Building Highly Available Systems: The Dell PowerEdge … · rently supports two-node clustering based on the compo-nents shown in Figure 1. The supported PowerEdge servers ... to

storage

DELL POWER SOLUTIONS | May 200770

G lobal commerce and the demand for 24/7 access

have made information availability vital to

enterprise success. A lack of data access during

maintenance, hardware or software failure, or any other

period of downtime can lead to decreased productivity,

customer dissatisfaction, and lost revenue. As high-

availability technology continues to advance, both the

cost and complexity associated with clustering for avail-

ability and failover have been reduced significantly,

making these configurations feasible for small businesses

as well as large enterprise-class data centers.

For data to be continuously available, IT systems must

maintain uninterrupted data access during both planned and

unplanned downtime. Host-based clustering, in conjunction

with high availability at the storage level, can greatly enhance

IT service levels, and represents a crucial step toward ensur-

ing data availability and minimizing downtime problems.

Allowing multiple servers (for example, a two-node cluster)

shared access to data on a single storage system helps pro-

vide application availability even if one of the servers

becomes unavailable.

The Dell PowerEdge Cluster SE600W solution and Dell

PowerVault MD3000 storage array are designed to allow

enterprises of all sizes to build highly available systems in

a cost-effective way. Deploying this solution can help main-

tain access to critical application data even following a hard-

ware or software failure.

Dell PowerEdge Cluster SE600W Dell PowerEdge clusters are based on Microsoft Cluster

Service software and designed to keep applications and

services available during any single failure within the cluster.

When Microsoft Cluster Service detects a failure, it automati-

cally moves cluster resources from the failed cluster node to

a healthy one and restarts the applications. PowerEdge clus-

ters support cluster-aware applications such as Microsoft

SQL Server™, Microsoft Exchange Server, and Oracle® Database

with Oracle Fail Safe software.

In addition to application-level availability, enterprises

should consider redundancy in the server-to-storage I/O

path. Because a failure of any component along this path

(such as a server, adapter, controller, cable, or disk drive)

jeopardizes system availability, the storage system’s ability

to maintain data access during such a failure is a key part of

a highly available storage deployment.

The PowerEdge Cluster SE600W is the latest addition to

the Dell family of feature-rich clustering solutions, and the

first based on Serial Attached SCSI (SAS) technology. It cur-

rently supports two-node clustering based on the compo-

nents shown in Figure 1. The supported PowerEdge servers

provide key high-availability features such as error-correcting

code memory; software or hardware RAID for the internal

drives; hot-swappable drives, power supplies, fans, and PCI

slots; optional dual host bus adapters (HBAs); and optional

redundant paths to the storage systems.

Related Categories:

Clustering

Dell PowerEdge servers

Dell PowerVault storage

Direct attach storage (DAS)

High availability (HA)

Microsoft WindowsServer 2003

Serial Attached SCSI (SAS)

Storage architecture

Visit www.dell.com/powersolutions

for the complete category index.

Building Highly Available Systems: The Dell PowerEdge Cluster SE600W and PowerVault MD3000The Dell™ PowerEdge™ Cluster SE600W is a Serial Attached SCSI (SAS)–based cluster solution comprising Dell PowerEdge servers, Microsoft® Windows® operating systems, and the Dell PowerVault™ MD3000 storage system. Deploying this solution enables enterprises to achieve highly available service levels in a cost-effective way.

By NaM NguyeN

DaNiel Moges

shaBaNa M.

Travis vigil

reprinted from Dell Power Solutions, May 2007. Copyright © 2007 Dell inc. all rights reserved.

http://www.dell.com/powersolutions

71www.dell.com/powersolutions

The PowerEdge Cluster SE600W offers three

configurations:

• Entry-level configuration: This configuration

offers only one path from each server to the

storage (see Figure 2). If a component in one

path fails, the server loses access to the

storage; when this happens, Microsoft

Cluster Service moves resources from the

affected server to the other server and

restarts the applications.

• Redundant configuration with dual-port

HBAs: This configuration provides higher

availability than the entry-level configura-

tion, incorporating two ports on each SAS

5/E HBA to create two paths from each server

to the storage (see Figure 3). If an HBA fails,

Microsoft Cluster Service handles applica-

tion migration the same way it does in the

entry-level configuration. If another storage

component fails, I/Os are transparently

rerouted to the alternate path without affect-

ing applications or clients.

• Fully redundant configuration: This configu-

ration provides the highest availability of

the three configurations, incorporating two

SAS 5/E HBAs and two paths from each

server to the storage (see Figure 4).

