NAS w/ multipath

[Tom Samplonius]

On Tue, 7 Feb 2006, Palle Girgensohn wrote:

> Hi!
>
> I'd like to connect a FreeBSD system to an IBM NAS system, using a 
> FibreChannel connection. I can find two supported drivers in the system, 
> mpt(4) and isp(4). It seems to me, none of them supports multipath, something 
> we need. Is this a false assumption? Are there other drivers that do support 
> multipath? Recommendations apprecieated.
>
> If not, an alternative would be iSCSI, but I have no experience and it seems 
> it is hardly implemented at all for FreeBSD, is that true?
>
> Thanks
> Palle
>

   Well, typically HBA drivers don't need to support mulitpathing, and that is 
implemented int the SCSI layer of the OS.  Windows is a bit different, since 
Windows tried to mount every device (no /etc/fstab file), so Windows HBA drivers 
typically need multipath control to hide the fact that a single logical disk is 
visible on multiple channels.

   With multipath, you typically just see the logical disk on two separate 
channels (separate LUNs).  Depending on your disk controller, you might need 
"trespass" support in the OS/driver, but that seems rare these days.

   So you can either do a cold multipath, by having the second path listed in 
your /etc/fstab, but leave it commented out, until you need to use the second 
path.

   Now, in FreeBSD you could also multipath in the GEOM layer.  GEOM knows about 
devices going away, and knows how to handle that (ex. gmirror).  There is some 
support in GEOM for round-robin IO to two devices.  However, phk has reported 
that the isp driver can hang forever on some timeouts, so it might not be 
useful.  And I don't even know if GEOM round-robin is even finished.

   It might an idea for a few people to get together some dollars for a 
sponsership on FreeBSD SAN support, specifically:

- isp driver updates (timeout handling)
- Multipath support support, either in CAM SCSI layer or in GEOM disk layer.


There has been great results on some other sponserships.


Tom