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

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Date:	Tue, 12 May 2009 09:38:21 +1000
From:	Neil Brown <neilb@...e.de>
To:	Greg Freemyer <greg.freemyer@...il.com>
Cc:	Matthew Wilcox <matthew@....cx>, Theodore Tso <tytso@....edu>,
	Ric Wheeler <rwheeler@...hat.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 Monday May 11, greg.freemyer@...il.com wrote:
> 
> And since the mdraid layer is not currently planning to track what has
> been discarded over time, when a re-shape comes along, it will
> effectively un-trim everything and rewrite 100% of the FS.

You might not call them "plans" exactly, but I have had thoughts
about tracking which part of an raid5 had 'live' data and which were
trimmed.  I think that is the only way I could support TRIM, unless
devices guarantee that all trimmed blocks read a zeros, and that seems
unlikely.
You are right that the granularity would have to be at least
one stripe.
And a re-shape would be interesting, wouldn't it!  We could probably
avoid instantiating every trimmed block, but in general quite a few
would get instantiated.. I hadn't thought about that...

NeilBrown
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