Assigning a segment manager
This chapter discusses when to use a segment manager, what the different types of segment managers are, and how to assign a segment manager to a disk.
When to assign a segment manager
Assigning a segment manager to a disk allows the disk to be subdivided into
smaller storage objects called disk segments. The
assign command causes a
segment manager to create appropriate metadata and expose freespace
that the segment manager
finds on the disk. You need to assign segment managers when you
have a new disk or
when you are switching from one partitioning scheme to another.
EVMS displays disk segments as the following types:
Data: a set of contiguous sectors that has been allocated
from a disk and can be used to construct a volume or object.
Freespace: a set of contiguous sectors that are unallocated
or not in use. Freespace can be used to create a segment.
Metadata: a set of contiguous sectors that contain
information needed by the segment manager.
Types of segment managers
There are five types of segment managers in EVMS: DOS, GPT, S/390, Cluster, and BSD.
DOS Segment Manager
The most
commonly used segment manager is the DOS Segment Manager. This plug-in
provides support for traditional DOS disk partitioning. The
DOS Segment Manager also recognizes and supports the following variations
of the DOS partitioning scheme:
OS/2: an OS/2 disk has additional metadata
sectors that contain information needed to reconstruct disk segments.
Embedded partitions: support for BSD, SolarisX86, and UnixWare
is sometimes found embedded in primary DOS partitions.
The DOS Segment Manager recognizes and supports these
slices as disk segments.
GUID Partitioning Table (GPT) Segment Manager
The GUID Partitioning Table (GPT) Segment Manager handles the
new GPT partitioning scheme
on IA-64 machines. The Intel
Extensible Firmware Interface Specification
requires that firmware be able to discover partitions and produce logical devices that
correspond to disk partitions. The partitioning scheme described in
the specification is called GPT due to the extensive use of
Globally Unique Identifier (GUID) tagging. GUID is a 128 bit long
identifier, also referred to as a Universally Unique Identifier (UUID).
As described in the Intel Wired For Management Baseline Specification,
a GUID is a combination of time and space fields that produce an
identifier that is unique across an entire UUID space.
These identifiers are used extensively on GPT partitioned disks
for tagging entire disks and individual partitions.
GPT partitioned disks serve several functions, such as:
keeping a primary and backup copy of metadata
replacing msdos partition nesting by allowing many partitions
using 64 bit logical block addressing
tagging partitions and disks with GUID descriptors
The GPT Segment Manager scales better to large disks. It provides more redundancy with added reliability and uses unique names. However, the GPT Segment Manager is not compatible with DOS, OS/2, or Windows®.
S/390 Segment Manager
The S/390 Segment Manager is used exclusively on System/390
mainframes. The S/390 Segment Manager has the ability to recognize
various disk layouts found on an S/390 machine, and provide
disk segment support for this architecture. The two most common disk
layouts are Linux Disk Layout (LDL) and Common Disk Layout (CDL).
The principle difference between LDL and CDL is that an LDL disk
cannot be further subdivided. An LDL disk will produce a single metadata
disk segment and a single data disk segment. There is no freespace on an
LDL disk, and you cannot delete or re-size the data segment. A CDL disk can
be subdivided into multiple data disk segments
because it contains metadata that is missing from an LDL disk, specifically
the Volume Table of Contents (vtoc) information.
The S/390 Segment Manager is the only segment manager plug-in
capable of understanding the unique S/390 disk layouts. The S/390 Segment Manager
cannot be assigned or unassigned from a disk.
Cluster segment manager
The cluster segment manager (CSM) supports high availability clusters. When the
CSM is assigned to a shared storage disk, it writes metadata on the disk that:
provides a unique disk ID (guid)
names the EVMS container the disk will reside within
specifies the cluster node (nodeid) that owns the disk
specifies the HA cluster (clusterid)
This metadata allows the CSM to build containers for supporting failover situations.
It does so by constructing an EVMS container object that consumes all shared objects
discovered by the CSM and belonging to the same container. These shared storage
objects are consumed by the container. A single segment object is produced by the
container for each consumed storage object. A failover of the EVMS resource is then
accomplished by simply reassigning the container to the standby cluster node and
having that node re-run its discovery process.
