add warning about over-provisioning

[pve-docs.git] / pvesm.adoc

diff --git a/pvesm.adoc b/pvesm.adoc
index 36be46e87948dc260f02e72988818ee42a72275e..462d9427c1965ecb2260c3a100adf03dd9541eb8 100644 (file)
--- a/pvesm.adoc
+++ b/pvesm.adoc
@@ -83,6 +83,31 @@ snapshots and clones.
 TIP: It is possible to use LVM on top of an iSCSI storage. That way
 you get a 'shared' LVM storage.
 
+Thin provisioning
+~~~~~~~~~~~~~~~~~
+
+A number of storages, and the Qemu image format `qcow2`, support _thin
+provisioning_.  With thin provisioning activated, only the blocks that
+the guest system actually use will be written to the storage.
+
+Say for instance you create a VM with a 32GB hard disk, and after
+installing the guest system OS, the root filesystem of the VM contains
+3 GB of data.  In that case only 3GB are written to the storage, even
+if the guest VM sees a 32GB hard drive. In this way thin provisioning
+allows you to create disk images which are larger than the currently
+available storage blocks. You can create large disk images for your
+VMs, and when the need arises, add more disks to your storage without
+resizing the VMs filesystems.
+
+All storage types which have the 'Snapshots' feature also support thin
+provisioning.
+
+CAUTION: If a storage runs full, all guests using volumes on that
+storage receives IO error. This can cause file system inconsistencies
+and may corrupt your data. So it is advisable to avoid
+over-provisioning of your storage resources, or carefully observe
+free space to avoid such conditions.
+
 Storage Configuration
 ---------------------