Messages in this thread | | | Date | Fri, 22 Nov 2013 22:05:38 +0100 | From | Stefan Priebe <> | Subject | Re: Why is O_DSYNC on linux so slow / what's wrong with my SSD? |
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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.
Thanks - but i have raw block devices the data goes to .
> 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
So i have to switch to write through but i'm still using the wb cache of the device?
echo temporary write through > /sys/class/scsi_disk/<disk>/cache_type
Thanks!
Stefan
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