BSD segment manager
BSD refers to the Berkeley Software Distribution UNIX® operating system. The EVMS
BSD segment manager is responsible for recognizing and producing EVMS segment
storage objects that map BSD partitions. A BSD disk may have a slice table in the
very first sector on the disk for compatibility purposes with other operating systems.
For example, a DOS slice table might be found in the usual MBR sector. The BSD disk
would then be found within a disk slice that is located using the compatibility slice table.
However, BSD has no need for the slice table and can fully dedicate the disk to
itself by placing the disk label in the very first sector. This is called a "fully dedicated
disk" because BSD uses the entire disk and does not provide a compatibility slice table.
The BSD segment manager recognizes such "fully dedicated disks" and provides mappings for
the BSD partitions.
Assigning a segment manager to an existing disk
When you assign a segment manager to a disk, the segment manager
needs to change the basic
layout of the disk. This change means that some sectors are
reserved for metadata and the remaining sectors are
made available for creating data disk segments. Metadata sectors
are written to disk to save information
needed by the segment manager; previous information found on the
disk is lost. Before assigning a
segment manager to an existing disk, you must remove any existing
volume management structures, including any previous segment manager.
Assigning a segment manager to a new disk
When a new disk is added to a system,
the disk usually contains no data and has not
been partitioned. If this is the case, the disk shows up in EVMS as a compatibility volume because
EVMS cannot tell if the disk is being used as a volume. To assign a segment manager to the disk so that it
can be subdivided into smaller disk segment objects, tell EVMS that the disk is not a
compatibility volume by deleting the volume information.
If the new disk was moved from another system, chances are good that the disk already contains
metadata. If the disk does contain metadata, the disk shows up in EVMS with storage objects that
were produced from the existing metadata. Deleting these objects will allow you to assign a different
segment manager to the disk, and you lose any old data.
Example: assign a segment manager
This section shows how to assign
a segment manager with EVMS.
EVMS initially displays the
physical disks it sees as volumes. Assume that you have added a new
disk to the system that EVMS
sees as sde.
This disk contains no data and has not been subdivided
(no partitions). EVMS assumes that this
disk is a compatibility volume known as /dev/evms/sde.
Assign the DOS Segment Manager
Assign the DOS Segment Manager to disk sde.
NOTE
In the following example, the DOS Segment
Manager creates two segments on the disk:
a metadata segment known as sde_mbr, and a
segment to represent the available space on
the drive, sde_freespace1. This freespace
segment (sde_freespace1) can be
divided into
other segments because it represents space on
the drive that is not in use.
Using the EVMS GUI
To assign the DOS Segment Manager to sde, first
remove the volume, /dev/evms/sde:
Select Actions
Delete
Volume.
Select /dev/evms/sde.
Click Delete.
Alternatively, you can remove the volume through the GUI context sensitive menu:
From the Volumes tab, right click /dev/evms/sde.
Click Delete.
After the volume is removed, assign the DOS Segment Manager:
Select Actions
Add
Segment Manager to Storage Object.
Select DOS Segment Manager.
Click Next.
Select sde
Click Add
Using Ncurses
To assign the DOS Segment Manager to sde, first remove the
volume /dev/evms/sde:
Select ActionsDelete
Segment Manager to Storage Object.
Select /dev/evms/sde.
Activate Delete.
Alternatively, you can remove the volume through the context sensitive menu:
From the Logical Volumes view, press Enter on
/dev/evms/sde.
Activate Delete.
After the volume is removed, assign the DOS Segment Manager:
Select ActionsAdd
Segment Manager to Storage Object
Select DOS Segment Manager.
Activate Next.
Select sde.
Activate Add.
Using the CLI
To assign the DOS Segment Manager to sde, first tell EVMS that this disk is not a
volume and is available for use:
Delete:/dev/evms/sde
Next, assign the DOS Segment Manager to sde by typing the following:
Assign:DosSegMgr={},sde