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=ham 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 0E4D7C433FE for ; Thu, 3 Dec 2020 17:57:20 +0000 (UTC) Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [63.128.21.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 4AEB6206F9 for ; Thu, 3 Dec 2020 17:57:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4AEB6206F9 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=tempfail smtp.mailfrom=dm-devel-bounces@redhat.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1607018238; h=from:from:sender:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:list-id:list-help: list-unsubscribe:list-subscribe:list-post; bh=b2qOL3Tbu4wtmumGnCfCxfDFNLU3wKFxFosqb6mqYxA=; b=M/SWd5w1Y4Sx87howLq+UcIZRZpN+bmlTp21HptD05xy5pSCosCOr6vaeIRbxfZL5Yzlil 5N9Q74qhB50Oj32/ZKC9I2Gkr4zgZwwbuF5cqKz6u2v6HM3LrGELlzVvBPmKwBAfmEoP/N ofFOj6sN2H98U+voTTidfN7f3K+cf1Q= 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-528-8Px1OmDwMsqY8kIziBw79w-1; Thu, 03 Dec 2020 12:57:16 -0500 X-MC-Unique: 8Px1OmDwMsqY8kIziBw79w-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 18B481081B22; Thu, 3 Dec 2020 17:57:11 +0000 (UTC) Received: from colo-mx.corp.redhat.com (colo-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.21]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8B2445D6AC; Thu, 3 Dec 2020 17:57:10 +0000 (UTC) Received: from lists01.pubmisc.prod.ext.phx2.redhat.com (lists01.pubmisc.prod.ext.phx2.redhat.com [10.5.19.33]) by colo-mx.corp.redhat.com (Postfix) with ESMTP id 38B864BB7B; Thu, 3 Dec 2020 17:57:09 +0000 (UTC) Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) by lists01.pubmisc.prod.ext.phx2.redhat.com (8.13.8/8.13.8) with ESMTP id 0B3Hv7u1008607 for ; Thu, 3 Dec 2020 12:57:07 -0500 Received: by smtp.corp.redhat.com (Postfix) id 5595660861; 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 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 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 X-loop: dm-devel@redhat.com Cc: axboe@kernel.dk, martin.petersen@oracle.com, jdorminy@redhat.com, bjohnsto@redhat.com, Ming Lei , linux-block@vger.kernel.org, dm-devel@redhat.com Subject: Re: [dm-devel] [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking X-BeenThere: dm-devel@redhat.com X-Mailman-Version: 2.1.12 Precedence: junk List-Id: device-mapper development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dm-devel-bounces@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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 -- dm-devel mailing list dm-devel@redhat.com https://www.redhat.com/mailman/listinfo/dm-devel