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=-10.2 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, 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 D46DBC433DB for ; Fri, 12 Feb 2021 12:13:54 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 6545A64DD7 for ; Fri, 12 Feb 2021 12:13:54 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6545A64DD7 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:38246 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lAXKZ-00049F-Vw for qemu-devel@archiver.kernel.org; Fri, 12 Feb 2021 07:13:52 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:55786) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lAXIa-0003Gp-UM for qemu-devel@nongnu.org; Fri, 12 Feb 2021 07:11:50 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:30688) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lAXIT-0003b2-AL for qemu-devel@nongnu.org; Fri, 12 Feb 2021 07:11:48 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1613131900; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=xiW5cixbTW8+IKJylX94EFLrMb2f7QgVYgaAzLlt5dc=; b=GZ3noCPnQBDPbCY4zTtXmgqQNKUVyoWjK97bUsJqNTGZDPFmh6F1RNccqLXJdiJkcX//cx u5BUWkPjK2GbpJzN3UTkwFFYln93ItEWNTrmK8BiPgEFo/Sy7U/84c9zHcePzvTdEWMn04 CvRCXYVJcoeQu6R3zc99mxwLS5ZfUX4= 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-540-ycT0-W3COgyCRIuJURF6Gg-1; Fri, 12 Feb 2021 07:11:35 -0500 X-MC-Unique: ycT0-W3COgyCRIuJURF6Gg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 6B18C107AD74; Fri, 12 Feb 2021 12:11:28 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-113-15.ams2.redhat.com [10.36.113.15]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6BFDE1002393; Fri, 12 Feb 2021 12:11:27 +0000 (UTC) Subject: Re: [PATCH 2/2] file-posix: Cache next hole To: Kevin Wolf References: <20210211172242.146671-1-mreitz@redhat.com> <20210211172242.146671-3-mreitz@redhat.com> <20210212102538.GD6221@merkur.fritz.box> From: Max Reitz Message-ID: <24d69153-5539-ff92-da94-862646f14a8d@redhat.com> Date: Fri, 12 Feb 2021 13:11:25 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <20210212102538.GD6221@merkur.fritz.box> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.569, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.119, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 12.02.21 11:25, Kevin Wolf wrote: > Am 12.02.2021 um 10:14 hat Max Reitz geschrieben: >> On 11.02.21 21:38, Vladimir Sementsov-Ogievskiy wrote: >>> 11.02.2021 20:22, Max Reitz wrote: >>>> We have repeatedly received reports that SEEK_HOLE and SEEK_DATA are >>>> slow on certain filesystems and/or under certain circumstances.  That is >>>> why we generally try to avoid it (which is why bdrv_co_block_status() >>>> has the @want_zero parameter, and which is why qcow2 has a metadata >>>> preallocation detection, so we do not fall through to the protocol layer >>>> to discover which blocks are zero, unless that is really necessary >>>> (i.e., for metadata-preallocated images)). >>>> >>>> In addition to those measures, we can also try to speed up zero >>>> detection by letting file-posix cache some hole location information, >>>> namely where the next hole after the most recently queried offset is. >>>> This helps especially for images that are (nearly) fully allocated, >>>> which is coincidentally also the case where querying for zero >>>> information cannot gain us much. >>>> >>>> Note that this of course only works so long as we have no concurrent >>>> writers to the image, which is the case when the WRITE capability is not >>>> shared. >>>> >>>> Alternatively (or perhaps as an improvement in the future), we could let >>>> file-posix keep track of what it knows is zero and what it knows is >>>> non-zero with bitmaps, which would help images that actually have a >>>> significant number of holes (where this implementation here cannot do >>>> much).  But for such images, SEEK_HOLE/DATA are generally faster (they >>>> do not need to seek through the whole file), and the performance lost by >>>> querying the block status does not feel as bad because it is outweighed >>>> by the performance that can be saved by special-cases zeroed areas, so >>>> focussing on images that are (nearly) fully allocated is more important. >>>> >>>> Signed-off-by: Max Reitz >>> >>> I'll look at it tomorrow... Just wanted to note that something similar >>> was proposed by Kevin some time ago: >>> >>> <20190124141731.21509-1-kwolf@redhat.com> >>> https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06271.html >> >> Interesting. The reasoning that it doesn’t matter whether anyone >> writes to the assumed-data regions makes sense. >> >> I can’t see a real reason why it was kind of forgotten, apparently... > > After qcow2 stopped recursively querying the file-posix layer, the > relevant case under discussion was fixed anyway, so it didn't have the > highest priority any more... > > I think the open question (and possibly work) in the old thread was > whether this should be moved out of file-posix into the generic block > layer. > > With your patch, I guess the other open question is whether we want to > try and cache holes anyway. I assume that in the common case, you may > have many consecutive data extents, but probably rarely many holes (I > guess you can have more than one if some areas are unallocated and > others are allocated, but unwritten?) Then it's probably not worth > caching holes. Yes, that’s what I think. If you have many small holes, caching won’t help much anyway (at least this way, where we’d only cache information about a single hole). So caching holes would mainly help when you have several large holes. But in that case, finding them will surely speed up whatever it is the block-status caller is doing (e.g. mirroring), so a slow SEEK_HOLE/DATA will be tolerable. Max