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

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Message-ID: <4A083D1E.2040807@redhat.com>
Date:	Mon, 11 May 2009 10:58:38 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Theodore Tso <tytso@....edu>
CC:	Greg Freemyer <greg.freemyer@...il.com>,
	Jörn 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 05/11/2009 10:50 AM, 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.  :-)
>
> 						- Ted


Probably not as uncommon as you would think, but not as you suggest to raid thin 
provisioned luns (those are done usually as RAID devices inside an array).

Think more of the array providing a thinly provisioned LUN made up out of T13 
TRIM enabled SSD's devices internally.  RAID makes sense here (data protection 
is still needed to avoid a single point of failure) and the relative expense of 
the SSD's devices makes "thin provisioning" really attractive to external users :-)

ric
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