From nobody Wed Dec 17 21:48:38 2025 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=fail(p=none dis=none) header.from=redhat.com Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1520018495087942.5604568211434; Fri, 2 Mar 2018 11:21:35 -0800 (PST) Received: from localhost ([::1]:37031 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1erqFK-0007iI-3x for importer@patchew.org; Fri, 02 Mar 2018 14:21:34 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52964) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1erpqY-0001gv-Hf for qemu-devel@nongnu.org; Fri, 02 Mar 2018 13:56:00 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1erpqX-00037R-0Q for qemu-devel@nongnu.org; Fri, 02 Mar 2018 13:55:58 -0500 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:60124 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1erpqT-00035Y-PY; Fri, 02 Mar 2018 13:55:53 -0500 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 515664022909; Fri, 2 Mar 2018 18:55:53 +0000 (UTC) Received: from localhost.localdomain.com (ovpn-117-66.ams2.redhat.com [10.36.117.66]) by smtp.corp.redhat.com (Postfix) with ESMTP id 34265AB583; Fri, 2 Mar 2018 18:55:51 +0000 (UTC) From: Kevin Wolf To: qemu-block@nongnu.org Date: Fri, 2 Mar 2018 19:54:36 +0100 Message-Id: <20180302185448.6314-26-kwolf@redhat.com> In-Reply-To: <20180302185448.6314-1-kwolf@redhat.com> References: <20180302185448.6314-1-kwolf@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.6]); Fri, 02 Mar 2018 18:55:53 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.6]); Fri, 02 Mar 2018 18:55:53 +0000 (UTC) for IP:'10.11.54.5' DOMAIN:'int-mx05.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'kwolf@redhat.com' RCPT:'' X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 66.187.233.73 Subject: [Qemu-devel] [PULL 25/37] docs: document how to use the l2-cache-entry-size parameter X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kwolf@redhat.com, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" From: Alberto Garcia This patch updates docs/qcow2-cache.txt explaining how to use the new l2-cache-entry-size parameter. Here's a more detailed technical description of this feature: https://lists.gnu.org/archive/html/qemu-block/2017-09/msg00635.html And here are some performance numbers: https://lists.gnu.org/archive/html/qemu-block/2017-12/msg00507.html Signed-off-by: Alberto Garcia Reviewed-by: Eric Blake Signed-off-by: Kevin Wolf --- docs/qcow2-cache.txt | 46 +++++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 43 insertions(+), 3 deletions(-) diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt index b0571de4b8..170191a242 100644 --- a/docs/qcow2-cache.txt +++ b/docs/qcow2-cache.txt @@ -1,6 +1,6 @@ qcow2 L2/refcount cache configuration =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D -Copyright (C) 2015 Igalia, S.L. +Copyright (C) 2015, 2018 Igalia, S.L. Author: Alberto Garcia =20 This work is licensed under the terms of the GNU GPL, version 2 or @@ -118,8 +118,8 @@ There are three options available, and all of them take= bytes: =20 There are two things that need to be taken into account: =20 - - Both caches must have a size that is a multiple of the cluster - size. + - Both caches must have a size that is a multiple of the cluster size + (or the cache entry size: see "Using smaller cache sizes" below). =20 - If you only set one of the options above, QEMU will automatically adjust the others so that the L2 cache is 4 times bigger than the @@ -143,6 +143,46 @@ much less often than the L2 cache, so it's perfectly r= easonable to keep it small. =20 =20 +Using smaller cache entries +--------------------------- +The qcow2 L2 cache stores complete tables by default. This means that +if QEMU needs an entry from an L2 table then the whole table is read +from disk and is kept in the cache. If the cache is full then a +complete table needs to be evicted first. + +This can be inefficient with large cluster sizes since it results in +more disk I/O and wastes more cache memory. + +Since QEMU 2.12 you can change the size of the L2 cache entry and make +it smaller than the cluster size. This can be configured using the +"l2-cache-entry-size" parameter: + + -drive file=3Dhd.qcow2,l2-cache-size=3D2097152,l2-cache-entry-size=3D40= 96 + +Some things to take into account: + + - The L2 cache entry size has the same restrictions as the cluster + size (power of two, at least 512 bytes). + + - Smaller entry sizes generally improve the cache efficiency and make + disk I/O faster. This is particularly true with solid state drives + so it's a good idea to reduce the entry size in those cases. With + rotating hard drives the situation is a bit more complicated so you + should test it first and stay with the default size if unsure. + + - Try different entry sizes to see which one gives faster performance + in your case. The block size of the host filesystem is generally a + good default (usually 4096 bytes in the case of ext4). + + - Only the L2 cache can be configured this way. The refcount cache + always uses the cluster size as the entry size. + + - If the L2 cache is big enough to hold all of the image's L2 tables + (as explained in the "Choosing the right cache sizes" section + earlier in this document) then none of this is necessary and you + can omit the "l2-cache-entry-size" parameter altogether. + + Reducing the memory usage ------------------------- It is possible to clean unused cache entries in order to reduce the --=20 2.13.6