LDP/LDP/guide/docbook/EVMSUG/appx-lvm.xml

<appendix id="appxlvm"><title>The LVM plug-in</title>

<para>The LVM plug-in combines storage objects into groups called containers.
From these containers, new storage objects can be created, with a variety of
mappings to the consumed objects.  Containers allow the storage capacity of
several objects to be combined, allow additional storage to be added in the future,
and allow for easy resizing of the produced objects.</para>
<sect1 id="lvmimp"><title>How LVM is implemented</title>
<para>The Linux LVM plug-in is compatible with volumes and volume groups from
the original Linux LVM tools from Sistina Software.  The original LVM is based on the
concept of volume groups.  A volume group (VG) is a grouping of physical volumes 
(PVs), which are usually disks or disk partitions.  The volume group is not directly
usable as storage space; instead, it represents a pool of available storage.
You create logical volumes (LVs) to use this storage.  The storage space of the LV can 
map to one or more of the group's PVs.</para>
<para>The Linux LVM concepts are represented by similar concepts in the EVMS LVM plug-in.
A volume group is called a container, and the logical volumes that are produced are
called regions.  The physical volumes can be disks, segments, or other regions.
Just as in the original LVM, regions can map to the consumed objects in a variety of ways.</para>
</sect1>

<sect1 id="containerops"><title>Container operations</title>

<sect2 id="createlvmconts"><title>Creating LVM containers</title>

<para>Containers are created with an initial set of objects.  In the LVM plug-in, the
objects can be disks, segments, or regions.  LVM has two options for creating containers.
The value of these options cannot be changed after the container has been created.  The
options are:</para>
<variablelist>
<varlistentry><term>name</term>
<listitem><para>The name of the new container.</para></listitem>
</varlistentry>

<varlistentry><term>pe_size</term>
<listitem><para>The physical extent (PE) size, which is the granularity with which regions can be created.  The default is 16 MB.  Each region must have a whole number of extents.  
Also, each region can have only up to 65534 extents.  Thus, the PE size for the container
limits the maximum size of a region in that container.  With the default PE size, an LVM
region can be, at most 1 TB.  In addition, each object consumed by the container must
be big enough to hold at least five extents.  Thus, the PE size cannot be arbitrarily large.  Choose wisely.</para></listitem>
</varlistentry>

</variablelist>
</sect2>

<sect2><title>Adding objects to LVM containers</title>
<para>You can add objects to existing LVM containers in order to increase the pool of
storage that is available for creating regions.  A single container can consume up to 256
objects.  Because the name and PE size of the containers are set when the container is
created, no options are available when you add new objects to a container.  Each object
must be large enough to hold five physical extents.  If an object is not large enough to
satisfy this requirement, the LVM plug-in will not allow the object to be added to the container.
</para>
</sect2>


<sect2><title>Removing objects from LVM containers</title>
<para>You can remove a consumed object from its container as long as no regions
are mapped to that object.  The LVM plug-in does not allow objects that are in use to
be removed their their container.  If an object must be removed, you can delete or
shrink regions, or move extents, in order to free the object from use.</para>
<para>No options are available for removing objects from LVM containers.</para>

</sect2>

<sect2><title>Deleting LVM containers</title>
<para>You can delete a container as long as the container does not have any produced
regions.  The LVM plug-in does not allow containers to be deleted if they have any
regions.  No options are available for deleting LVM containers.</para>
</sect2>
</sect1>

<sect1><title>Region operations</title>

<sect2><title>Creating LVM regions</title>
<para>You create LVM regions from the freespace in LVM containers.  If there is at least
one extent of freespace in the container, you can create a new region.</para>
<para>The following options are available for creating LVM regions:</para>

<variablelist>
<varlistentry><term>name</term>
<listitem><para>The name of the new region.</para></listitem>
</varlistentry>

