Ports vs Drives

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Quartzeye

New Member
Jul 29, 2013
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Hey everyone. I am confused. When looking at RAIS adapters I notice that there is a difference between the number of ports and the number of drives supported. Such as the following off of eBay:

Adaptec 5405 RAID controller

4 internal port, low profile, PCI-Express (PCIe) Unified Serial RAID controller with Intelligent Power Management delivers exceptional performance, advanced data protection, and maximum scalability for enterprise-class, high-density server applications

RAID levels 0, 1, 1E, 5, 5EE, 6, 10, 50*, 60*, JBOD

Key RAID Features
• Supports 4 direct-attached or up to 256 SATA or SAS disk drives using SAS expanders
• RAID levels 0, 1, 1E, 5, 5EE, 6, 10, 50* and 60*


But when I look at RAID setups, it appears that there is one drive per port attached.

There are a number of 8 port adapters that say they support RAID 60 and more than 24 drives. I assume that I would use a breakout cable that plugs in 4-6 SATA drives into a single port on the adapter. If that is the case, why would you want to load up a server with 3-4 RAID adapters and plug one drive per port. I don't think fault tolerence is the issue as a bad card takes off line all the drives attached. Granted that may be only one RAID set but that isn't my question.

My goal is to build (2) 12 disk RAID 60 sets. If I want to load up a 24 disk storage server, do I need a 24 port adapter, one per dive, or an adapter that can support 24 drives, 4 drives per port? Maybe two 4 port adapters with breakout cables?
 

Rhinox

Member
May 27, 2013
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What you need is SAS/SATA port expander. Description says it all:

Supports 4 direct-attached or up to 256 SATA or SAS disk drives using SAS expanders
 

dba

Moderator
Feb 20, 2012
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San Francisco Bay Area, California, USA
With RAID cards, there are a number of different terms: Connectors, Ports, and disks.

First are Connectors. On a server-grade RAID card or HBA, these are SAS connectors, either SFF-8088, SFF-8087, SFF-8643, or SFF-8644. Most cards have 1-2 of these, with some having up to six.

Next are Ports. It's simple: Each of the above connectors has, internally, four ports.

Lastly are disks. You know what those are.

A disk can be connected to a single port or, in a more complex setup, multiple disks can be connected to a single port via a SAS expander. With enough expanders, which can be chained, a single connector on a RAID card can talk to a thousand disks, depending on the capabilities of the card.

So for your 24-disk setup you need any of:

1) A RAID card or HBA with six SAS connectors, which will be sold as a "24-port" card. This card will have six SAS connectors. You'll use a 1xSAS to 4xSATA forward breakout cable on each connector to turn those six SAS connectors into 24 SATA connectors that can plug directly into your drives.

2) A RAID card or HBA with at least one connector, plus a 24-disk backplane with a built-in expander - something like this. You'll connect your card to the expander using a single SAS cable, and the expander in the backplane with do the magic of talking to the 24 drives.

3) A RAID card or HBA as above, a disk chassis without a built-in expander, plus an external expander card. The external expander card will need to be a "36 port" model. You'll need one or two SAS connectors to connect to your RAID card or HBA, and six SAS connectors to connect to your drives with the appropriate cables. This sort of chassis is commonly used with a separate expander card.

Also see:

http://www.servethehome.com/external-sassata-disk-chassis-wiring-part-1/
http://www.servethehome.com/sas-expanders-diy-cheap-low-cost-jbod-enclosures-raid/

and be sure to search this forum for many others who have built similar solutions.
 
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EloquentSrv

New Member
Jan 21, 2014
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Sure so think of a port as a direct attach channel. 1 port of SAS2 = 6.0gb/s so 4 ports = 24gb/s

You can use expanders to "breakout" a 4 port SAS2 interface into many more disks, and you can chain expanders. Think of this in networking like you took a cable with 4x gigabit links and connected a quad port Ethernet adapter in a server to a 24 port switch. There are 20 ports remaining but only 4 ports of bandwidth back to the server.

So if you got a 28 port expander, or bigger, you could do one cable from the server to the expander. Then from the expander (think switch) you can connect up to 24 more drives (28-4 with a 28 port expander).

The catch is, you only get a maximum of the backhaul links so you may have 24 SSDs capable of 500MB/s but you will only get a maximum of about 2GB/s with a 4 port RAID card.

You can do an 8 port RAID card and get more ports for the backhaul to the expander.
You can get 3x 8 port cards then use software raid to make your RAID 6 arrays
 

Lost-Benji

Member
Jan 21, 2013
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The arse end of the planet
Sure so think of a port as a direct attach channel. 1 port of SAS2 = 6.0gb/s so 4 ports = 24gb/s
You can use expanders to "breakout" a 4 port SAS2 interface into many more disks, and you can chain expanders. Think of this in networking like you took a cable with 4x gigabit links and connected a quad port Ethernet adapter in a server to a 24 port switch. There are 20 ports remaining but only 4 ports of bandwidth back to the server.

So if you got a 28 port expander, or bigger, you could do one cable from the server to the expander. Then from the expander (think switch) you can connect up to 24 more drives (28-4 with a 28 port expander).

The catch is, you only get a maximum of the backhaul links so you may have 24 SSDs capable of 500MB/s but you will only get a maximum of about 2GB/s with a 4 port RAID card.
About 3GB/sec.
What was missed above is that a SAS expander usually has a SAS connector (usually a SFF-8087 or SFF-8088) uplink (goes to the HBA or RAID card and then the rest of the ports are for down-link/downstream devices. What to be careful with is the port-count, most decent Expanders will likely have external SFF-8088 connectors and 8 of the ports in the port-count are not usable for devices in the norm. 4 ports are up-link, 4 ports are down-link to another expander or chassis, also named as IN and OUT.
https://www.google.com.au/search?q=...soGgAw&sqi=2&ved=0CAcQ_AUoAQ&biw=2560&bih=972

I think some readers will be getting a little caught up on Ports, Channels (single SATA/SAS devices) and the actual names for the "SAS" connectors.
Serial attached SCSI - Wikipedia, the free encyclopedia
 

EloquentSrv

New Member
Jan 21, 2014
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Good clarifications. Thanks. I was typing on my phone so not a complete dissertation on this.

I would mention that we have used expander ports to link to drives rather than chassis in many occasions and the result was it worked each time. I think most expanders work more or less like switches.