linux-kernel - Re: Why is O_DSYNC on linux so slow / what's wrong with my SSD?

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Message-ID: <20131120155507.GA5380@fieldses.org>
Date:	Wed, 20 Nov 2013 10:55:07 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Theodore Ts'o <tytso@....edu>, Chinmay V S <cvs268@...il.com>,
	Stefan Priebe - Profihost AG <s.priebe@...fihost.ag>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>, matthew@....cx
Subject: Re: Why is O_DSYNC on linux so slow / what's wrong with my SSD?

On Wed, Nov 20, 2013 at 10:37:03AM -0500, Theodore Ts'o wrote:
> On Wed, Nov 20, 2013 at 08:52:36PM +0530, Chinmay V S wrote:
> > 
> > If you have confirmed the performance numbers, then it indicates that
> > the Intel 530 controller is more advanced and makes better use of the
> > internal disk-cache to achieve better performance (as compared to the
> > Intel 520). Thus forcing CMD_FLUSH on each IOP (negating the benefits
> > of the disk write-cache and not allowing any advanced disk controller
> > optimisations) has a more pronouced effect of degrading the
> > performance on Intel 530 SSDs. (Someone with some actual info on Intel
> > SSDs kindly confirm this.)
> 
> You might also want to do some power fail testing to make sure that
> the SSD is actually flusing all of its internal Flash Translation
> Layer (FTL) metadata to stable storage on every CMD_FLUSH command.

Some SSD's are also claim the ability to flush the cache on power loss:

	http://www.intel.com/content/www/us/en/solid-state-drives/ssd-320-series-power-loss-data-protection-brief.html

Which should in theory let them respond immediately to flush requests,
right?  Except they only seem to advertise it as a safety (rather than a
performance) feature, so I probably misunderstand something.

And the 520 doesn't claim this feature (look for "enhanced power loss
protection" at http://ark.intel.com/products/66248), so that wouldn't
explain these results anyway.

--b.

> 
> There are lots of flash media that don't do this, with the result that
> I get lots of users whining at me when their file system stored on an
> SD card has massive corruption after a power fail event.
> 
> Historically, Intel has been really good about avoiding this, but
> since they've moved to using 3rd party flash controllers, I now advise
> everyone who plans to use any flash storage, regardless of the
> manufacturer, to do their own explicit power fail testing (hitting the
> reset button is not good enough, you need to kick the power plug out
> of the wall, or better yet, use a network controlled power switch you
> so you can repeat the power fail test dozens or hundreds of times for
> your qualification run) before being using flash storage in a mission
> critical situation where you care about data integrity after a power
> fail event.
> 
> IOW, make sure that the SSD isn't faster because it's playing fast and
> loose with the FTL metadata....
> 
> Cheers,
> 
> 						- Ted
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