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Storage area networks (SANs) offer an effective means

of storing and sharing data. As the amount of data

stored on a SAN increases, however, backup windows

lengthen and disaster recovery requires more time. EMC

SnapView 2.1 and MirrorView 1.7 storage management

software can help facilitate efficient backups and disaster

recovery in Dell | EMC SAN environments.

To demonstrate how IT administrators can improve SAN

management in a typical data center environment using

EMC storage management software, this article presents a

scenario using a fictional company called Acme. In this sce-

nario, Acme has a local data center that uses a Dell |EMC

SAN for consolidated, redundant, high-availability storage.

In addition, the company has heterogeneous servers shar-

ing the storage and tape library.

To protect the companys valuable business data, the

Acme IT department implemented a disaster recovery

plan. The plan involved deploying a fully redundant

Dell|EMC SAN, which allows Acme to achieve high avail-

ability at the hardware level. To prevent failures at the oper-

ating system (OS) and application levels, the IT department

deployed the companys main application in a Microsoft

Cluster Service (MSCS) environment. Every application

server that is connected to the SAN has two host bus

adapter (HBA) cards to provide redundancy and increase

bandwidth. Acme connected a tape library to the SAN for

backup and restore; tapes are stored in a remote site after

tapes at the production site are backed up and verified.

The remote site also acts as a disaster recovery site for

the primary (production) site, and the IT department

established a secondary SAN for the applications running

at the remote location.

Currently, Acme faces three business challenges:

Increasing backup window: As the databasegrows, the backup window will soon exceed the

time available for the daily backups.

Lengthy time to recovery: The length of timerequired for disaster recoveryreferred to as mean

time to recovery (MTTR)is growing because

larger databases require longer restore times.

The companys main application is inaccessible

during restoration.

Overhead on the production environment: Acmeuses its production database to perform application

development work; however, developer access to

the production database creates overhead on the

database engine. Performing online backups also

incurs overhead that affects the performance of the

production servers.

POWER SOLUTIONS August 200368

Data Replication and Recovery Using

EMC SnapView and MirrorViewEMC SnapView 2.1 and MirrorView 1.7 software help administrators protect and

recover their data in Dell|EMC storage area networks (SANs). This article explains the

differences between these applications, demonstrating how they can be used to achievehigh availability in a disaster recovery environment, reduce backup windows, and

remove the processing overhead for backups from production servers.

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To resolve these issues, Acme decided to update its disasterrecovery plan. The Acme IT department connected the primary-site

SAN with the secondary-site SAN, as shown in Figure 1. The

company plans to use SnapView snapshots and clones to create

replicas for online backups and for development use. Acme will use

MirrorView software across the Dell| EMC SANs to create remote

copies for disaster recovery. Using SnapView and MirrorView will

also enable Acme to create a plan for recovery at the file, logical

unit number (LUN), and array levels, as well as to complete online

backups without affecting the production environment.

Creating snapshots and clones for backups using SnapView

SnapView 2.1 creates either a virtual point-in-time copy (snapshot)

of the original data or a full, physical point-in-time copy (clone) of

the original data. Currently, SnapView is supported on Dell | EMC

FC4700-2, CX400, and CX600 storage arrays as a nondisruptive

upgrade, meaning that the software can be added at any time with-

out disturbing the production environment.

Snapshots depend on source LUN

At Acme, a long backup window consumes resources from the data-

base engine and creates overhead on the production environment.

The necessity for Acme development engineers to access the production

database for development work contributes to these two problems.

SnapView can help resolve both of these business issues.

Using SnapView, administrators can create up to eight point-

in-time snapshots of a LUN, which can subsequently be made

accessible to as many as eight hosts. For example, the Acme SAN

administrator can make a snapshot accessible to a backup server,

allowing the production server to continue processing without the

downtime traditionally associated with backup processes. An admin-

istrator also can create additional snapshot sessions for use by

the development engineers without affecting the data on the pro-

duction, or source, LUN.

