linux-ext4 - Re: Is TRIM/DISCARD going to be a performance problem?

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Message-ID: <20090511101815.GR4694@kernel.dk>
Date:	Mon, 11 May 2009 12:18:15 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jörn Engel <joern@...fs.org>
Cc:	Theodore Tso <tytso@....edu>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ric Wheeler <rwheeler@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: Is TRIM/DISCARD going to be a performance problem?

On Mon, May 11 2009, Jörn Engel wrote:
> On Mon, 11 May 2009 04:37:54 -0400, Theodore Tso wrote:
> > 
> > Well, no one has actually implemented the low-level TRIM support yet;
> 
> Iirc dwmw2 did so for some of the FTL drivers.  More a curiosity than a
> useful device, though.
> 
> > Well, no, Matthew's changes didn't do any of that, I suspect because
> > most SSD's, including X25-M, are expected to have a granularity size
> > of 1 block.  It's the crazy people in the SCSI standards world who've
> > been pushing for granlarity sizes in the 1-4 megabyte range; as I
> > understand things, the granularity issue was not going to be a problem
> > for the ATA TRIM command.
> 
> I am not sure about this part.  So far Intel has been the only party to
> release any information about their dark-grey box.  All other boxes are
> still solid black.  And until I'm told otherwise I'd consider them to be
> stupid devices that use erase block size as trim granularity.
> 
> Given the total lack of information, your guess is as good as mine,
> though.
> 
> > As far as thinking that the proposal is ludicrous --- what precisely
> > did you find ludicrous about it?
> 
> Mainly the idea that discard requests should act as barriers and instead
> of fixing that, you propose a lot of complexity to work around it.

But the command is effectively a barrier at the device level anyway,
since you need to drain the hardware queue before issuing.

> > The only problem with SSD's is the people who designed the ATA TRIM
> > command requires us to completely drian the I/O queue before issuing
> > it.  Because of this incompetence, we need to be a bit more careful
> > about how we issue them.
> 
> And this bit that I wasn't aware of.  Such a requirement in the standard
> is a trainwreck indeed.

Precisely, but that's how basically anything works with SATA NCQ, only
read/write dma commands may be queued. Anything else requires an idle
drive before issue.

-- 
Jens Axboe

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