cluster: network: clarify and ease latency requirements

[pve-docs.git] / pvecm.adoc

diff --git a/pvecm.adoc b/pvecm.adoc
index 26b52cf26f0b7de08f25562ce0d38f54b65157de..97a665be2504f8cf129e65d9994c5a164620c303 100644 (file)
--- a/pvecm.adoc
+++ b/pvecm.adoc
@@ -511,11 +511,18 @@ file system (`pmxcfs`).
 [[pvecm_cluster_network_requirements]]
 Network Requirements
 ~~~~~~~~~~~~~~~~~~~~
-This needs a reliable network with latencies under 2 milliseconds (LAN
-performance) to work properly. The network should not be used heavily by other
-members; ideally corosync runs on its own network. Do not use a shared network
-for corosync and storage (except as a potential low-priority fallback in a
-xref:pvecm_redundancy[redundant] configuration).
+
+The {pve} cluster stack requires a reliable network with latencies under 5
+milliseconds (LAN performance) between all nodes to operate stably. While on
+setups with a small node count a network with higher latencies _may_ work, this
+is not guaranteed and gets rather unlikely with more than three nodes and
+latencies above around 10 ms.
+
+The network should not be used heavily by other members, as while corosync does
+not uses much bandwidth it is sensitive to latency jitters; ideally corosync
+runs on its own physically separated network.  Especially do not use a shared
+network for corosync and storage (except as a potential low-priority fallback
+in a xref:pvecm_redundancy[redundant] configuration).
 
 Before setting up a cluster, it is good practice to check if the network is fit
 for that purpose. To ensure that the nodes can connect to each other on the
@@ -974,9 +981,9 @@ the cluster and to have a corosync-qnetd package available. We provide a package
 for Debian based hosts, and other Linux distributions should also have a package
 available through their respective package manager.
 
-NOTE: In contrast to corosync itself, a QDevice connects to the cluster over
-TCP/IP. The daemon may even run outside of the cluster's LAN and can have longer
-latencies than 2 ms.
+NOTE: Unlike corosync itself, a QDevice connects to the cluster over TCP/IP.
+The daemon can also run outside the LAN of the cluster and isn't limited to the
+low latencies requirements of corosync.
 
 Supported Setups
 ~~~~~~~~~~~~~~~~
@@ -1273,7 +1280,7 @@ Migration Type
 
 The migration type defines if the migration data should be sent over an
 encrypted (`secure`) channel or an unencrypted (`insecure`) one.
-Setting the migration type to insecure means that the RAM content of a
+Setting the migration type to `insecure` means that the RAM content of a
 virtual guest is also transferred unencrypted, which can lead to
 information disclosure of critical data from inside the guest (for
 example, passwords or encryption keys).
@@ -1286,7 +1293,7 @@ NOTE: Storage migration does not follow this setting. Currently, it
 always sends the storage content over a secure channel.
 
 Encryption requires a lot of computing power, so this setting is often
-changed to "unsafe" to achieve better performance. The impact on
+changed to `insecure` to achieve better performance. The impact on
 modern systems is lower because they implement AES encryption in
 hardware. The performance impact is particularly evident in fast
 networks, where you can transfer 10 Gbps or more.