linux-ext4 - Re: Severe slowdown caused by jbd2 process

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Message-ID: <20110124201603.GA5131@thunk.org>
Date:	Mon, 24 Jan 2011 15:16:03 -0500
From:	Ted Ts'o <tytso@....edu>
To:	torn5 <torn5@...ftmail.org>
Cc:	Josef Bacik <josef@...hat.com>,
	Jon Leighton <j@...athanleighton.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Severe slowdown caused by jbd2 process

On Sun, Jan 23, 2011 at 07:43:10PM +0100, torn5 wrote:
> I thought it was possible via the completion notifications from the disk.
> AFAIK if a disk is in NCQ mode it will return completion for a
> command only when the write was really delivered to the platters.
> While in non-NCQ mode the disk immediately returns completion and
> caches the write. Is this correct?

No, that's not correct.  The completion notification from the disk is
merely that the DMA has completed.  It does not mean that the data has
hit the platters.  This is true in both NCQ and non-NCQ mode.

You can disable the write cache (which is what I think you're thinking
about), but the performance hit is pretty significant on standard
HDD's.

						- Ted
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