From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stefan Priebe Subject: Re: Why is O_DSYNC on linux so slow / what's wrong with my SSD? Date: Fri, 22 Nov 2013 22:05:38 +0100 Message-ID: <528FC722.7010703@profihost.ag> 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> 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: Ric Wheeler , Christoph Hellwig , Chinmay V S Return-path: In-Reply-To: <528FC09E.5090004@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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//cache_type Thanks! Stefan