<varlistentry><term>extents</term>
<listitem><para>The number of extents to allocate to the new region.  A new region must
have at least one extent and no more than the total available free extents in the container,
or 65534 (whichever is smaller).  If you use the <filename>extents</filename> option, the appropriate value
for the size option is automatically calculated.  By default, a new region uses all
available extents in the container.</para></listitem>
</varlistentry>


<varlistentry><term>size</term>
<listitem><para>The size of the new region.  This size must be a multiple of the
container's PE size.  If you use the <filename>size</filename> option, the appropriate value for the
extents options is automatically calculated.  By default, a new region uses all
available freespace in the container.</para>
</listitem></varlistentry>

<varlistentry><term>stripes</term>
<listitem><para>If the container consumes two or more objects, and each object has
unallocated extents, then the new region can be striped across multiple objects.
This is similar to RAID-0 striping and achieves an increased amount of I/O
throughput across multiple objects.  This option specifies how many objects the
new region should be striped across.  By default, new regions are not striped, and
this value is set to 1.</para>
</listitem>
</varlistentry>

<varlistentry><term>stripe_size</term>
<listitem><para>The granularity of striping.  The default value is 16 KB.  Use this option
only if the <filename>stripes</filename> option is greater than 1.</para>
</listitem>
</varlistentry>

<varlistentry><term>contiguous</term>
<listitem><para>This option specifies that the new region must be allocated on a single
object, and that the extents on that object must be physically contiguous.  By default,
this is set to false, which allows regions to span objects.  This option cannot be used
if the <filename>stripes</filename> option is greater than 1.</para>
</listitem>
</varlistentry>

<varlistentry><term>pv_names</term>
<listitem><para>A list of names of the objects the new region should map to.  By default,
this list is empty, which means all available objects will be used to allocate space
to the new region.</para>
</listitem>
</varlistentry>

</variablelist>
</sect2>

<sect2><title>Expanding LVM regions</title>
<para>You can expand an existing LVM region if there are unused extents in the
container.  If a region is striped, you can expand it only by using free space on
the objects it is striped across.  If a region was created with the contiguous option,
you can only expand it if there is physically contiguous space following the
currently allocated space.</para>
<para>The following options are available for expanding LVM regions:</para>

<variablelist>
<varlistentry><term>add_extents</term>
<listitem><para>The number of extents to add to the region.  If you specify this
option, the appropriate value for the add_size option is automatically
calculated.  By default, the region will expand to use all free extents in the
container.</para></listitem>
</varlistentry>

<varlistentry><term>add_size</term>
<listitem><para>The amount of space to add to the region.  If you specify this option,
the appropriate value for the add_extents option is automatically calculated.
By default, the region will expand to use all freespace in the container.</para></listitem>
</varlistentry>

<varlistentry><term>pv_names</term>
<listitem><para>A list of names of the objects to allocate the additional space from.
By default,  this list is empty, which means all available objects will be used to
allocate new space to the region.</para></listitem>

</varlistentry>
</variablelist>
</sect2>

<sect2><title>Shrinking LVM regions</title>
<para>You can shrink an existing LVM region by removing extents from the end of the
region.  Regions must have at least one extent, so regions cannot be shrunk to zero.</para>
<para>The following options are available when shrinking LVM regions.  Because regions
are always shrunk by removing space from the end of the region, a list of objects
cannot be specified in this command.</para>

<variablelist>
<varlistentry><term>remove_extents</term>
<listitem><para>The number of extents to remove from the region.  If you specify this option,
the appropriate value for the <filename>remove_size</filename> option is automatically calculated.  By
default, one extent is removed from the region.</para>
</listitem></varlistentry>

<varlistentry><term>remove_size</term>
<listitem><para>The amount of space to shrink the region by.  If you specify this option,
the appropriate value for the <filename>remove_extents</filename> option is automatically calculated.</para></listitem>
</varlistentry>

</variablelist>
</sect2>

<sect2><title>Deleting LVM regions</title>
<para>You can delete an existing LVM region as long as it is not currently a 
compatibility volume, an EVMS volume, or consumed by another EVMS plug-in.
No options are available for deleting LVM regions.</para>
</sect2>

