linux-kernel - Re: about TRIM/DISCARD support and barriers

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Message-Id: <1227480776.25499.3.camel@localhost.localdomain>
Date:	Mon, 24 Nov 2008 07:52:56 +0900
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Matthew Wilcox <matthew@....cx>, Tejun Heo <teheo@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	Jeff Garzik <jeff@...zik.org>,
	Dongjun Shin <djshin90@...il.com>, chris.mason@...cle.com
Subject: Re: about TRIM/DISCARD support and barriers

On Sun, 2008-11-23 at 13:39 +0000, David Woodhouse wrote:
> > We don't attempt to put non-contiguous ranges into a single TRIM yet.
> 
> We don't even merge contiguous ranges -- I still need to fix the
> elevators to stop writes crossing writes,

I don't think we want to do that ... it's legal if the write isn't a
barrier and it will inhibit merging.  That may be just fine for a SSD,
but it's not for spinning media since they get better performance out of
merged writes.

>  before we can stop discards
> from also being barriers. (Discards are just writes, for the purpose of
> that conversation).

Perhaps they shouldn't be ... they have different characteristics.  For
instance, a discard may cross a read or write that has no sectors in
common with it; a discard may be merged as a non contiguous range
(assuming the drive supports multiple ranges), etc.

I think it might be better to give it its own type for the elevators.

James

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