From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B8831C433EF for ; Sat, 28 May 2022 07:22:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229636AbiE1HWu (ORCPT ); Sat, 28 May 2022 03:22:50 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60862 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229446AbiE1HWt (ORCPT ); Sat, 28 May 2022 03:22:49 -0400 Received: from smtp.mfedv.net (smtp.mfedv.net [IPv6:2a04:6c0:2::19]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 25509237EA for ; Sat, 28 May 2022 00:22:43 -0700 (PDT) Received: from suse92host.mfedv.net (suse92host.mfedv.net [IPv6:2a04:6c0:2:3:0:0:0:ffff]) by smtp.mfedv.net (8.15.2/8.15.2/Debian-10) with ESMTP id 24S7MUr5010091; Sat, 28 May 2022 09:22:31 +0200 Received: from xoff (klappe2.mfedv.net [192.168.71.72]) by suse92host.mfedv.net (Postfix) with SMTP id 44BD4C809A; Sat, 28 May 2022 09:22:30 +0200 (CEST) (envelope-from bcache@mfedv.net) Date: Sat, 28 May 2022 09:22:30 +0200 From: Matthias Ferdinand To: Eric Wheeler Cc: Adriano Silva , Coly Li , Bcache Linux Subject: Re: Bcache in writes direct with fsync. Are IOPS limited? Message-ID: References: <958894243.922478.1652201375900.ref@mail.yahoo.com> <958894243.922478.1652201375900@mail.yahoo.com> <9d59af25-d648-4777-a5c0-c38c246a9610@ewheeler.net> <681726005.1812841.1653564986700@mail.yahoo.com> <8aac4160-4da5-453b-48ba-95e79fb8c029@ewheeler.net> <532745340.2051210.1653624441686@mail.yahoo.com> <5b164113-364b-76a8-5bcc-94c1cec868db@ewheeler.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <5b164113-364b-76a8-5bcc-94c1cec868db@ewheeler.net> Precedence: bulk List-ID: X-Mailing-List: linux-bcache@vger.kernel.org On Fri, May 27, 2022 at 06:27:53PM -0700, Eric Wheeler wrote: > > I can say that the performance of tests after the write back command for > > all devices greatly worsens the performance of direct tests on NVME > > hardware. Below you can see this. > > I wonder what is going on there! I tried the same thing on my system and > 'write through' is faster for me, too, so it would be worth investigating. In Ceph context, it seems not unusual to disable SSD write back cache and see much improved performance (or the other way round: see surprisingly low performance with write back cache enabled): https://yourcmc.ru/wiki/Ceph_performance#Drive_cache_is_slowing_you_down Disk controllers seem to interpret FLUSH CACHE / FUA differently. If bcache would set FUA for cache device writes while running fio directly on the nvme device would not, that might explain the timing difference. Regards Matthias