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Re: [Ksummit-2005-discuss] Summary of 2005 Kernel Summit Proposed Topics

To: "open-iscsi@xxxxxxxxxxxxxxxx" <open-iscsi@xxxxxxxxxxxxxxxx>
Subject: Re: [Ksummit-2005-discuss] Summary of 2005 Kernel Summit Proposed Topics
From: Dmitry Yusupov <dmitry_yus@xxxxxxxxx>
Date: Sun, 27 Mar 2005 13:49:11 -0800
Cc: Mike Christie <michaelc@xxxxxxxxxxx>, mpm@xxxxxxxxxxx, andrea@xxxxxxx, James.Bottomley@xxxxxxxxxxxxxxxxxxxxx, ksummit-2005-discuss@xxxxxxxxx, netdev@xxxxxxxxxxx
In-reply-to: <20050327103115.5c746472.davem@xxxxxxxxxxxxx>
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On Sun, 2005-03-27 at 10:31 -0800, David S. Miller wrote:
> On Sun, 27 Mar 2005 10:26:29 -0800
> Mike Christie <michaelc@xxxxxxxxxxx> wrote:
> > reliable receive is ciritical for WRITEs. Even if the WRITE is executed
> > successfully on the remote device, if we cannot receive the return status
> > from the device the operation will fail at the iscsi driver side due to a
> > SCSI timeout.
> I keep hearing this word "reliable", it means something very
> different for TCP over a transport like IP than it does
> for the SCSI layer.
> It is, in fact, the whole difficulty of implementing iSCSI:
> being able to cope with this difference in expectations.

actually, you absolutely right!

there are two ways to achieve reasonable "reliability" of iSCSI

1) ERL=2(see RFC3720) + multiple connections (read: single iSCSI session
with multiple connections)
2) ERL=0 + device multipath (read: multiple iSCSI sessions with single
connection per session)

I think we should slow down on "reliability" feature since 1) and 2)
basically covers the case. All we need is to make sure that deadlock
situations during OOM will never happen.

> All I can see is that the SCSI layer's timeout is inappropriate
> for something like iSCSI, not that TCP or networking needs
> to change in some way.

right. looks like there is the way to "postpone" SCSI timeout until
iSCSI session will finally recover.

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