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

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Message-ID: <CAK-9PRCQ6qWvzpdvK8BswpS+TfgM7NeoRaVdChFHhyDjwtUGTw@mail.gmail.com>
Date:	Wed, 20 Nov 2013 22:41:54 +0530
From:	Chinmay V S <cvs268@...il.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	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 Wilcox <matthew@....cx>
Subject: Re: Why is O_DSYNC on linux so slow / what's wrong with my SSD?

On Wed, Nov 20, 2013 at 9:25 PM, J. Bruce Fields <bfields@...ldses.org> wrote:
> 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.

FYI, nowhere does Intel imply that the CMD_FLUSH is instantaneous. The
product brief for Intel 320 SSDs (above link), explains that it is
implemented by a power-fail detection circuit that detects drop in
power-supply, following which the on-disk controller issues an internal
CMD_FLUSH equivalent command to ensure data is moved to the
non-volatile area from the disk-cache. Large secondary capacitors
ensure backup supply for this brief duration.

Thus applications can always perform asynchronous I/O upon the disk,
taking comfort in the fact that the physical disk ensures that all
data in the volatile disk-cache is automatically transferred to the
non-volatile area even in the event of an external power-failure. Thus
the host never has to worry about issuing a CMD_FLUSH (which is still
a terribly expensive performance bottleneck, even on the Intel 320
SSDs).
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