NASdisks

SAS cables, connectors and expanders for a NAS

How to actually wire an HBA to your drives: SFF-8087 vs SFF-8643 connectors, forward vs reverse breakout cables, port math, and when you need a SAS expander (Intel RES2SV240 / RES3TV360) instead of a bigger card.

You bought the HBA. Now you have a card with two odd-looking square ports and a stack of drives with plain SATA connectors, and nothing plugs into anything. This is the part the buying guides skip: which cable, how many drives per port, and what to do when you outgrow the card. Get the cable wrong and it either won't fit or, worse, it fits and quietly does nothing.

The connectors

An HBA's internal ports are one of two types, decided by the chip generation:

  • SFF-8087 (Mini-SAS) - the SAS2 6Gb/s connector, on SAS2008 and SAS2308 cards (9211-8i, 9207-8i, M1015, H310, H220).
  • SFF-8643 (Mini-SAS HD) - the SAS3 12Gb/s connector, on SAS3008 and newer (9300-8i, HBA330, 9305-16i).

They look similar and are not interchangeable. Each one carries four drives' worth of lanes, which is why an 8-port card has two of them and a 16-port card has four. If you're buying externally-connected gear you'll also meet SFF-8088 (external SAS2) and SFF-8644 (external SAS3); the ancient SFF-8484 shows up only on very old cards.

Breakout cables (one port, four drives)

A single SFF-8087 or SFF-8643 port fans out to four drives with a forward breakout cable: one Mini-SAS plug on the card end, four SATA (or four SFF-8482 for SAS drives) plugs on the drive end. Two of these off an 8i card wire up eight drives directly. That's the normal home-NAS setup.

The trap that catches people: forward breakout and reverse breakout are different cables and are not interchangeable. A reverse breakout runs the other direction (four SATA ports on a motherboard to one Mini-SAS backplane input). Buy the wrong one and the connectors physically mate but no drive ever appears. If your drives hang off the HBA, you want forward breakout. If your drives sit in a backplane, you want a straight SFF-8087-to-SFF-8087 (or 8643-to-8643) cable from the card to the backplane instead.

Port math

The arithmetic is simple once the connectors click:

  • 8i card = 2 ports = 8 drives directly (two forward breakouts, or two cables to a backplane).
  • 16i card = 4 ports = 16 drives.
  • 24i card (9305-24i) = 6 ports = 24 drives.

So the first question when you're planning past eight drives is whether to buy a bigger card or add an expander.

SAS expanders (going past the ports you have)

A SAS expander is a port multiplier for storage. It takes a few lanes from your HBA and fans them out to many more drives, the same way a network switch fans out one uplink. You keep your existing 8i card and hang 16, 24 or more drives off the expander.

The common ones a home builder meets:

  • Intel RES2SV240 - a SAS-2 (6Gb/s) expander built on the LSISAS2x24 chip, with eight SFF-8087 ports for up to 24 SAS or SATA devices. Cheap on the used market and the classic budget pick.
  • Intel RES3TV360 - the SAS-3 (12Gb/s) generation, SFF-8643, up to 36 ports. Any port can be SAS or SATA.

Two things worth knowing:

  • SATA drives work fine behind a SAS expander. The old "SATA behind an expander is unstable" warnings trace back to specific early SATA-tunneling issues that modern LSI expanders handle cleanly. Plain SATA HDDs on a RES2SV240 are a standard, boring setup.
  • You share the uplink bandwidth. All the drives on the expander split the lanes feeding it from the HBA. For spinning disks this never matters - a shelf of HDDs won't saturate a four-lane SAS uplink. For an SSD array it can, so wide the uplink (feed the expander from two HBA ports) or skip the expander and use a native high-port card.

Expander or a bigger card?

For a big HDD pool, an 8i card plus a cheap expander is usually the lowest cost per drive, and bandwidth isn't a concern. For SSDs, or if you just want the simplest possible wiring, a single high-port card like a 9305-16i or 9305-24i - 16 or 24 ports on one chip, no expander, no shared uplink to reason about - is cleaner. My default for a home HDD build is the expander; for anything with SSDs, the bigger card.

Pick the HBA first from the IT-mode HBA reference and the buying guide, then work out the cabling from there - the card's connector type tells you which cables and which expander generation to match.