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Date: Tue, 17 Jun 2014 08:14:05 +0200
From: Peter Lieven <pl@kamp.de>
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Subject: Re: [Qemu-devel] [PATCH] block/iscsi: use 16 byte CDBs only when
 necessary
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To: Michael Tokarev <mjt@tls.msk.ru>, ronnie sahlberg <ronniesahlberg@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>

On 05.06.2014 11:12, Michael Tokarev wrote:
> 04.06.2014 18:00, ronnie sahlberg wrote:
>> That would mean you get to use the 10 version of the cdb even for very
>> large devices (as long as the IO is for blocks at the beginning of the
>> device) and thus provide partial avoidance of this issue for those
>> large devices.
> That may make some bugs "ghosty", so to say.  Ie, if there's a bug in/with
> 16 version of a command, you'll hit it only when you actually try to access
> a "far" area of a drive.  Which means you're unlikely to hit it while trying
> to reproduce in a clean environment, even after using a large device.  Or,
> the bug will be triggered at random, since data placement on the filesystem
> is effectively (from user PoV) random.
>
> To my taste it is better to make it a bit more deterministic.

BTW, while debugging a case with a bigger storage supplier I found
that open-iscsi seems to do exactly this undeterministic behaviour.
I have a 3TB LUN. If I access < 2TB sectors it uses READ10/WRITE10 and
if I go beyond 2TB it changes to READ16/WRITE16.

Peter