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

[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

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