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

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Message-ID: <5290F386.5040806@profihost.ag>
Date:	Sat, 23 Nov 2013 19:27:18 +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 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
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