Good writeup.
I would also add that more then one target in a target group is
less-then-worthless UNLESS you have a cluster aware FS. When you have a FS
designed to handle locking for it, having more then one host with access to
the same disk can be vary useful. It is practically a requirement to do
vMotion on VMWare.
With the proper feature you can have multiple machines in a cluster working
on the same data, without the proper features it can destroy your data.
Andrew Hettinger
http://Prominic.NET || ahettin...@prominic.net
Tel: 866.339.3169 (toll free) -or- +1.217.356.2888 x.110 (int'l)
Fax: 866.372.3356 (toll free) -or- +1.217.356.3356 (int'l)
Sumit Gupta wrote on 11/10/2010 01:17:23 PM:
>
> Hi Peter
>
> Before going into details, overall what you are doing is emulating
> SCSI disks for access by initiators on the SAN. So for "each disk",
> one should be able to say things like:
>
> - Which initiators can access it.
> - Through which targets can the initiators access it.
> - What is the SCSI LUN number a given initiator sees when they
> access this disk.
>
> These setting are known as "lun mappings".
>
> Also you should be able to specify the storage where the data for
> this emulated disk is stored. This can be a zvol, a file etc. This
> is known as "backing store".
>
> Also even though you are specifying the LUN mappings for each lun,
> COMSTAR does not also you to specify initiators and targets
> individually. You have to specify a list of initiators and targets
> known as "host groups" and "target groups" (even if the list has
> only one member).
>
> So to put everything together:
>
> A LU is an actual emulated SCSI device (mostly a disk, but you can
> also emulate other SCSI devices like tape etc.). If the LU is a
> disk, then it needs a backing store which is mostly specified as a
> zvol (also known as volume).
>
> A target is simply an access point for an initiator to come in. When
> an initiator logs into a target, it sees a bunch of LUs depending
> upon the LUN mappings you specify.
>
> A view is simply another name for a single LUN mapping entry.
>
> Target portal group is an iSCSI construct. It is basically a way to
> tie in a bunch of IP addresses with an iSCSI target. I think it is
> described in COMSTAR Admin guide. But it is different from "target
> group" which is basically a list or targets which is then used
> inside LUN mappings.
>
> Sumit
>
> >
> > Folks,
> >
> > I have gone through various articles on setting up iSCSI initiator and
> > have successfully managed to share an iSCSI volume following the steps.
> > This was a simple test scenario. Now, I am trying to set up a specific
> > scenario but I am confused on proper relationship between various terms
> > that are used. I would appreciate your help in making me understand the
> > relationship between the following terms:
> >
> > - Volume
> > - LU
> > - Target
> > - Target Group
> > - Target Portal Group
> > - Host Group
> > - View
> >
> > As I understand:
> > 1. There is a one-to-one relationship between a volume and its LU.
> > 2. A target group consists of targets and a host group consists of
> > hosts.
> > 2. A view consists of one host group, one target group, and one or more
> > LUs.
> >
> > What I don't understand:
> > 1. At first, I thought there is a one-to-one relationship between an LU
> > and a target. However, this doesn't seem to be the case. Why would you
> > need more than one target in a target group?
> > 2. Where does the target portal group fit in?
> >
> > In my scenario, I would like to share volumes v1 and v2 with hosts h1
> > and h2. I would also like to limit the network to go through ip
> > addresses p1 and p2 only. Can someone please give me just the broad
> > steps;''
> >
> > Thank you in advance for your help.
> >
> > Regards,
> > Peter
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