From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate001.proxmox.com (gate001.proxmox.com [45.144.208.40]) by lore.proxmox.com (Postfix) with ESMTPS id 965091FF138 for ; Mon, 20 Jul 2026 11:47:23 +0200 (CEST) Received: from gate001.proxmox.com (localhost.localdomain [127.0.0.1]) by gate001.proxmox.com (Proxmox) with ESMTP id 56F16214B8; Mon, 20 Jul 2026 11:47:17 +0200 (CEST) From: Erik Fastermann To: pve-devel@lists.proxmox.com Subject: [PATCH pve-docs v2 3/3] partially fix #1989: qm: document qcow2 cache size config Date: Mon, 20 Jul 2026 11:46:38 +0200 Message-ID: <20260720094638.113056-4-e.fastermann@proxmox.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260720094638.113056-1-e.fastermann@proxmox.com> References: <20260720094638.113056-1-e.fastermann@proxmox.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-SPAM-LEVEL: Spam detection results: 2 AWL -0.076 Adjusted score from AWL reputation of From: address DMARC_MISSING 0.1 Missing DMARC policy KAM_DMARC_STATUS 0.01 Test Rule for DKIM or SPF Failure with Strict Alignment (newer systems) KAM_LAZY_DOMAIN_SECURITY 1 Sending domain does not have any anti-forgery methods RDNS_NONE 1.274 Delivered to internal network by a host with no rDNS SPF_HELO_NONE 0.001 SPF: HELO does not publish an SPF Record SPF_NONE 0.001 SPF: sender does not publish an SPF Record Message-ID-Hash: GM6GYAGMB7BTNI55TECT67W5VUVO7XYO X-Message-ID-Hash: GM6GYAGMB7BTNI55TECT67W5VUVO7XYO X-MailFrom: efastermann@ruth.proxmox.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; loop; banned-address; emergency; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: Erik Fastermann X-Mailman-Version: 3.3.10 Precedence: list List-Id: Proxmox VE development discussion List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: Add a new subsection to document the new qcow2 cache config options. Signed-off-by: Erik Fastermann --- changes since v1: * documented the based on disk calculation for the default case * also some small rewording qm.adoc | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/qm.adoc b/qm.adoc index 93372c5..da5122c 100644 --- a/qm.adoc +++ b/qm.adoc @@ -341,6 +341,19 @@ underlying storage. Another benefit is reduced latency (hangs) in the guest for very I/O-intensive host workloads, since neither the main thread nor a vCPU thread can be blocked by disk I/O. +[[qm_hard_disk_qcow2_cache]] +Cache Size (qcow2) +^^^^^^^^^^^^^^^^^^ +Fine-tuning the qcow2 L2/refcount cache behavior can provide significant +performance gains. See the +https://gitlab.com/qemu-project/qemu/-/blob/master/docs/qcow2-cache.txt[QEMU documentation] +for a detailed explanation of these options. You can either set the cache size +directly or let {pve} choose it automatically based on the disk image size. When +sizing the cache automatically, with the default 64 KiB cluster size, 16 +refcount bits and no extended L2 entries, the cache uses at most roughly 160 KiB +of memory per GiB of disk. The remaining cache options can only be configured +through the API. + [[qm_cpu]] CPU ~~~ -- 2.47.3