New availability features in
Red Hat� Enterprise Linux� 4
by Matthew T. O'Keefe
Introduction
New features in Red Hat Enterprise Linux 4 have increased the availability and
ease-of-management of servers running Red Hat's most recent enterprise OS. Most
of the new availability features are a result of the release of LVM2, the
latest version of the Linux Logical Volume Manager; all the new features are
not yet available in Enterprise Linux 4, but they are scheduled for release in
mid-summer 2005. These features include:
a.. Online volume and file system resizing
b.. Volume mirroring and I/O multi-pathing
c.. Volume snapshots for online file system backup, application testing and
restart
d.. Data migration from an ailing storage device
e.. Clustering software for application failover and data sharing
This article explores these new availability features and how they might be
useful to increase the availability of Red Hat Enterprise Linux servers.
Server availability
Server availability is often considered only in the narrow context of migrating
applications from one failed server to another. Although application failover
technologies are an important component in increasing availability (and are
discussed later in the article), Red Hat Enterprise Linux comes with other
important features that increase overall system availability by avoiding
scheduled as well as unscheduled downtime. This is accomplished via techniques
such as volume mirroring and I/O multi-pathing that avoid single points of
failure. In addition, techniques such as volume snapshots and data migration
allow for additional redundancy and protection against potential faults; more
importantly, they can be performed while the Red Hat Enterprise Linux server
remains available to run application workloads. In the next section, we discuss
these specific volume management features.
Linux Logical Volume Manager (LVM)
The Linux Logical Volume Manager (LVM) is a software tool for managing disks.
LVM allows multiple physical disks or partitions to be treated as a single
logical disk. Logical Volumes (LVs) are created by partitioning an LVM Volume
Group (VG) into separate volumes. Volume groups are formed by aggregating
Physical Volumes (PVs). PVs are physical storage devices that are integrated
into LVM by labeling them as Physical Volumes (PVs). The three-layer
abstraction used in LVM can be seen in Figure 1, LVM three layer abstraction.
Figure 1. LVM three layer abstraction
When using LVM, logical volumes replace raw physical partitions. File systems
and database tables can be mapped to logical volumes instead of raw partitions.
A key advantage of logical volumes over raw partitions is that they can easily
be resized on-the-fly without stopping server operations. With this capability,
it is possible to add a new storage device, integrate its storage into a
logical volume, and then make that storage available to a database or file
system that is approaching its maximum size.
A snapshot of a logical volume can be taken at any time. A snapshot preserves
an exact copy of the logical volume at the point-in-time when the snapshot is
taken. The snapshot volume copy can be used for several purposes. As shown in
Figure 2, Logical volume snapshot, a consistent view of the file system or
database residing on the snapshot volume can be backed up without any later
changes to the file system disrupting the backup. This approach allows a
consistent file system or database backup to be taken without disrupting server
operations, which increases server availability.
LVM snapshots can be read from, written to, and resized on-the-fly just like
standard LVM volumes, allowing great flexibility in their usage. In addition to
allowing consistent, non-disruptive backups, snapped volumes can be used for
application testing without disrupting current operations. They can also be
used to back up to a known, consistent state of the file system or database in
case of system errors. Snapshots are also useful in development environments
where accessing prior versions of test data or programs before more recent
modifications that may have introduced errors is an important capability. LVM
snapshots use copy-on-write techniques to reduce the number of block copies
between the original volume and its snapshots, so that volumes that are only
slightly modified from the original require only an additional 3-5% of disk
storage.
Figure 2. LVM snapshot
It's important to understand that logical volumes are created by aggregating
multiple physical disks into a single logical disk. Red Hat Enterprise Linux
has mechanisms to monitor the health and status of its physical storage
devices. If a disk is beginning to malfunction, it is prudent to migrate its
data to another, healthier disk in the volume group. This can be accomplished
using the LVM PVMOVE command, which migrates data from one mounted physical
device to another. As shown in Figure 3, PVMOVE, this data movement can be
accomplished while the original physical disk is in use by the server, without
shutting down server operations during the move, thereby increasing server
availability.
