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=-13.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable 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 1D1F7C4361A for ; Thu, 3 Dec 2020 17:58:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B256E207A2 for ; Thu, 3 Dec 2020 17:58:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729343AbgLCR6i (ORCPT ); Thu, 3 Dec 2020 12:58:38 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:41140 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726670AbgLCR6h (ORCPT ); Thu, 3 Dec 2020 12:58:37 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1607018230; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=gWp36VufSubdlmAvb6F/9rBQn5CM1OS7g9pplIoqYFw=; b=OFv+DjjAh7r+KI6jXNLUwDwrtS38XPGxPG8r7LFdaRVT7fmiB6mtm98hixvgQC9UBRJqyB DA8rvbPr1TCav1J5dQSzBdxYdW1uTAqjQ1AIwornYhF13nHpga4SoNo9vDeCoResR1iuk7 AmqrgH9XRdhSdfKk9aExl4aHkParXGQ= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-62-usm6uAdLOj-E2Ore2JG19w-1; Thu, 03 Dec 2020 12:57:08 -0500 X-MC-Unique: usm6uAdLOj-E2Ore2JG19w-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5700C107ACE3; Thu, 3 Dec 2020 17:57:07 +0000 (UTC) Received: from localhost (unknown [10.18.25.174]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7016460854; Thu, 3 Dec 2020 17:56:58 +0000 (UTC) Date: Thu, 3 Dec 2020 12:56:57 -0500 From: Mike Snitzer To: Keith Busch Cc: Ming Lei , axboe@kernel.dk, martin.petersen@oracle.com, linux-block@vger.kernel.org, dm-devel@redhat.com, jdorminy@redhat.com, bjohnsto@redhat.com Subject: Re: [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking Message-ID: <20201203175657.GA29623@redhat.com> References: <20201130171805.77712-1-snitzer@redhat.com> <20201201160709.31748-1-snitzer@redhat.com> <20201203032608.GD540033@T590> <20201203143359.GA29261@redhat.com> <20201203162738.GA3404013@dhcp-10-100-145-180.wdc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201203162738.GA3404013@dhcp-10-100-145-180.wdc.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Thu, Dec 03 2020 at 11:27am -0500, Keith Busch wrote: > On Thu, Dec 03, 2020 at 09:33:59AM -0500, Mike Snitzer wrote: > > On Wed, Dec 02 2020 at 10:26pm -0500, > > Ming Lei wrote: > > > > > I understand it isn't related with correctness, because the underlying > > > queue can split by its own chunk_sectors limit further. So is the issue > > > too many further-splitting on queue with chunk_sectors 8? then CPU > > > utilization is increased? Or other issue? > > > > No, this is all about correctness. > > > > Seems you're confining the definition of the possible stacking so that > > the top-level device isn't allowed to have its own hard requirements on > > IO sizes it sends to its internal implementation. Just because the > > underlying device can split further doesn't mean that the top-level > > virtual driver can service larger IO sizes (not if the chunk_sectors > > stacking throws away the hint the virtual driver provided because it > > used lcm_not_zero). > > I may be missing something obvious here, but if the lower layers split > to their desired boundary already, why does this limit need to stack? The problematic scenario is when the topmost layer, or layers, are the more constrained. _That_ is why the top-level's chunk_sectors limit cannot be relaxed. For example (in extreme where chunk_sectors is stacked via gcd): dm VDO target (chunk_sectors=4K) on dm-thin (ideally chunk_sectors=1280K, reality chunk_sectors=128K) on 10+2 RAID6 (chunk_sectors=128K, io_opt=1280K) on raid members (chunk_sectors=0) Results in the following bottom up blk_stack_limits() stacking: gcd(128K, 0) = 128K -> but MD just sets chunk_sectors, no stacking is done afaik gcd(1280K, 128K) = 128K -> this one hurts dm-thin, needless splitting gcd(4K, 128K) = 4K -> vdo _must_ receive 4K IOs, hurts but "this is the way" ;) So this is one extreme that shows stacking chunk_sectors is _not_ helpful (if the resulting chunk_sectors were actually used as basis for splitting). Better for each layer to just impose its own chunk_sectors without concern for the layers below. Think I'd be fine with block core removing the chunk_sectors stacking from blk_stack_limits()... (and as you see below, I've been forced to revert to _not_ using stacked chunk_sectors based splitting in DM) > Won't it also work if each layer sets their desired chunk_sectors > without considering their lower layers? The commit that initially > stacked chunk_sectors doesn't provide any explanation. Yes, I think it would work. The current stacking doesn't have the luxury of knowing which layer a blk_stack_limits() maps too. BUT within a layer chunk_sectors really does need to be compatible/symbiotic. So it is unfortunately all or nothing as you build up the stack. And that all-or-nothing stacking of chunk_sectors is why I've now (just last night, based on further review by jdorminy) had to punt on using stacked chunk_sectors and revert DM back to doing its own fine-grained (and varied) splitting on a per DM target basis, see: https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.10-rcX&id=6bb38bcc33bf3093c08bd1b71e4f20c82bb60dd1 Kind of depressing that I went so far down the rabbit hole, of wanting to lean on block core, that I lost sight of an important "tenet of DM": + * Does the target need to split IO even further? + * - varied (per target) IO splitting is a tenet of DM; this + * explains why stacked chunk_sectors based splitting via + * blk_max_size_offset() isn't possible here. And it is because of this that DM is forced to lean on human creation of an optimal IO stack.. which is prone to human error when a particular thinp "blocksize" is selected, etc. Mike