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 X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2C1D3C43331 for ; Thu, 26 Mar 2020 22:21:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F0B792077D for ; Thu, 26 Mar 2020 22:21:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726359AbgCZWVW (ORCPT ); Thu, 26 Mar 2020 18:21:22 -0400 Received: from syrinx.knorrie.org ([82.94.188.77]:42652 "EHLO syrinx.knorrie.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726067AbgCZWVV (ORCPT ); Thu, 26 Mar 2020 18:21:21 -0400 Received: from [IPv6:2a02:a213:2b80:f000::12] (unknown [IPv6:2a02:a213:2b80:f000::12]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by syrinx.knorrie.org (Postfix) with ESMTPSA id CD9A36088D555; Thu, 26 Mar 2020 23:21:18 +0100 (CET) Subject: Re: Q: what exactly does SSD mode still do? To: =?UTF-8?Q?Holger_Hoffst=c3=a4tte?= , linux-btrfs References: <8dcb2f1b-7cb4-cfd4-04ba-7fe4f3c3940b@applied-asynchrony.com> From: Hans van Kranenburg Autocrypt: addr=hans@knorrie.org; keydata= mQINBFo2pooBEADwTBe/lrCa78zuhVkmpvuN+pXPWHkYs0LuAgJrOsOKhxLkYXn6Pn7e3xm+ ySfxwtFmqLUMPWujQYF0r5C6DteypL7XvkPP+FPVlQnDIifyEoKq8JZRPsAFt1S87QThYPC3 mjfluLUKVBP21H3ZFUGjcf+hnJSN9d9MuSQmAvtJiLbRTo5DTZZvO/SuQlmafaEQteaOswme DKRcIYj7+FokaW9n90P8agvPZJn50MCKy1D2QZwvw0g2ZMR8yUdtsX6fHTe7Ym+tHIYM3Tsg 2KKgt17NTxIqyttcAIaVRs4+dnQ23J98iFmVHyT+X2Jou+KpHuULES8562QltmkchA7YxZpT mLMZ6TPit+sIocvxFE5dGiT1FMpjM5mOVCNOP+KOup/N7jobCG15haKWtu9k0kPz+trT3NOn gZXecYzBmasSJro60O4bwBayG9ILHNn+v/ZLg/jv33X2MV7oYXf+ustwjXnYUqVmjZkdI/pt 30lcNUxCANvTF861OgvZUR4WoMNK4krXtodBoEImjmT385LATGFt9HnXd1rQ4QzqyMPBk84j roX5NpOzNZrNJiUxj+aUQZcINtbpmvskGpJX0RsfhOh2fxfQ39ZP/0a2C59gBQuVCH6C5qsY rc1qTIpGdPYT+J1S2rY88AvPpr2JHZbiVqeB3jIlwVSmkYeB/QARAQABtCZIYW5zIHZhbiBL cmFuZW5idXJnIDxoYW5zQGtub3JyaWUub3JnPokCTgQTAQoAOBYhBOJv1o/B6NS2GUVGTueB 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text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org Hi! On 3/26/20 7:16 PM, Holger Hoffstätte wrote: > > could someone explain what SSD mode *actually* still does? Not ssd_spread, > that's clear and unrelated. A recent commit removed the thread-offloaded > bio submission (avoiding context switches etc.) Can you share the commit id? > - which I thought was the > reason for SSD mode? - and looking through the code I couldn't find any > bits that helped clarify the difference. After the change in 2017 to change the extent allocator in ssd mode for data to behave like nossd already did before, there are two differences between ssd and nossd left: 1) This if statement in tree-log.c: cd354ad613a39 (Chris Mason 2011-10-20 15:45:37 -0400 3042) /* when we're on an ssd, just kick the log commit out */ 0b246afa62b0c (Jeff Mahoney 2016-06-22 18:54:23 -0400 3043) if (!btrfs_test_opt(fs_info, SSD) && 2) Metadata "cluster allocator" write behavior: *empty_cluster = SZ_64K # nossd *empty_cluster = SZ_2M # ssd This happens in extent-tree.c. For 1) I guess this is ok if you can do "seek free writes"? For 2) I initially wanted to start more research on the behavioral difference, but when upgrading from Linux 4.9 to 4.19, the majority of the problems with exploding extent tree metadata writes were already gone (in ssd mode), so that never happened. So, there's still those two hard coded values without any proper recent explanation why they should be at that value. Hans