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

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Message-ID: <528FB71C.5020304@profihost.ag>
Date:	Fri, 22 Nov 2013 20:57:16 +0100
From:	Stefan Priebe <s.priebe@...fihost.ag>
To:	"J. Bruce Fields" <bfields@...ldses.org>,
	Theodore Ts'o <tytso@....edu>, Chinmay V S <cvs268@...il.com>,
	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?

Am 20.11.2013 16:55, schrieb J. Bruce Fields:
> 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.

Yes but they all should make use and support CMD_FLUSH so it's slow on 
them too.

> 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.

Correct i think intel simply ignores CMD_FLUSH on that drive - no idea 
why an they fixed this for their 330, 530, DC S3500 (all tested)

> --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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