Re: Data Digest Errors

["Nicholas A. Bellinger" <nab@xxxxxxxxxxxxxxx>]

On Mon, 2012-02-06 at 07:25 -0800, jrepac@xxxxxxxxx wrote:
> I noted something interesting with the digest error problem noted
> over the  past two months with the Emulex initiator and the target.
> It seems that problem occurs just after the target sends out a sequence
> of multiple tightly spaced R2Ts for outstanding write commands.  The
> problem occurs while simply trying to initialize or format the drive
> with the MS iSCSI initiator (using the Emulex adapter instance). Errors
> are reported in /var/log/messages as data digest failures but the wireshark
> trace shows the digest is actually good. The MS drive initialization
> completes fine when both data and header digest are disabled.
> 

Hi Joe,

Thanks for continuing to look into this issue.

> This problem only seems to occur when a group of R2T's are sent
> out with <20us between PDUs.

So thinking about this a bit more, you may want to verify that the
MaxOutstandingR2T value is being negotiated to '1' during iSCSI session
login.

Note that mainline iscsi_target_mod code does support >1 operation, but
I'm pretty sure the MSFT login logic should be negotiating
MaxOutstandingR2T=1..  Definitely worth checking at this point.

> I did four trace captures on this last night and the behavior seems to be consistent.
> The magic number so far is three R2Ts.  Nicholas fixed a list corruption issue (noted below)
> that was occurring when the first mis-detected digest error report.  However, the mis-detected
> digest error remains.  It seems like data for another outstanding write command (released by
> one of the R2Ts) is interfering with a digest calculation in progress. 
> 
> Can someone familiar with this area of the take a look at this?  I plan to cut the
> queue depth on the target to 1 today to see if the problem goes away.
> 

I assume you mean iSCSI CmdSN session depth here, yes..?  By default
this value is currently set to 16, and can be changed via
default_queue_depth (for TPG demo-mode operation) or via queue_depth
attribute for explict iSCSI initiator NodeACLs.

--nab

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