Re: iSCSI virtualization proposal

["Jim Hafner/Almaden/IBM" <hafner@almaden.ibm.com>]

Yaron,

Is this something specific to iSCSI or more general (e.g., applicable to
SVP or FCP or even parallel)?  If so, I would suggest that this is an issue
for T10.

Additionally, this can be interpreted in some sense as the initiator being
a copy manager and receives (as a target) from the manager (now an
initiator) an  EXTENDED COPY command in which the destination of the data
is the initiator. This point of view  changes the sematics a bit (and
doesn't quite fit the suggestion from Charles to pass status back through
the manager).  Do we get the same net function this way? Is there anything
that iSCSI needs to do?  Can it all be accomplished with implementations
within the existing spec?

I honestly don't know the answer to these questions (as I haven't really
thought about this that hard).

Jim Hafner


Yaron Klein <klein@eng.tau.ac.il>@ece.cmu.edu on 10-12-2000 02:54:12 AM

Sent by:  owner-ips@ece.cmu.edu


To:   ips@ece.cmu.edu
cc:
Subject:  iSCSI virtualization proposal



Proposal for iSCSI virtualization:

The problem:

In order to implement iSCSI virtualization in a local network, we need the
following topology:

  -----------
  |
  | host    |
  |
  -----------



---------------------------------------------------------
     |              |                |
     |              |                |
     |              |                |
----------      ----------      ----------      ----------
|        |      |        |      |        |      |
| Manager|      | Disk A |      | Disk B |      | Disk C |
|        |      |        |      |        |      |
----------      ----------      ----------      ----------

When the host is an iSCSI initiator, the disks are iSCSI targets and the
manager is with iSCSI target port to the host and iSCSI initiator port to
the disks.

The host considers the manager as a "flat" disk space with iSCSI port and
is unaware of the disks. The manager manages the disks in some algorithm to
construct a combined virtual volume.

Consider the following example:

Each disk contains 1000 blocks. The virtual volume is thus 3000 blocks. The
hosts sends an iSCSI command to the manager to read 40 blocks from address
500. The physical addresses are: A ? 400:409, B ? 300:319 and C ? 600:609.

In the current iSCSI protocol, the traffic scenario is:

Host -> manager: iSCSI command, read from 500 size 40.
Manager -> A: iSCSI command, read from 400 size 10.
A -> manager: iSCSI data.
A -> manager: iSCSI status.
Manager -> host: iSCSI data.
Manager -> B: iSCSI command, read from 300 size 20.
B -> manager: iSCSI data.
B -> manager: iSCSI status.
Manager -> host: iSCSI data.
Manager -> C: iSCSI command, read from 600 size 10.
C -> manager: iSCSI data.
C -> manager: iSCSI status.
Manager -> host: iSCSI data.
Manager -> host: iSCSI status.

Problem 1: Traffic on the line is double! Each data packet is transferred
twice (from disk to manager and from manager to host).

Problem 2: The manager is a bottleneck. Both data and commands of all the
system (assuming many hosts and disks) is transferred via it.

Solution:

Lets add in the iSCSI status message, in the "iSCSI status" field, the
following option:

2 - iSCSI reflection

Which means that the status contains add-ons of iSCSI command that the host
should implement. These commands will implement the original request. In
our example it will look as:

Host -> manager: iSCSI command, read from 500 size 40.
Manager -> host: iSCSI status (with reflection), iSCSI commands (for A, B
and C)
host -> A: iSCSI command, read from 400 size 10.
A -> host: iSCSI data.
A -> host: iSCSI status.
host -> B: iSCSI command, read from 300 size 20.
B -> host: iSCSI data.
B -> host: iSCSI status.
host -> C: iSCSI command, read from 600 size 10.
C -> host: iSCSI data.
C -> host: iSCSI status.

Benefits:

* Data traffic on the line is single.
* No bottleneck on the manager.


Note: The manager is a software pack. It can be an independent unit, in the
host or in one of the disk. It is just schematically stated as independent
unit.

In conclusion, the addition of the reflection feature in the protocol is
minor change, can be optional and will enable the enormous potential of
virtualization.

Comments are more than welcome,

Yaron Klein
SANRAD

klein@sanrad.com