linux-kernel - Re: scsi_debug: shared dev context, BUG or FEATURE?

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Message-ID: <87o9wl3cxh.fsf@dmlp.sw.ru>
Date:   Tue, 28 Mar 2017 13:36:58 +0300
From:   Dmitry Monakhov <dmonakhov@...nvz.org>
To:     "Martin K. Petersen" <martin.petersen@...cle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, dgilbert@...erlog.com,
        martin.petersen@...cle.com
Subject: Re: scsi_debug: shared dev context, BUG or FEATURE?

"Martin K. Petersen" <martin.petersen@...cle.com> writes:

> Dmitry Monakhov <dmonakhov@...nvz.org> writes:
>
> Dmitry,
>
>> scsi_debug has very strange structure from one point it supports
>> dynamic number of devices but from other point context is common for
>> all devices:
>
>> So basically we may have many devices with single context which refers
>> common data. Are any sane reason to share context between devices?
>> Who use such behaviour?
>
> As the name implies, scsi_debug was conceived to debug the SCSI layer.
> Among other things, the intent was to be able to test hundreds of
> controllers and LUNs without having physical hardware or storage to back
> that up. Plus to have a target whose reporting could easily be tweaked
> to test the SCSI core code.
>
> So that's the reason for the oddball shared buffer setup. scsi_debug
> wasn't really meant to be a "useful" storage target.
>
> If you want something with a per-device backing store I suggest you look
> at the SCSI target subsystem. With tcm_loop and ramdisk you get
> essentially the same thing as scsi_debug. With the added bonus that you
> can use files or block devices if you actually want the data to be
> persistent.
Wow this is really awesome. This is exactly what I need. Thank you.
>
>> IMHO this is a pure bug. Please correct me if I'm wrong, I'll plan to
>> fix that by allocation separate context for each dev. 
>
> I don't have a problem with allowing it as an option as long as the
> original behavior can be preserved. But again, I think target mode is a
> better bet if you actually care about what's being stored on the
> "media".

>
> -- 
> Martin K. Petersen	Oracle Linux Engineering