RE: iSCSI Target Reset

["Elliott, Robert" <Robert.Elliott@compaq.com>]

[iSCSI added to subject]

> -----Original Message-----
> From: Eddy Quicksall [mailto:ESQuicksall@hotmail.com]
> Sent: Monday, April 23, 2001 11:44 AM
> Cc: ips@ece.cmu.edu
> Subject: Re: Target Reset
> 
> I am wondering how clustering will work on NT without some 
> sort of reset.
> 
> On parallel SCSI, NT will issue a SCSI BUS RESET to break reservations
> during a challenge for the quorum drive.
> 
> On iSCSI, there is no full equivalent to the SCSI BUS RESET so I would
> assume the NT driver would have to issue a TARGET RESET to 
> each target that it is supporting.
> 
> How would you propose this would be done without a TARGET RESET?
> 
> Eddy

LOGICAL UNIT RESET breaks the reservations with the correct scope.

According to a WinHEC presentation, the new STORPORT port driver will 
attempt resets in this order:
1. LOGICAL UNIT RESET
2. TARGET RESET
3. Bus reset 

LOGICAL UNIT RESET isn't supported by all existing logical units, so 
a fallback scheme (with its unfortunate side effects) is necessary.  
This is mandatory as of SAM-2 revision 16, so new devices should 
start supporting it (on all protocols).

TARGET RESET is the first level of fallback (medium hammer).  This 
is optional for protocols to support as of SAM-2 revision 16.  
Protocols that don't support it require their logical units to play 
nicely and support LOGICAL UNIT RESET.  Protocols that do support
it leave themselves open to all the problems Fibre Channel went
through.

Bus reset is the final level of fallback (big hammer), which doesn't
exist on serial busses.

A miniport driver for SCSIPORT (the existing port driver) can convert 
a "target reset" request into LOGICAL UNIT RESETs for each logical 
unit known to that system to help limit the damage.

From what I understand, Linux also relies on target resets and 
bus resets and will need some mid-layer work to play well in a fabric.

---
Rob Elliott, Compaq Server Storage
Robert.Elliott@compaq.com