SAS Hard Drives

10K SAS as a replacement: for an existing array, choose the same spindle speed and at least the same capacity. A larger drive works, but the controller only uses the original size. Don't mix speeds within an array – it always runs at the pace of its slowest drive. The Dell 2.4 TB 10K already comes in the right caddy for PowerEdge 14G and 15G.

Why fast drives stay small

The faster the platters spin, the smaller and lighter they have to be: centrifugal force, vibration, heat and power draw all rise with speed. That's why 10K and 15K drives use the 2.5-inch format with a few small platters. Capacity drives at 7,200 rpm stack many large platters instead; modern models such as the Exos X12 are filled with helium, which cuts friction and heat.

Since SSDs took over the role of fast drives, 10K and 15K disks have barely been developed further, and 10K tops out at 2.4 TB. For new systems that need performance, server SSDs are the better choice: a single SATA SSD already manages many times the IOPS of an entire 10K array.

Nearline SAS or SATA for 3.5-inch drives?

Nearline SAS (NL-SAS) combines capacity-drive mechanics with the SAS interface. Compared with the SATA version of the same drive family, it adds two ports for redundant paths, more robust error handling and trouble-free operation behind SAS expanders – in JBOD enclosures or storage servers such as the Supermicro SSG-6049P with 36 bays. SATA drives cost a little less and are fine for simple NAS boxes. Watch the workload rating: enterprise models such as the Exos X12 are rated for 550 TB a year, typical NAS and desktop drives for considerably less.

Our guide SAS vs SATA vs NVMe compares the interfaces in depth. To run SAS drives you need a SAS-capable RAID controller or HBA.

Large drives and RAID: allow for rebuild time

When a drive fails, the controller has to rewrite its entire contents. At 12 TB and an average of 200 MB/s that takes about 17 hours on paper, and often one to two days under production load. During that time a RAID 5 array has no protection against a second failure. For large nearline drives, RAID 6 or RAID 10 is therefore the norm; see our guide RAID 10, RAID 5 or RAID 6.