linux-kernel - Re: New TRIM/UNMAP tree published (2009-05-02)

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Date:	Sun, 03 May 2009 18:54:06 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Matthew Wilcox <matthew@....cx>
CC:	Boaz Harrosh <bharrosh@...asas.com>,
	Hugh Dickins <hugh@...itas.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jeff Garzik <jgarzik@...hat.com>, linux-scsi@...r.kernel.org,
	Jens Axboe <jens.axboe@...cle.com>,
	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>,
	Mark Lord <lkml@....ca>, David Woodhouse <dwmw2@...radead.org>
Subject: Re: New TRIM/UNMAP tree published (2009-05-02)

Matthew Wilcox wrote:
>> (3) when creating a discard request, use block helpers w/ queue-specific  
>> knowledge to create either
>> 	(a) one request, REQ_TYPE_FS, with discard flag or
>> 	(b) two requests, REQ_TYPE_FS followed by REQ_TYPE_DISCARD
> 
> I'm not sure we need option 3b.


Well -- it is a hard requirement to map 1:1 struct request with the 
underlying hardware device's command set.

If a device command set lacks a READ-and-DISCARD operation, then you 
_must_ create two struct request.  Otherwise you break block layer 
tagging and other 1:1-based assumptions that exist today.	

Thus, given that all of ATA|SCSI|NVMHCI have a DISCARD operation 
distinct from other commands...

option 3b is the overwhelming common case.

	Jeff



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