Administrators can configure I/Os to use

any path for load balancing. If a storage

path fails, I/Os are transparently rerouted

to the alternate path without affecting

applications or clients.

Dell PowerVault MD3000The Dell PowerVault MD3000 storage array, with

two high-performance, active/active external

RAID controllers and a mirrored cache, can be a

key part of designing a cost-effective, highly

available cluster system. It is based on SAS

technology and offers high-availability features

such as hot spares; hot-swappable drives,

power supplies, cooling fans, and storage con-

trollers; redundant storage controllers; redun-

dant back-end paths with dual-port SAS drives;

and optional premium features such as snap-

shots and virtual disk copy.

To appreciate the potential value of cluster-

ing with PowerVault MD3000 storage, enter-

prises should first understand the host-based

RAID method of clustering. This method uses a

direct connection from a storage device to each

server, and the RAID controller itself resides on

a PCI RAID card installed in the server. In an

external RAID configuration, like that of the

PowerVault MD3000, the RAID controller resides

outside the server in the external storage

system itself.

Among the issues associated with host-

based RAID clustering, one of the most signifi-

cant is related to the cache and performance.

The cache is high-speed memory designed to

reduce the time required to read or write data.

In host-based RAID configurations, the cache

resides on the RAID adapter card located in the

server. To help ensure that in the event of a

server failure the unwritten cache data is writ-

ten to disk, vendors disable the use of the

cache on RAID adapters to maintain cache

coherency for the cluster. Unfortunately, doing

Building Highly Available Systems: The Dell PowerEdge Cluster SE600W and PowerVault MD3000

Figure 1. Supported components for the Dell PowerEdge Cluster SE600W

operating systems Microsoft Windows server 2003 and Windows server 2003 release 2 (r2) enterprise edition and enterprise x64 edition

servers Dell Poweredge models 1800, 1850, 2800, 2850, 6800, 6850, 1950, 2900, 2950, 2970, and 6950

storage Dell Powervault MD3000

Figure 3. Redundant Dell PowerEdge Cluster SE600W configuration with dual-port HBAs

Figure 2. Entry-level Dell PowerEdge Cluster SE600W configuration

Figure 4. Fully redundant Dell PowerEdge Cluster SE600W configuration


LAN DellPowerEdge

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DellPowerEdge

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HBA HBA

Controller 1 Controller 2

Dell PowerVault MD3000

Disk drives

LAN DellPowerEdge

server

DellPowerEdge

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HBA HBA



Disk drives

LAN DellPowerEdge

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storage

DELL POWER SOLUTIONS | May 200772

so can cause a significant degradation in clus-

ter performance.

When using external RAID, the cache

resides on the external controller within the

storage system, and cache coherency can be

maintained without disabling the cache func-

tionality. For this reason, external systems

are preferable for a clustered configuration.

Storing the cache separately from the server

helps eliminate the risks associated with a

server failure in a host-based RAID cluster.

As an external storage system, the PowerVault

MD3000 is ideal for highly available two-

node clustering applications on PowerEdge

servers when performance and access to data

are critical.

Cluster features and advantagesThe PowerEdge Cluster SE600W and PowerVault

MD3000 are designed to build upon the reli-

ability of parallel SCSI while addressing that

technology’s performance, reliability, and scal-

ability limitations. Figure 5 summarizes the

cluster features of the SAS-based PowerEdge

Cluster SE600W compared with the parallel

SCSI–based PowerEdge Cluster SE500W.

Performance The PowerVault MD3000 can provide better

performance than PowerVault 22xS storage

because of its increased data transfer rate over

the SAS link and its enabled write cache. While

the maximum data transfer rate for Ultra320

SCSI is 320 Mbps (and the complications of the

shared bus architecture make increased speeds

unlikely), SAS currently offers speeds of up to

3 Gbps, and the SCSI Trade Association plans

for it to eventually support up to 12 Gbps.

In a PowerEdge Cluster SE500W, the write

cache is disabled in the RAID controller to main-

tain cache consistency, because the RAID intel-

ligence is part of the controller residing on the

host. In a PowerEdge Cluster SE600W, the write

cache is enabled in the RAID controller, because

the RAID intelligence is part of the controller

residing on the external storage device. Each

storage controller has 512 MB of cache, which

helps significantly improve overall system

performance.

reliabilityWhen deployed in an environment using

parallel SCSI and a shared bus architecture, a

misbehaving device can bring down the entire

system. The PowerVault MD3000 offers a point-

to-point architecture in which such devices can

be easily isolated without affecting the entire

system. In addition, the fully redundant PowerEdge

Cluster SE600W configuration provides two paths

from the servers to the storage, and includes

dual-port SAS drives to allow data access from

either port from one of the two storage controllers.

