ganeti: add a section about disk cache configuration

diff --git a/input/howto/install-ganeti.creole b/input/howto/install-ganeti.creole
index 2f21981..d88f024 100644 (file)
--- a/input/howto/install-ganeti.creole
+++ b/input/howto/install-ganeti.creole
@@ -483,6 +483,16 @@ both the network and disk speed allow that.
 }}}
 
 
+=== Change the disk cache ===
+
+When using raw volumes or partitions, it is best to avoid the host cache completely to reduce data copies
+and bus traffic. This can be done using:
+
+{{{
+    gnt-cluster modify -H kvm:disk_cache=none
+}}}
+
+
 === Change the CPU type ===
 
 Modern processors come with a wide variety of additional instruction sets (SSE, AES-NI, etc.) which vary from processor to processor, but can greatly improve the performance depending on the workload. Ganeti and QEMU default to a compatible subset of cpu features called qemu64, so that if the host processor is changed, or a live migration is performed, the guest will see its CPUfeatures unchanged. This is great for compatibility but comes at a performance cost.