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

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Date:	Sat, 23 Nov 2013 21:43:16 -0500
From:	Ric Wheeler <ricwheeler@...il.com>
To:	Pavel Machek <pavel@....cz>
CC:	Howard Chu <hyc@...as.com>, 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 07:22 PM, Pavel Machek wrote:
> On Sat 2013-11-23 18:01:32, Ric Wheeler wrote:
>> 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...
>> 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.
> Yes, and I'm arguing that is a bug (as in, < 0.01% people are using
> hdparm correctly).

Almost no end users use hdparm. Those who do should read the man page and add 
the -K flag :)

Or system scripts that tweak should invoke it with the right flags.....

Ric

> So you used hparm to disable write cache so that ext3 can be safely
> used on your hdd. Now you have glitch on power. Then, system continues
> to operate in dangerous mode until reboot.
>
> I guess it would be safer not to reattach drives after power
> fail... (also I wonder what this does to data integrity. Drive lost
> content of its writeback cache, but kernel continues... Journal will
> not prevent data corruption in this case).
>
> 									Pavel

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