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From: Dave Chinner <david@fromorbit.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Goldwyn Rodrigues <rgoldwyn@suse.de>,
	Damien Le Moal <damien.lemoal@wdc.com>,
	Naohiro Aota <naohiro.aota@wdc.com>,
	Johannes Thumshirn <jth@kernel.org>,
	Matthew Wilcox <willy@infradead.org>,
	linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	cluster-devel@redhat.com, linux-ext4@vger.kernel.org,
	linux-xfs@vger.kernel.org
Subject: Re: RFC: iomap write invalidation
Date: Wed, 15 Jul 2020 11:47:34 +1000	[thread overview]
Message-ID: <20200715014734.GE2005@dread.disaster.area> (raw)
In-Reply-To: <20200713074633.875946-1-hch@lst.de>

On Mon, Jul 13, 2020 at 09:46:31AM +0200, Christoph Hellwig wrote:
> Hi all,
> 
> this series has two parts:  the first one picks up Dave's patch to avoid
> invalidation entierly for reads, picked up deep down from the btrfs iomap
> thread.  The second one falls back to buffered writes if invalidation fails
> instead of leaving a stale cache around.  Let me know what you think about
> this approch.

Either we maintain application level concurrency for direct IOs and
ignore the stale data in the page cache, or we kill application IO
concurrency and keep the page cache coherent.

It's a lose-lose choice and I'm on the fence as to which is the
lesser of two evils.

The main factor is whether the buffered IO fallback can be
diagnosed. There's a new tracepoint for that case, so at least we
will be able to tell if the fallback co-incides with application
performance cratering. Hopefully this will only be a rare event.

So, to hoist myself on my own petard: correctness first, performance
second.

Acked-by: Dave Chinner <dchinner@redhat.com>

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

  parent reply	other threads:[~2020-07-15  1:47 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-13  7:46 RFC: iomap write invalidation Christoph Hellwig
2020-07-13  7:46 ` [PATCH 1/2] iomap: Only invalidate page cache pages on direct IO writes Christoph Hellwig
2020-07-13  7:46 ` [PATCH 2/2] iomap: fall back to buffered writes for invalidation failures Christoph Hellwig
2020-07-13 11:55   ` Matthew Wilcox
2020-07-14 11:00     ` Christoph Hellwig
2020-07-13 12:20   ` Goldwyn Rodrigues
2020-07-13 16:09     ` David Sterba
2020-07-13 15:39   ` Darrick J. Wong
2020-07-14 11:00     ` Christoph Hellwig
2020-07-14  1:41   ` Damien Le Moal
2020-07-15  1:47 ` Dave Chinner [this message]
2020-07-20 21:51 ` RFC: iomap write invalidation Goldwyn Rodrigues
2020-07-21 14:53   ` Christoph Hellwig
2020-07-21 14:59     ` Darrick J. Wong
2020-07-21 15:04     ` Matthew Wilcox
2020-07-21 15:06       ` Christoph Hellwig
2020-07-21 15:14         ` Matthew Wilcox
2020-07-21 15:16           ` Christoph Hellwig
2020-07-21 15:27             ` Darrick J. Wong
2020-07-21 15:41               ` Christoph Hellwig
2020-07-21 15:59                 ` Darrick J. Wong
2020-07-21 16:01                   ` Christoph Hellwig
2020-07-21 16:05                     ` Darrick J. Wong
2020-07-21 15:31             ` Matthew Wilcox
2020-07-21 15:42               ` Christoph Hellwig
2020-07-21 15:52                 ` Matthew Wilcox
2020-07-21 16:03                   ` Darrick J. Wong

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