From nobody Fri Oct 24 09:35:07 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; dkim=fail; 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 Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1519052261913457.71288385134255; Mon, 19 Feb 2018 06:57:41 -0800 (PST) Received: from localhost ([::1]:50675 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1enmsv-0002uf-7K for importer@patchew.org; Mon, 19 Feb 2018 09:57:41 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39845) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1enmrP-00025O-9u for qemu-devel@nongnu.org; Mon, 19 Feb 2018 09:56:08 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1enmrL-0000LM-21 for qemu-devel@nongnu.org; Mon, 19 Feb 2018 09:56:07 -0500 Received: from fanzine.igalia.com ([91.117.99.155]:35570) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1enmrK-0008Os-MD; Mon, 19 Feb 2018 09:56:02 -0500 Received: from [194.100.51.2] (helo=perseus.local) by fanzine.igalia.com with esmtpsa (Cipher TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim) id 1enmqi-0004An-0a; Mon, 19 Feb 2018 15:55:24 +0100 Received: from berto by perseus.local with local (Exim 4.89) (envelope-from ) id 1enmqQ-00028B-EV; Mon, 19 Feb 2018 16:55:06 +0200 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=igalia.com; s=20170329; h=Message-Id:Date:Subject:Cc:To:From; bh=0OuKgf2PtiMIKHyikkieoGLszssy+hcxgq3xIeUh/gw=; b=BmOEDN2sFqjNguAalmQgPjXuYTR2pc24OwgoOs5IwlmB9/ortQuK/fZINPDb5XjJRrQUuozatVRwSXDqXWeNw4LVjuAHD4C1q/sKfxblEqdpbDKFsDzY6zkfQ2w26uIDK+giqOmXHsatOZAm7OVLRHG0YAvVJBcU0sykXqL9CagxxJD3Y3c4Ps2JoaZ3doO5Efh1nLs8JvBI7ZvmtDOZh6hVwqZSh7DYdafQXVAFoifFlY8h7hQ7eIEIz1d38fQzJDWjghG5zp7YcJHJHjZaq71p3mrugIDoSX1wGNOe8UwQ19CUlOSY46urhkz+AXElmirzn8UKm/pv03l4YeJYWA==; From: Alberto Garcia To: qemu-devel@nongnu.org Date: Mon, 19 Feb 2018 16:54:59 +0200 Message-Id: <20180219145459.8143-1-berto@igalia.com> X-Mailer: git-send-email 2.11.0 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x (no timestamps) [generic] [fuzzy] X-Received-From: 91.117.99.155 Subject: [Qemu-devel] [PATCH] 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: Kevin Wolf , "Denis V . Lunev" , Alberto Garcia , qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: fail (Header signature does not verify) X-ZohoMail: RDKM_2 RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" 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 --- 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 2.11.0