[libvirt] [PATCH 1/1] Add detail to documentation on storage pools and volumes.

Dave Allan dallan at redhat.com
Thu Feb 2 22:06:48 UTC 2012

The storage pools page contains details about the capabilities of the
various pool types, but not an overview of how they are intended to be
used.  This patch adds some explanation of what pools and volumes can
be used for and why an administrator might want to use them.
 docs/storage.html.in |   82 ++++++++++++++++++++++++++++++++++++++++++++++++--
 1 files changed, 79 insertions(+), 3 deletions(-)

diff --git a/docs/storage.html.in b/docs/storage.html.in
index a9c7f1c..0e3e289 100644
--- a/docs/storage.html.in
+++ b/docs/storage.html.in
@@ -3,9 +3,85 @@
     <h1 >Storage Management</h1>
-This page describes the backends for the storage management capabilities in
+      Libvirt provides storage management on the physical host through
+      storage pools and volumes.
+    </p>
+    <p>
+      A storage pool is a quantity of storage set aside by an
+      administrator, often a dedicated storage administrator, for use
+      by virtual machines.  Storage pools are divided into storage
+      volumes either by the storage administrator or the system
+      administrator, and the volumes are assigned to VMs as block
+      devices.
+    </p>
+    <p>
+      For example, the storage administrator responsible for an NFS
+      server creates a share to store virtual machines' data.  The
+      system administrator defines a pool on the virtualization host
+      with the details of the share
+      (e.g. nfs.example.com:/path/to/share should be mounted on
+      /vm_data).  When the pool is started, libvirt mounts the share
+      on the specified directory, just as if the system administrator
+      logged in and executed 'mount nfs.example.com:/path/to/share
+      /vmdata'.  If the pool is configured to autostart, libvirt
+      ensures that the NFS share is mounted on the directory specified
+      when libvirt is started.
+    </p>
+    <p>
+      Once the pool is started, the files in the NFS share are
+      reported as volumes, and the storage volumes' paths may be
+      queried using the libvirt APIs.  The volumes' paths can then be
+      copied into the section of a VM's XML definition describing the
+      source storage for the VM's block devices.  In the case of NFS,
+      an application using the libvirt APIs can create and delete
+      volumes in the pool (files in the NFS share) up to the limit of
+      the size of the pool (the storage capacity of the share).  Not
+      all pool types support creating and deleting volumes.  Stopping
+      the pool (somewhat unfortunately referred to by virsh and the
+      API as "pool-destroy") undoes the start operation, in this case,
+      unmounting the NFS share.  The data on the share is not modified
+      by the destroy operation, despite the name.  See man virsh for
+      more details.
+    </p>
+    <p>
+      A second example is an iSCSI pool.  A storage administrator
+      provisions an iSCSI target to present a set of LUNs to the host
+      running the VMs.  When libvirt is configured to manage that
+      iSCSI target as a pool, libvirt will ensure that the host logs
+      into the iSCSI target and libvirt can then report the available
+      LUNs as storage volumes.  The volumes' paths can be queried and
+      used in VM's XML definitions as in the NFS example.  In this
+      case, the LUNs are defined on the iSCSI server, and libvirt
+      cannot create and delete volumes.
+    </p>
+    <p>
+      Storage pools and volumes are not required for the proper
+      operation of VMs.  Pools and volumes provide a way for libvirt
+      to ensure that a particular piece of storage will be available
+      for a VM, but some administrators will prefer to manage their
+      own storage and VMs will operate properly without any pools or
+      volumes defined.  On systems that do not use pools, system
+      administrators must ensure the availability of the VMs' storage
+      using whatever tools they prefer, for example, adding the NFS
+      share to the host's fstab so that the share is mounted at boot
+      time.
+    </p>
+    <p>
+      If at this point the value of pools and volumes over traditional
+      system administration tools is unclear, note that one of the
+      features of libvirt is its remote protocol, so it's possible to
+      manage all aspects of a virtual machine's lifecycle as well as
+      the configuration of the resources required by the VM.  These
+      operations can be performed on a remote host entirely within the
+      libvirt API.  In other words, a management application using
+      libvirt can enable a user to perform all the required tasks for
+      configuring the host for a VM: allocating resources, running the
+      VM, shutting it down and deallocating the resources, without
+      requiring shell access or any other control channel.
+    </p>
+    <p>
+      Libvirt supports the following storage pool types:
+    </p>
         <a href="#StorageBackendDir">Directory backend</a>

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