From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ric Wheeler Subject: Re: Why is O_DSYNC on linux so slow / what's wrong with my SSD? Date: Sat, 23 Nov 2013 14:35:42 -0500 Message-ID: <5291038E.4000908@redhat.com> References: <528CA73B.9070604@profihost.ag> <20131120125446.GA6284@infradead.org> <528CC36A.7080003@profihost.ag> <20131120153703.GA23160@thunk.org> <20131120155507.GA5380@fieldses.org> <20131120175807.GC5380@fieldses.org> <20131121101101.GA18404@infradead.org> <528FB828.5000301@profihost.ag> <528FC09E.5090004@redhat.com> <5290F386.5040806@profihost.ag> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: "J. Bruce Fields" , "Theodore Ts'o" , linux-fsdevel@vger.kernel.org, Al Viro , LKML , Matthew Wilcox To: Stefan Priebe , Christoph Hellwig , Chinmay V S Return-path: In-Reply-To: <5290F386.5040806@profihost.ag> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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