The snapshot feature uses a cache-and-pointer design, where achunk map table keeps track of data chunks (groups of blocks)

based on their state at a given time. As the first write request to a

block is made to the source LUN, the chunk to be modified is copied

to a snapshot cache on private LUNsa process known as copy on

first write (COFW). The source LUN, the snapshot cache, and the

chunk map table work together to create the virtual snapshot LUN.

The snapshot LUN is an exact copy of the production LUN, and

thus the snapshot must be accessed by a different host, such as a

development or backup server. The backup server can read from

and write to a snapshot LUN, but any changes made to the

snapshot LUN do not replicate back to the source LUN. When

the snapshot session is deactivated, the virtual snapshot LUN will

be invisible to the server.

As Figure 2 indicates, every source LUN can have as many as

eight sessions and eight snapshots. Snapshots have a one-to-one

relationship with a server. Each snapshot must be assigned to a

different server, whereas sessions can be related to any server,

depending on which session is activated and when it is activated.

The most common use of a snapshot is to produce a backup

copy of a large database. Performing an online backup of a data-

base can help to shorten the backup window without interrupting

STORAGE ENVIRONMENT

www.dell.com/powersolutions POWER SOLUTIONS 69

NAS serverStand-alone

application serverClustered application server group

Primary storage array

Primary site

Backup/restoreserver

Remoteapplication server

Remote storage array

Secondary site

Remote backup/restore tape library

Figure 1. Primary and secondary SANs connected for disaster recovery

One sourceLUN

Multiplesnapshot cache

Up to eightsessions

Up to eightsnapshots

Up to eightservers

Possible relationshipbetween snapshot

LUN and the session

Storage group 1

Storage group 2

Storage group 3

Storage group 8

Figure 2. Source LUN, session, and snapshot LUN relationship

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production access to the database. However, online backups create

overhead on the production database server, sometimes even requir-

ing that the database be stopped during the backup window. A

SnapView snapshot allows the database to be replicated instanta-

neously. The replica can then be used for online backups, as well

as for development work, without putting additional overhead onthe application server.

SnapView snapshots also improve and simplify file-level recov-

ery. Administrators can maintain a repository of snapshot sessions

across multiple days on the network attached storage (NAS) server

connected to the SAN, as shown in Figure 3. If, for example, a user

wants to access files from the Friday snapshot session, the SAN

administrator can simply activate the Friday session and share that

snapshot LUN with the user. The user can then retrieve the needed

files by copying files from the snapshot LUN to the source LUN.

Clones produce full, independent copies

Although SnapView reduces the backup window and removes

backup overhead from the production server, the snapshot fea-

tures cache-and-pointer design means that snapshot LUNs depend

on the existence of the source LUN. If the source LUN is damaged

or destroyed, administrators would need to rebuild the source LUN

and recover the data from tape or another backup medium (assum-

ing that the local Dell | EMC storage array is still up and running).

The MTTR after such an event might take hours depending on the

size of the LUN and the speed of the tape technology. For a com-

pany requiring fast disaster recovery, as in the Acme scenario,

snapshot LUNsthat is, virtual LUNsare not an ideal solution.

To decrease MTTR, administrators can use the SnapView clone

function to create LUN copies that are independent of the source

LUN. Unlike snapshots, which are point-in-time views of a

source LUN, clones are synchronous copies of the source LUN.

Each clone LUN consumes exactly the same amount of physical

space as the source LUN. Essentially a local mirror of the source

LUN, a clone offers high availability and can withstand storage

processor failures or source LUN failures, as well as path failures,

provided that EMC PowerPath or Application Transparent Failover

(ATF) software is installed and properly configured. Clones, there-

fore, are business continuance volumes (BCVs).

To create a clone, the initial data is copied, or synchronized,

to the clone (see Figure 4). During synchronization, any host write

requests made to the source LUN are copied to the clone. Once the

clone is 100 percent synchronized, it is fractured manually at a point

in time to create a stand-alone BCV that is independent of

the source LUN. Servers cannot access the clone LUN until it is

fracturedthough application I/O can still access the source

LUN during synchronization.

