linux-ext4 - Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super

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Date:	Thu, 18 Nov 2010 17:07:02 -0500
From:	Mark Lord <kernel@...savvy.com>
To:	James Bottomley <James.Bottomley@...e.de>
CC:	Greg Freemyer <greg.freemyer@...il.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Matthew Wilcox <matthew@....cx>,
	Josef Bacik <josef@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>, tytso@....edu,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, sandeen@...hat.com
Subject: Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super_operation

On 10-11-18 04:50 PM, James Bottomley wrote:
>
> Before we go gung ho on this, there's no evidence that N discontiguous
> ranges in one command are any better than the ranges sent N times ...
> the same amount of erase overhead gets sent on SSDs.

No, we do have evidence:  execution time of the TRIM commands on the SSD.

The one-range-at-a-time is incredibly slow compared to multiple ranges at a time.
That slowness comes from somewhere, with about 99.9% certainty that it is due
to the drive performing slow flash erase cycles.

I think perhaps we should do the batching as much as possible,
and then split them into single ranges for LLDs that cannot handle multi-ranges.

Way more efficient that way.

Cheers
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