RE: iSCSI virtualization proposal

[Charles Monia <cmonia@NishanSystems.com>]

Hi:

 

I like the basic idea of farming out the data transfer traffic to the real devices. To preserve SCSI semantics however, I believe completion status for each implicit I/O should be directed to the "manager", who then returns a normal SCSI completion status to the initiator.  Having the manager act as the go-between simplifies the problem of exception handling as well.

 

Charles

-----Original Message-----
From: Yaron Klein [mailto:klein@eng.tau.ac.il]
Sent: Thursday, October 12, 2000 2:54 AM
To: ips@ece.cmu.edu
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