linux-kernel - Re: [PATCH 4/7] sd: add support for WRITE SAME (16) with unmap bit

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Date:	Sat, 29 Aug 2009 22:43:44 -0400
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org, liml@....ca, jens.axboe@...cle.com,
	matthew@....cx, dwmw2@...radead.org
Subject: Re: [PATCH 4/7] sd: add support for WRITE SAME (16) with unmap bit

Christoph Hellwig wrote:
> On Sat, Aug 29, 2009 at 08:43:27PM -0400, Douglas Gilbert wrote:
>>>  +	if (buffer[14] & 0x80)
>>> +		sdkp->thin_provisioning = 1;
>>> +
>> So you are checking the TPE bit (Thin Provisioning Enabled) but
>> not the TPRZ bit (Thin Provisioning Read Zeros). Shouldn't
>> there also be a sdkp->thin_provisioning_read_zeros bit defined
>> in struct scsi_disk?
>>
>> Is your application well defined when TPRZ==0 ?
> 
> Filesystems do not care if these blocks are in a defined state, as
> they must never return the content of uninitilized blocks to userspace.
> Now if we do want to support discard through raid arrays we might start
> to care, and will have check the TPRZ bit.

Another reason to note the TPRZ bit is that if it is 1 then
the data given to WRITE SAME (16 and 32) must be a logical
block of zeros for the UNMAP bit to be honoured (sbc3r19.pdf
section 4.6.3.2 last paragraph).
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