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

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Date:	Mon, 11 May 2009 09:00:40 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Theodore Tso <tytso@....edu>
Cc:	Ric Wheeler <rwheeler@...hat.com>,
	Greg Freemyer <greg.freemyer@...il.com>,
	J?rn Engel <joern@...fs.org>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	Linux RAID <linux-raid@...r.kernel.org>
Subject: Re: Is TRIM/DISCARD going to be a performance problem?

On Mon, May 11, 2009 at 10:50:59AM -0400, Theodore Tso wrote:
> On Mon, May 11, 2009 at 10:29:51AM -0400, Ric Wheeler wrote:
> > The key is not at the FS layer - this is an issue for people who RAID 
> > these beasts together and want to actually check that the bits are what 
> > they should be (say doing a checksum validity check for a stripe).
> 
> Good point, yes I can see why they need that.  In that case, the
> storage device can't just silently truncate a TRIM request; it would
> have to expose to the OS its alignment requirements.  The risk though
> is that more they try push this compleixity into the OS, the higher
> the risk that the OS will simply decide not to take advantage of the
> functionality.  Of course, there is the question why anyone would want
> to build a software-raid device on top of a thin-provisioned hardware
> storage unit.  :-)

It's not a problem for people who use Thin Provisioning, it's a problem
for people who want to run RAID-5 on top of SSDs.  If you have a sector
whose reads are indeterminate, your parity for that stripe will always
be wrong.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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