Comments on: SAS initiator and target mix http://storageadvisors.adaptec.com/2005/12/06/sas-initiator-and-target-mix/ Storage Solutions for Real World IT Professionals Thu, 18 Mar 2010 09:40:32 +0000 http://wordpress.org/?v=1.5.2 Comment on SAS initiator and target mix by: Tom http://storageadvisors.adaptec.com/2005/12/06/sas-initiator-and-target-mix/#comment-151 Fri, 24 Mar 2006 19:17:59 +0000 http://storageadvisors.adaptec.com/2005/12/06/sas-initiator-and-target-mix/#comment-151 Joseph, An HBA is allowed to straddle multiple SAS domains. As in your example, an HBA could have a 4X connector to one domain and another 4X connect to a different domain. Each of these domains can have >16K devices. The limit of 100 disks is not a limit of expanders. It's simply limited by the controller hardware. One reason is that each disk typically has a structure defining that disk, and that structure is usually kept on-chip for performance reasons. Also, each command to those disks has a structure. All this adds up to signficant memory. It appears that most of the chip vendors have chosen "~100" as a big enough disk count. But eventually we'll see this change. As SAS becomes more prevalent, I'm sure high-end SAS controllers will start to support a LOT more than 100 drives. TT Joseph,

An HBA is allowed to straddle multiple SAS domains. As in your example, an HBA could have a 4X connector to one domain and another 4X connect to a different domain. Each of these domains can have >16K devices.

The limit of 100 disks is not a limit of expanders. It’s simply limited by the controller hardware. One reason is that each disk typically has a structure defining that disk, and that structure is usually kept on-chip for performance reasons. Also, each command to those disks has a structure. All this adds up to signficant memory. It appears that most of the chip vendors have chosen “~100″ as a big enough disk count.

But eventually we’ll see this change. As SAS becomes more prevalent, I’m sure high-end SAS controllers will start to support a LOT more than 100 drives.

TT

]]> Comment on SAS initiator and target mix by: Joseph Sonnier http://storageadvisors.adaptec.com/2005/12/06/sas-initiator-and-target-mix/#comment-142 Tue, 14 Mar 2006 23:57:55 +0000 http://storageadvisors.adaptec.com/2005/12/06/sas-initiator-and-target-mix/#comment-142 I have been confounded by this particular question. If an HBA is limited to a 100 or so disk drives, can those disk exist in different Edge Domains? So the typical SAS HBA has 4 or 8 ports - could it connect to 4 or 8 separate Edge Domains - as long as the total does not exceed the card's limit? The SAS diagrams always show a host connected to the fan-out expander - which in turn connects multiple edge domains. What is limiting the host from seeing only a 100 disk in the total SAS network? I have been confounded by this particular question. If an HBA is limited to a 100 or so disk drives, can those disk exist in different Edge Domains?
So the typical SAS HBA has 4 or 8 ports - could it connect to 4 or 8 separate Edge Domains - as long as the total does not exceed the card’s limit? The SAS diagrams always show a host connected to the fan-out expander - which in turn connects multiple edge domains. What is limiting the host from seeing only a 100 disk in the total SAS network?

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