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 14:35:42 -0500
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Stefan Priebe <s.priebe@...fihost.ag>,
	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?

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

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