<sect2><title>Moving LVM regions</title>
<para>The LVM plug-in lets you change the logical-to-physical mapping
for an LVM region and move the necessary data in the process.
This capability is most useful if a PV needs to be removed from a container.
There are currently two LVM plug-in functions for moving regions: 
<command>move_pv</command> and <command>move_extent</command>.</para>

<sect3><title>move_pv</title>
<para>When a PV needs to be removed from a container, all PEs on that PV that are
allocated to regions must be moved to other PVs.  The <command>move_pv</command> command lets you move PEs to other PVs. 
<command>move_pv</command> is targeted at the LVM container and the desired PV is used as 
the selected object.  The following options are available:</para>
<variablelist>
<varlistentry><term>target_pvs</term>
<listitem><para>By default, all remaining PVs in the container are used to find
available extents to move the PEs.  You can specify a subset of the PVs with this option.</para>
</listitem>
</varlistentry>
<varlistentry><term>maintain_stripes</term>
<listitem><para>When the target PV contains striped regions, there are three choices
for handling moving extents that belong to those regions:</para>

<variablelist>
<varlistentry><term>no</term>
<listitem><para>Don't bother to maintain true striping.  This choice allows
extents to be moved to PVs that the region already uses for other stripes.
This means that the performance will not be as optimal as it is with true
striping, but allows the most flexibility in performing the move operation.
This choice is the default for the <command>maintain_stripes</command> option.</para>
</listitem>
</varlistentry>
<varlistentry><term>loose</term>
<listitem><para>Ensure that moved extents do not end up on any PVs that the
striped region already uses.  However, this does not ensure that all moved
extents end up on the same PV.  For example, a region with three stripes may
end up mapping to four or more PVs.</para>
</listitem>
</varlistentry>
<varlistentry><term>strict</term>
<listitem><para>Ensure that all moved extents end up on the same PV, thus ensuring
true striping with the same number of PVs that the striped region originally used.
This is the most restricted choice, and may prevent the <command>move_pv</command> operation from
proceeding (depending on the particular configuration of the container).</para>
</listitem>
</varlistentry>
</variablelist>

<para>If the target PV has no striped regions, the <command>maintain_stripes</command> option is ignored.</para>
</listitem>
</varlistentry>
</variablelist>

</sect3>

<sect3><title>move_extent</title>
<para>In addition to moving all the extents from one PV, the LVM plug-in provides
the ability to move single extents.
This allows a fine-grain tuning of the allocation of extents.
This command is targeted at the region owning the extent to move.  There are
three required options for the <command>move_extent</command> command:</para>

<variablelist>
<varlistentry><term>le</term>
<listitem><para>The number of the logical extent to move.  LE numbers start at 0.</para></listitem>
</varlistentry>
<varlistentry><term>pv</term>
<listitem><para>The target object to move the extent to.</para></listitem>
</varlistentry>
<varlistentry><term>pe</term>
<listitem><para>The target physical extent on the target object.  PE numbers
also start at 0.</para></listitem>
</varlistentry>
</variablelist>
<para>To determine the source LE and target PE, it is often helpful to view the
extended information about the region and container in question.
The following are command-line options that can be used to gather this
information:</para>
<para>To view the map of LEs in the region, enter this command:</para>
<programlisting>query:ei,&lt;region_name&gt;,Extents</programlisting>

<para>To view the list of PVs in the container, enter this command:</para>
<programlisting>query:ei,&lt;container_name&gt;,Current_PVs</programlisting>

<para>To view the current PE map for the desired target PV, enter this command:</para>
<programlisting>query:ei,&lt;container_name&gt;,PEMapPV#</programlisting>
<para># is the number of the target PV in the container.</para>

<para>This information is also easily obtainable in the GUI and Text-Mode UIs by
using the &quot;Display Details&quot; item in the context-popup menus for the desired
region and container.</para>
</sect3>

</sect2>


</sect1>
	      
</appendix>