As a result, if a component in a storage path fails,

the multipath software can automatically reroute

the I/Os to the alternate path without affecting

the applications or clients.

scalabilityParallel SCSI can support up to 16 devices on

the shared bus, while a SAS domain can sup-

port up to 16,384 devices without performance

degradation. The PowerEdge Cluster SE600W

can support up to 45 SAS hard drives by adding

PowerVault MD1000 disk expansion enclosures

to the PowerVault MD3000 using a daisy-chain

topology (see Figure 6).Figure 5. Feature comparison of Dell PowerEdge Cluster SE500W and PowerEdge Cluster SE600W

Dell Poweredge Cluster se500W Dell Poweredge Cluster se600W

HBas up to two Poweredge expandable raiD Controller (PerC) 4/DC or PerC 4e/DC adapters per node

up to two sas 5/e hBas per node

storage up to four Powervault 22xs enclosures one Powervault MD3000 enclosure connected to up to two Powervault MD1000 enclosures

Cables 68-pin shielded P-type sCsi cable for connection to the host, up to 12 meters long

Compact cable and connector, up to 6 meters long

Hard drives 13 sCsi hard drives per enclosure 15 sas hard drives per enclosure (with expansion to up to 45 drives by adding Powervault MD1000 enclosures)

redundant path to the storage system

No yes, including redundant storage controllers with failover capability, dual-port sas drives, and a multipath failover driver

raID raiD-1, raiD-5, raiD-10, and raiD-50 raiD-0, raiD-1, raiD-5, and raiD-10

Write cache Disabled in cluster mode enabled in cluster mode

storage manageability

Dell openManage storage services and Dell openManage array Manager, installed on a host directly connected to the storage

Dell Modular Disk storage Manager with advanced features such as snapshots and virtual disk copy, installed on a management station (either in-band through the sas link or out-of-band over the network)


“The Poweredge Cluster se600W and Powervault MD3000 are designed to build upon the reliability of parallel sCsi while addressing that technology’s per-formance, reliability, and scalability limitations.”

73www.dell.com/powersolutions

Highly available cluster solutionThe Dell PowerEdge Cluster SE600W and

PowerVault MD3000 take advantage of SAS

technology to offer enhanced performance, reli-

ability, and scalability, and include additional

features not found in parallel SCSI–based

PowerEdge Cluster SE500W configurations. This

cost-effective clustering solution can benefit

enterprises of all sizes that require high avail-

ability for critical applications such as data-

bases, messaging systems, and file, print, and

Web servers.

Nam Nguyen is a senior consultant in the High-

Availability Cluster Development Group at Dell,

and the lead engineer for SAS, Internet SCSI

(iSCSI), and Fibre Channel Dell PowerEdge Cluster

products. His current interests include business

continuity, clustering, and storage technologies.

He has a B.S. and an M.S. in Electrical Engineering

from the University of Texas at Austin.

Daniel Moges is a systems engineer adviser in the

High-Availability Cluster Development Group at

Dell. His responsibilities include developing SCSI-,

iSCSI-, and Fibre Channel–based high-availability

clustering products; his current interests related to

high-availability clustering include enterprise stor-

age technologies and database systems. Daniel

has a B.S. in Electrical Engineering from the

University of Illinois at Urbana-Champaign.

Shabana M. is an engineering analyst in the High-

Availability Cluster Development Group at Dell.

Her interests include the development of SCSI-

and Fibre Channel–based clustering solutions and

application performance. She has a B.E. in

Computer Science and Engineering.

Travis Vigil is a senior product marketing consultant

for Dell PowerVault storage. He has a B.S. from

Stanford University and an M.B.A. from Northwestern

University’s Kellogg School of Management.

Figure 6. Fully redundant Dell PowerEdge Cluster SE600W configuration with two daisy-chained Dell PowerVault MD1000 enclosures


LAN DellPowerEdge

server

DellPowerEdge

server

HBA 1 HBA 2 HBA 1 HBA 2


Dell PowerVaultMD3000

Disk drives



Building Highly Available Systems: The Dell PowerEdge … · rently supports two-node clustering based on the compo-nents shown in Figure 1. The supported PowerEdge servers ... to

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