[PATCH pve-docs v2 3/3] partially fix #1989: qm: document qcow2 cache size config

[Erik Fastermann <e.fastermann@proxmox.com>]

Add a new subsection to document the new qcow2 cache config options.

Signed-off-by: Erik Fastermann <e.fastermann@proxmox.com>
---

 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