linux-kernel - Re: [Scst-devel] Fwd: Re: linuxcon 2010...

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Date:	Sat, 21 Aug 2010 22:42:16 +0400
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
CC:	Dirk Meister <dmeister@...-paderborn.de>,
	linux-scsi@...r.kernel.org,
	James Bottomley <James.Bottomley@...e.de>,
	Chetan Loke <chetanloke@...il.com>,
	Chetan Loke <generationgnu@...oo.com>,
	scst-devel <scst-devel@...ts.sourceforge.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [Scst-devel] Fwd: Re: linuxcon 2010...

Nicholas A. Bellinger, on 08/20/2010 02:29 AM wrote:
> "We will be extending the scsi_tgt_[lib,if].c mapped ring interface to
> allow TCM to access userspace backstores transparently with existing
> kernel level TCM fabric modules, and using the generic configfs fabric
> module infrastructure in target_core_fabric_configfs.c for the port and
> I_T nexus control plane just as you would with any TCM backstore
> subsystem today.
>
> Again, in open source you have to build upon what already exists and
> move forward.  The original STGT kernel<->  userspace ring abstraction
> and logic in drivers/scsi/scsi_tgt_lib.c:scsi_tgt_queue_command() ->
> scsi_tgt_uspace_send_cmd() is already going to do the vast majority of
> what is required for handling fabric I/O processing and I_T Nexus and
> Port management in kernel space with a userspace backstore.  It is
> really just a matter of allowing the STGT ring request to optionally be
> sent out to userspace as a standalone LUN instead of as a target port."

You forgot to mention one small thing. You are going to (re)use current 
STGT interface for user space backend, which in 5 years of being 
mainline has not gained any noticeable interest, because it is 
fundamentally slow. It needs a complete redesign to become fast. In 
contrast, interface provided by scst_user has just 3% overhead comparing 
with fully in-kernel backend and allows to build storage capable of 
handling 1,500,000 IOPSes (Kaminario).

Vlad
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