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spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:55876 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lClm6-0000CS-F0 for qemu-devel@archiver.kernel.org; Thu, 18 Feb 2021 11:03:30 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:49468) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lCllH-00082Y-Qu for qemu-devel@nongnu.org; Thu, 18 Feb 2021 11:02:39 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:59237) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lCllC-0003Ct-3v for qemu-devel@nongnu.org; Thu, 18 Feb 2021 11:02:38 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1613664152; 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=P+YdJCbSeNsPTbBjVriRKgk5sXfu5L627490OfHJZc4=; b=bUQ4qJ5t2mXUp8rWBSfKj+1umU+tpqX66FCgpMBkzX0IG7LfEJiOh7vEn1RkobIPxu5Jgf LCBbYvtHEX9FRvg/Bpy2kWI1IabTut1lTxyz0UfAeam7XY1eQqFRiQEjE4hDYmtVc8QtfF eWO/W+aqlv/U9FpqkAFUNix22g00PmM= 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-497-EWjHju-0NcGCZCQAmY5vzg-1; Thu, 18 Feb 2021 11:02:30 -0500 X-MC-Unique: EWjHju-0NcGCZCQAmY5vzg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 2FA7A80197D; Thu, 18 Feb 2021 16:02:29 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-114-35.ams2.redhat.com [10.36.114.35]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 84ECB19714; Thu, 18 Feb 2021 16:02:27 +0000 (UTC) Subject: Re: [PATCH 3/7] block/qcow2: use compressed write cache To: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org References: <20210129165030.640169-1-vsementsov@virtuozzo.com> <20210129165030.640169-4-vsementsov@virtuozzo.com> <446ebfd5-ac72-dc18-fde3-6cc7ffa73176@redhat.com> From: Max Reitz Message-ID: <8b50ed82-5ed5-1d8e-2725-756665acfdcd@redhat.com> Date: Thu, 18 Feb 2021 17:02:25 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 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=216.205.24.124; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham 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: kwolf@redhat.com, den@openvz.org, qemu-devel@nongnu.org, armbru@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 11.02.21 13:53, Vladimir Sementsov-Ogievskiy wrote: > 10.02.2021 20:11, Max Reitz wrote: >> On 29.01.21 17:50, Vladimir Sementsov-Ogievskiy wrote: >>> Introduce a new option: compressed-cache-size, with default to 64 >>> clusters (to be not less than 64 default max-workers for backup job). >>> >>> Signed-off-by: Vladimir Sementsov-Ogievskiy >>> --- >>>   qapi/block-core.json   |  8 +++- >>>   block/qcow2.h          |  4 ++ >>>   block/qcow2-refcount.c | 13 +++++++ >>>   block/qcow2.c          | 87 ++++++++++++++++++++++++++++++++++++++++-- >>>   4 files changed, 108 insertions(+), 4 deletions(-) >>> >>> diff --git a/qapi/block-core.json b/qapi/block-core.json >>> index 9f555d5c1d..e0be6657f3 100644 >>> --- a/qapi/block-core.json >>> +++ b/qapi/block-core.json >>> @@ -3202,6 +3202,11 @@ >>>   #             an image, the data file name is loaded from the image >>>   #             file. (since 4.0) >>>   # >>> +# @compressed-cache-size: The maximum size of compressed write cache in >>> +#                         bytes. If positive must be not less than >>> +#                         cluster size. 0 disables the feature. Default >>> +#                         is 64 * cluster_size. (since 6.0) >> >> Do we need this, really?  If you don’t use compression, the cache >> won’t use any memory, right?  Do you plan on using this option? >> >> I’d just set it to a sane default. > > OK for me > >> >> OTOH, “a sane default” poses two questions, namely whether 64 * >> cluster_size is reasonable – with subclusters, the cluster size may be >> rather high, so 64 * cluster_size may well be like 128 MB.  Are 64 >> clusters really necessary for a reasonable performance? >> >> Second, I think I could live with a rather high default if clusters >> are flushed as soon as they are full.  OTOH, as I briefly touched on, >> in practice, I suppose compressed images are just written to >> constantly, so even if clusters are flushed as soon as they are full, >> the cache will still remain full all the time. >> >> >> Different topic: Why is the cache disableable?  I thought there are no >> downsides? >> > > to compare performance for example.. Well :D Doesn’t seem like a reason to expose it to the outside, though, I don’t know. Max