[OpenIndiana-discuss] HBA failover

[Richard Elling]

On Jun 17, 2013, at 7:12 AM, Sebastian Gabler <sequoiamobil at gmx.net> wrote:

> Hi,
> 
> it occured to me that obviously some ZFS Storage systems only feature a single SAS HBA, including the ZFSSA 7320. At least, as far as I understand.
> From what I saw in the 7320 documentation, each of the two HBA ports is connected to each of the two ports of a shelf, which should protect from the failure of a single cable, SAS Expander or drive controller (using dual ported drives). My understanding is that this topology is drawing upon MultiPath. But that still should make the HBA a SPOF, shouldn't it? As HBAs are a quite cheap commodity, I wonder why I even don't see a secondary HBA as an option for SUN ZFSSAs, or other appliances like the Netgear ReadyNas.
> 
> Thanks for any hints on how to exclude the HBA as a SPOF without using a cluster.

If you go one step further, these systems usually have a SPOF at the IOH. For more
recent Intel processors, the IOH is integrated onto the CPU, which is also a SPOF.
You can't get full redundancy with those systems, you have to move up to a SPARC
or Power based system. In short, it can become a simple economic decision rather than
a RAS decision -- adding another HBA does little to increase the overall availability
but does add costs. There are mathematical models that can be used to evaluate the
impact of these decisions.
 -- richard

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Richard.Elling at RichardElling.com
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