iSCSI boot mussings

[John Nielsen]

On Friday 16 March 2007 07:34, Danny Braniss wrote:
> Hi,
> 	Now that I have my hands on a server that can boot iSCSI,
> I started to look into it. After figuring out what magic is needed
> in the dhcpd.conf (just add option root-path
> "iscsi:target-ip::::target-name") I can boot FreeBSD to the point that it
> can't find a root device, and assuming that some more magic can be applied
> (ala NFS), I'm just
> wondering aloud, if it's realy worth the efford.
> 	For a PXE based diskless solution, you need
> 		1 - a working dhcpd
> 		2 - a working tftpd
> 		3 - a working NFS server with the exported root fs.
> 		    appplying some minor magic, you can have only one read-only fs.
> 	For an iSCSI based diskless solution, you need
> 		1- a working dhcpd
> 		2- a working iscsi-initiator, unless the BIOS can be used.
> 		3- a working target with a root fs
> 		   (one for each client, unless applying 3 from the above).
> 	Hybrid solution:
> 		boot via PXE, but mount root via iSCSI
>
> So, what say you all?

From the kernel's perspective (at the moment just prior to mounting root), is 
there a difference between the last two approaches? The situation as I see it 
(in both cases) is that the kernel is loaded into memory (by some magical 
means which is at this point irrelevant), and now has to locate a root device 
using only what it already has to bootstrap the process. If what it already 
has includes BOOTP code then it's possible to get some additional information 
externally. Whether the initial magic was PXE or BIOS-based iSCSI, the kernel 
has to have its own storage drivers and do its own network setup, right?

I think there are some benefits to being able to do this, but perhaps 90% of 
them could be realized with what we already have (iscontrol and the 
iscsi_initiator kernel module) plus some rc and fstab glue. Any kind of 
diskless server farm needs at least one "master" server to run dhcpd, and if 
you have it doing that you might as well have it do NFS and tftpd as well. 
Pretty much any client you'd want to use as an iSCSI initiator will have a 
decent NIC which nowadays implies PXE support. So it shouldn't be hard to get 
root mounted using currently available means (local disk or PXE+NFS), and 
from there have the option of using iSCSI for other partitions 
(including /usr).

A truly standalone iSCSI client will most likely want to use a TOE card, which 
to the OS looks like any other SCSI adapter. (I'm unsure which if any such 
cards are currently supported in FreeBSD, but that's a tangential question.)

Machines with iSCSI-capable BIOS'es are an inbetween case. Allowing such 
machines to be standalone clients would require things like the initiator 
name, the initiator's IP address and netmask, the target's IP address, and 
the target's (volume) name to be hard-coded in the kernel. It would be nice 
to support this scenario, but IMO it's the one with the lowest benefit/cost 
ratio.

Making it easy to integrate iSCSI into existing environments (diskless or not) 
is IMO the biggest hole in the current implementation (the missing rc and 
fstab bits I mentioned before).

JN