Figure 3. PVMOVE
Red Hat Enterprise Linux 4 provides additional fault tolerance for storage
subsystem failures via LVM volume mirroring and multi-pathing. When deployed
with Enterprise Linux, these two capabilities significantly increase the
availability of Red Hat Enterprise Linux servers.
LVM volumes can be mirrored so that the data on a single logical volume is
copied onto up to 32 separate physical volumes. Each physical volume gets a
copy of each disk block written to the LV, as shown in Figure 4, Volume
mirroring. These mirrors can be used to recover from disk failures and to
create point-in-time mirror copies that can be removed from a mirror set and
mounted as a separate volume. Also note that combining snapshots with mirroring
allows recovery from both disk errors as well as file system or database errors.
Figure 4. Volume mirroring
Red Hat Enterprise Linux supports I/O multi-pathing, which allows servers to
tolerate failures in storage host bus adapters, storage area network switch
paths, and storage array ports. By exploiting storage network path, host bus
adapter, and storage port redundancy and routing around failures in any of
these system components, I/O multi-pathing increases server uptime. It is also
possible to utilize the redundant storage paths to increase the rate of data
transfer between the Enterprise Linux server and its storage.
In addition to these new features to increase availability, the latest version
of LVM (known as LVM 2, and supported only in Red Hat Enterprise Linux 4 and
later releases) supports more physical devices (thousands versus only 256 in
prior versions), larger volumes (up to 8 Exabytes in 64-bit systems, and 16
Terabytes in 32-bit systems), and transactional metadata updates to simplify
recovery after server crashes. Readable/writable/resizable snapshots, volume
mirroring, multi-pathing, and data migration via PVMOVE are only available in
LVM 2 and Red Hat Enterprise Linux 4, not in LVM 1.
Increased availability via Red Hat Cluster Suite and GFS
Red Hat Cluster Suite increases application availability by providing an
automated way to migrate applications from a one server to another in a Red Hat
Enterprise Linux cluster in the event of a hardware or software failure or for
purposes of server maintenance as directed by the system administrator. Cluster
Suite monitors server availability via heartbeats and application availability
via service monitoring: the loss of either server or application availability
results in a restart operation for applications depending on the failed server
or a restart of the monitored application. Scripts are created to define the
steps necessary to both start and stop an application.
Cluster Suite's primary benefit is an automated, scripted, controlled sequence
of steps for migrating applications from one server to another for continuation
of application operation. This results in increased uptime at low cost using
industry-standard hardware components (servers, networks, and storage),
building on the availability and ease-of-management capabilities in Red Hat
Enterprise Linux and Red Hat Network. The lowest cost hardware configuration
can be achieved in Enterprise Linux 4 with Cluster Suite without a Quorum Disk
or a storage area network. It is also possible to continue to share data via an
NFS server in this configuration.
Cluster Suite is commonly used to provide application fail-over for databases
such as Oracle and MySQL, file services via NFS or Samba, and web services via
Apache and Red Hat Application Server. Scripts are provided for starting and
stopping most of these applications.
Figure 5. A GFS cluster
It is also possible to share data directly on a storage area with Red Hat
Global File System (GFS), a cluster file system for Linux, as shown in Figure
5, A GFS cluster. GFS and Cluster Suite are integrated applications and in Red
Hat Enterprise Linux 4 use the same cluster infrastructure software components.
System architects can use Cluster Suite without shared storage for applications
that do not require high performance or data sharing, but achieve higher
performance and data sharing with GFS if required.
Summary
Table 1 summarizes Red Hat Enterprise Linux 4 availability features and the
failures conditions that are handled by each feature. The new capabilities in
Red Hat Enterprise Linux, if used properly by system implementers, can
significantly increase server uptime and availability.
RHEL Feature What the RHEL Feature protects against
Disk errors Filesystem/ Database corruption HBA/SAN failure
Filesystem overflow Server crash Virus/ Application failure
LVM2 Mirroring X
LVM Snapshots X X
Multi-pathing X
LVM/ext3fx resize X
Cluster Suite X X
Table 1. Red Hat Enterprise Linux 4 availability features
[Non-text portions of this message have been removed]
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/Indo-Linux/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
