linux-kernel - Re: [PATCH 3.16 106/217] sd: disable discard_zeroes

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Date:	Mon, 9 May 2016 16:17:15 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Rafael David Tinoco <rafael.tinoco@...onical.com>,
	Ben Hutchings <ben@...adent.org.uk>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	akpm@...ux-foundation.org,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 3.16 106/217] sd: disable discard_zeroes_data for UNMAP



On 29/04/2016 06:00, Rafael David Tinoco wrote:
> Actually, It was an objection.
> 
> Knowing that WRITESAME(16), used as the discard mechanism, can cause
> storage servers to misbehave (like QEMU's SCSI WRITESAME
> implementation, workaround-ed by commit e461338b6cd4) and those
> storage servers can't  rely on LBPRZ flag to opt out from WRITESAME as
> discard mechanism (like QEMU does) since it is out of spec...

I think e461338b6cd4 is a perfectly fine change---independent of whether
it's a workaround for QEMU---and should be backported to stable kernels too.

Is there a reason to use WRITE SAME if LBPRZ=0?  You risk doing a real
write which breaks thin provisioning and will probably take a huge time too.

Paolo