Resynchronization can occur in either direction. To recover data

from the clone to the source LUN, administrators can use the

reverse synchronization feature while I/O continues to the source

LUN. A clone becomes available for read and write access once it

is fractured. Administrators also can access a clone by creating

a snapshot and then assigning the snapshot to a second server

storage group as long as the snapshot is in a different storage

group than the source LUN. This manner of implementation not

only removes the overhead on the server, but it also enables thesource LUN to access snapshots without I/O overhead.

After synchronization and fracturing, a clone becomes a fully

populated, physical copy of its source LUN. Because clones are not

pointer-based replicas, they are not affected by the COFW perfor-

mance penalty; the data is replicated to the clone instead of being

copied to nonvolatile memory along with the modified chunks.

This process results in lower performance overhead for clones

than snapshots.

A clone is commonly used in environments that require quick

MTTR or online backups based on the point-in-time copies that have

zero impact on the production data. A server can read from and

write to a fractured clone without affecting the source LUN. Also,

resynchronizing the clone is fast because clones use a space in

memory called the clone private log (CPL) to keep track of the

changes that occur after they have been fractured. For efficiency,

100 percent resynchronization is avoided; only post-fracture changes

are resynchronized.

Enabling array-level disaster recovery through MirrorView

The Acme disaster recovery plan protects critical business data

by outlining a procedure for recovery when the primary site is

Productionstorage group

Snapshot LUN

Clone group(Up to eight clones)

Fracture aftersynchronization

Figure 4. Clone creation and access

Source LUN

Monday 6:00 P.M. session

Tuesday 6:00 P.M. session

Wednesday 6:00 P.M. session

Thursday 6:00 P.M. session

Friday 6:00 P.M. session

NAS server

Snapshot LUN

Backup/restoreserver

Local area network

Figure 3. File recovery from a snapshot LUN to the source LUN

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down. The plan also addresses the replication of data from the

primary location to the secondary location so that applications run-

ning at the secondary site can access the same business data. To

implement these processes, the Acme scenario uses the EMC

MirrorView add-on software option. MirrorView is similar to the

SnapView clone option, but works between Dell | EMC arrays

instead of within a single array. Because MirrorView is array-

based software, it does not use server I/O or CPU resources, and

it supports all of the operating systems used on the array.

Provision for disaster recovery is the major benefit of

MirrorView mirroring. As shown in Figure 5, multiple arrays in dif-

ferent locations can mirror to a common disaster recovery site,

which makes it the central mirroring site for disaster recovery. If a

disaster cripples the primary site, a MirrorView secondary image can

be used to recover data and operations at the disaster recovery site.

MirrorView runs redundantly across arrays. If one storage

processor fails, MirrorViewrunning on the other storageprocessorwill take ownership of the mirrored LUNs. If the host

can fail over I/O to the remaining storage processor (using

PowerPath software), then mirroring will continue as normal.

After the primary-site array has been recovered, the data at the

secondary site can be synchronized back to the primary site.

Although the mirrored target cannot be directly assigned to a

server while it is acting as a mirrored target, SnapView software

can be used to take a snapshot of the secondary mirrored LUN and

then assign the snapshot to the servers on the secondary

site for immediate access, even if the two sites are mirroring.

MirrorView mirroring is synchronous, thus the longer the distance,

the longer the delay, because the application must wait for a

commitment to be returned from the remote array. For disaster

recovery, primary and secondary storage systems should be

relatively far apart (within 10 km) and connected through dedicated

redundant pairs of fiber-optic cabling for Fibre Channelbased

mirroring. For longer distances, other solutions exist.

MirrorView can ensure that data from the primary storage

system replicates to the secondary array (see Figure 6). The host

(if any) connected to the secondary array might normally sit idle

until the primary site fails. With SnapView at the secondary site,

the host at the secondary site can take snapshot copies of the

mirror images (that is, secondary LUNs) and back them up to other

media. This technique provides point-in-time snapshots of pro-

duction data with little impact to production server performance.

