linux-ext4 - Re: [PATCH 2/2] Add batched discard support for ext4.

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Date:	Sat, 24 Apr 2010 14:30:54 -0400
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	Eric Sandeen <sandeen@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Mark Lord <kernel@...savvy.com>, linux-ext4@...r.kernel.org,
	Edward Shishkin <eshishki@...hat.com>,
	Eric Sandeen <esandeen@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 2/2] Add batched discard support for ext4.

On Sat, Apr 24, 2010 at 1:04 PM, Ric Wheeler <rwheeler@...hat.com> wrote:
<snip>
>
> Let's summarize.
>
> 1. Everyone agrees that doing larger discard instead of little discards is a
> good thing.
>
> 2. Some devices care about this more than others (various types of SSD's and
> arrays have different designs and performance with discards). Some devices
> do small discards well, others don't.
>
> 3. How you get to those bigger discards in our implementation - using a
> series of single range requests, using vectored requests, tracking extents
> that can be combined in an rbtree or not - is something that we are working
> on. Using rbtrees versus a bitmap efficiency is about DRAM consumption, not
> performance of the resulting discard on the target.
>
> 4. Devices (some devices) can export their preferences in a standard way
> (look in /sys/block/....).
>
> If you want to influence the code, please do try the various options on
> devices you have at hand and report results.  That is what we are doing (we
> includes Lukas, Eric, Jeff and others on this thread) will real devices from
> vendors that have given us access. We are talking to them directly and
> trying out different work loads but certainly welcome real world results and
> suggestions.
>
> Thanks!
>
> Ric

Ric,

Is it also agreed by all that the current ext4 kernel implementation
of calling discard is a poor solution for most hardware / block layers
stacks / workloads and therefore is not worth retaining nor performing
further benchmarks?

I've not seen anyone arguing to keep the current kernel implementation
and I for one accept the previously posted benchmarks that show it is
not adding any value relative to the traditional non-discard case.

Therefore benchmarks between the current hdparm/wiper.sh userspace
implementation and a proposed new kernel implementation would be the
most beneficial?

I have yet to see any of those benchmarks posted.

Greg



-- 
Greg Freemyer
Head of EDD Tape Extraction and Processing team
Litigation Triage Solutions Specialist
http://www.linkedin.com/in/gregfreemyer
CNN/TruTV Aired Forensic Imaging Demo -
   http://insession.blogs.cnn.com/2010/03/23/how-computer-evidence-gets-retrieved/

The Norcross Group
The Intersection of Evidence & Technology
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