentiator. There is no reason not to have 15K RPM SATA drives, but no one makes them.

The controllers that drive the SAS and SATA connections are as varied as the SCSI RAID of old. Some have rather complex cache and battery backups (or flash-backed cache with a high capacity capacitor to commit the cash to flash when power drops). Some are just SAS/SATA connections onna card and don't bother with any kind of caching.

SSD's talk over SAS, SATA, or even something completely different like an PCIe card. RAID cards are variously able to handle TRIM, this capacity is still evolving. However, the raw throughputs SSDs are able to pump through can rapidly overrun the RAID card's ability to keep up; when that happens the RAID card itself becomes the biggest bottleneck to performance. The PCIe cards are the fastest SSDs around and present to the OS like a HBA.

RAID systems are beginning to handle things like storage tiering, features only really available in high end SAN arrays. Get a pile of 7.2K/10K disks and a few SSDs, and the RAID card will move the most frequently accessed blocks to the SSDs.