[OpenIndiana-discuss] HBA failover

[Peter Tribble]

On Mon, Jun 17, 2013 at 3:12 PM, 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.
>

My experience here is that the ports on HBAs are independent,
and fail independently. (Don't think I've had a SAS HBA fail on me,
but I've had several failures of SCSI and FC parts over the years.)

Which means that the physical HBA is less of a SPOF than you
might think. As others have noted, there are plenty of others.

The one case where using multiple HBAs makes sense is if you
can hot-swap the card, because then you can replace the failed
one while leaving the good one running. But that's not all that
common with current commodity hardware.

-- 
-Peter Tribble
http://www.petertribble.co.uk/ - http://ptribble.blogspot.com/