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

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Message-ID: <529133CC.4070904@gmail.com>
Date:	Sat, 23 Nov 2013 18:01:32 -0500
From:	Ric Wheeler <ricwheeler@...il.com>
To:	Pavel Machek <pavel@....cz>, Howard Chu <hyc@...as.com>
CC:	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 11/23/2013 03:36 PM, Pavel Machek wrote:
> On Wed 2013-11-20 08:02:33, Howard Chu wrote:
>> Theodore Ts'o wrote:
>>> 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.
>> Speaking of which, what would you use to automate this sort of test?
>> I'm thinking an SSD connected by eSATA, with an external power
>> supply, and the host running inside a VM. Drop power to the drive at
>> the same time as doing a kill -9 on the VM, then you can resume the
>> VM pretty quickly instead of waiting for a full reboot sequence.
> I was just pulling power on sata drive.
>
> It uncovered "interesting" stuff. I plugged power back, and kernel
> re-estabilished communication with that drive, but any settings with
> hdparm were forgotten. I'd say there's some room for improvement
> there...
>
> 								Pavel

Hi Pavel,

When you drop power, your drive normally loses temporary settings (like a change 
to write cache, etc).

Depending on the class of the device, there are ways to make that permanent 
(look at hdparm or sdparm for details).

This is a feature of the drive and its firmware, not something we reset in the 
device each time it re-appears.

Ric

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