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

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Date:	Mon, 25 Nov 2013 08:37:50 +0100
From:	Stefan Priebe <s.priebe@...fihost.ag>
To:	Ric Wheeler <rwheeler@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Chinmay V S <cvs268@...il.com>
CC:	"J. Bruce Fields" <bfields@...ldses.org>,
	Theodore Ts'o <tytso@....edu>, 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?

Hi Ric,

Am 23.11.2013 20:35, schrieb Ric Wheeler:
> On 11/23/2013 01:27 PM, Stefan Priebe wrote:
>> Hi Ric,
>>
>> Am 22.11.2013 21:37, schrieb Ric Wheeler:
>>> On 11/22/2013 03:01 PM, Stefan Priebe wrote:
>>>> Hi Christoph,
>>>> Am 21.11.2013 11:11, schrieb Christoph Hellwig:
>>>>>>
>>>>>> 2. Some drives may implement CMD_FLUSH to return immediately i.e. no
>>>>>> guarantee the data is actually on disk.
>>>>>
>>>>> In which case they aren't spec complicant.  While I've seen countless
>>>>> data integrity bugs on lower end ATA SSDs I've not seen one that
>>>>> simpliy
>>>>> ingnores flush.  If you'd want to cheat that bluntly you'd be better
>>>>> of just claiming to not have a writeback cache.
>>>>>
>>>>> You solve your performance problem by completely disabling any chance
>>>>> of having data integrity guarantees, and do so in a way that is not
>>>>> detectable for applications or users.
>>>>>
>>>>> If you have a workload with lots of small synchronous writes disabling
>>>>> the writeback cache on the disk does indeed often help, especially
>>>>> with
>>>>> the non-queueable FLUSH on all but the most recent ATA devices.
>>>>
>>>> But this isn't correct for drives with capicitors like Crucial m500,
>>>> Intel DC S3500, DC S3700 isn't it? Shouldn't the linux kernel has an
>>>> option to disable this for drives like these?
>>>> /sys/block/sdX/device/ignore_flush
>>>
>>> If you know 100% for sure that your drive has a non-volatile write
>>> cache, you can run the file system without the flushing by mounting "-o
>>> nobarrier".  With most devices, this is not needed since they tend to
>>> simply ignore the flushes if they know they are power failure safe.
>>>
>>> Block level, we did something similar for users who are not running
>>> through a file system for SCSI devices - James added support to echo
>>> "temporary" into the sd's device's cache_type field:
>>>
>>> See:
>>>
>>> https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?id=2ee3e26c673e75c05ef8b914f54fadee3d7b9c88
>>>
>>
>> At least to me this does not work. I get the same awful speed as
>> before - also the I/O waits stay the same. I'm still seeing CMD
>> flushes going to the devices.
>>
>> Is there any way to check whether the temporary got accepted and works?
>>
>> I simply executed:
>> for i in /sys/class/scsi_disk/*/cache_type; do echo $i; echo temporary
>> write back >$i; done
>>
>> Stefan
>
> What kernel are you running?  This is a new addition....
>
> Also, you can "cat" the same file to see what it says.
>
> Regards,
>
> Ric
>

Is the output i sent to you fine? Anything wrong?

Stefan

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