From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: raid5-cache I/O path improvements Date: Wed, 9 Sep 2015 17:59:41 +0200 Message-ID: <20150909155941.GA28608@lst.de> References: <1441603250-5119-1-git-send-email-hch@lst.de> <20150908002840.GA3196542@devbig257.prn2.facebook.com> <20150908061215.GA23833@lst.de> <20150908165611.GA371379@devbig257.prn2.facebook.com> <20150908170226.GI13749@mtj.duckdns.org> <20150908170736.GA423191@devbig257.prn2.facebook.com> <20150908173420.GJ13749@mtj.duckdns.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from verein.lst.de ([213.95.11.211]:38647 "EHLO newverein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752147AbbIIP7n (ORCPT ); Wed, 9 Sep 2015 11:59:43 -0400 Content-Disposition: inline In-Reply-To: <20150908173420.GJ13749@mtj.duckdns.org> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Tejun Heo Cc: Shaohua Li , Christoph Hellwig , neilb@suse.de, linux-raid@vger.kernel.org, Kernel-team@fb.com, dan.j.williams@intel.com, "Martin K. Petersen" , linux-ide@vger.kernel.org On Tue, Sep 08, 2015 at 01:34:20PM -0400, Tejun Heo wrote: > Hmmm... grep tells me that dm and md actually are branching on whether > the underlying device supports FUA. This is tricky. I didn't even > mean flush_flags to be used directly by upper layers. For rotational > devices, doing multiple FUAs compared multiple writes followed by > REQ_FLUSH is probably a lot worse - the head gets moved multiple times > likely skipping over data which can be written out while traversing > and it's not like stalling write pipeline and draining write queue has > much impact on hard drives. Well, that's what we'd need to do for the raid cache as well, given the resulst that Shaohua sees. Unless you have a good idea for another way to handle the issue we'll need to support both behaviors there.