MirrorView provides a synchronous mirroring solution, which

can help ensure that any write to the primary array also is com-

mitted on the secondary array before the production server gets an

acknowledgment. Although this technique is commonly imple-

mented on most mirroring technologies, it also requires that latency

between two storage arrays be calculated and considered to pre-

vent any performance degradation. Currently, MirrorView runs

through either Fibre Channel (using dedicated fiber-optic cables)

or Fibre Channel over IP (using routers and sufficient dedicatedbandwidth on an IP wide area network, or WAN).

Selecting the appropriate data-protection strategy

SnapView snapshots, SnapView clones, and MirrorView mirrors

provide different levels of data protection. Snapshots are most likely

to be used in a parallel processing environment to provide online

backups or file-level recovery, whereas clones and mirrors are more

often used in disaster recovery situations.

Clones may be used for fast recovery of local corrupt LUNs;

clones support read and write access to both source LUN and clone

once the clone has been fractured. Mirrors usually enable recov-

ery of arrays or sites. Mirrors also can be used to replicate data to

multiple sites, and then used with snapshots for remote access.

Mirroring provides read and write capability only to the source

LUN, but read and write access to the remote copy of the data can

be accomplished by using SnapView on the target array to take a

snapshot of the mirror.

To support either MirrorView or SnapView, administrators must

install the EMC Access Logix tool. This software masks source and

target LUNs to different servers to prevent LUN corruption.

Primary location A

Primary location B

Primary location C

Primary location D

Snapshot storage group

Disaster recovery site

Figure 5. Central mirroring for disaster recovery

Secondary location B

Secondary location A

Production storage group

Primary location A

Snapshot storage group

Snapshot of thesecondary

image

Snapshot storage group

Snapshot of thesecondary

image

Figure 6. Using MirrorView for data replication

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Combined solutions reduce backup window

and production server overhead

In the Acme scenario, administrators were able to use both SnapView

and MirrorView to solve the three business problems that the com-

pany faced. The company now uses its NAS server, to which any

user can map, for storing snapshots. This server enables adminis-

trators to recover data from a specific point in time without a large

backup window. The company created a local clone as a develop-

ment server for its main clustered application, removing overhead

from the production environment. Acme also mirrored its data to

the remote site and created a snapshot of the mirror to enable online

backups that will not affect the production environment.

Mirroring the companys main application to the remote site

provides quick MTTR and allows for remote backups in case of dis-

aster at the primary site. Through snapshots, data can be assigned

to servers at the remote location for other applications. Figure 7

provides a decision tree to help administrators choose the right repli-

cation and recovery tools for their own companys specific imple-

mentations.

Enabling comprehensive data-recovery plans

using EMC software

Dell |EMC SANs provide a reliable environment for data consoli-

dation. The optional SnapView and MirrorView software add-ons

enable administrators to create a comprehensive data-recovery plan

for different disaster scenarios. When administrators use the fea-

tures provided in SnapView and MirrorView, they enable online

development work or data mining to be performed without

affecting the production environment. These features also provide

a way to replicate data to multiple locations as well as maintain

data consistency.

Richard Hou (richard_hou@dell.com) is a systems engineer and consultant for the Dell

Enterprise Technology and Education Center (ETEC), part of the Dell Enterprise Services

and Support Group, where he specializes in SAN and Microsoft solutions. Richard has an

M.S. in Electrical and Computer Engineering from The University of Texas at Austin and a

B.S. in Mechanical Engineering from Zhejiang University, Hangzhou, China.

Steve Feibus (steve_feibus@dell.com) has been a storage enterprise technologist in

the Advanced Systems Group at Dell for the past two years and was recently promoted to

manager of the Client Technologist team at Dell. Steve has a B.S. in Electrical Engineering

from the University of Florida and has spent many years solving customer storage issues

using the latest technologies and products.

Patty Young (patty_young@dell.com) is a storage enterprise technologist in the Advanced

Systems Group at Dell. She has been working with storage solutions for many years,

supporting field system consultants in architecting storage solutions for their customers and

providing feedback from customers to Dell regarding storage challenges and requirements.

Patty has a B.A. from North Carolina State University.

STORAGE ENVIRONMENT

www.dell.com/powersolutions POWER SOLUTIONS 73

CX400, CX600,or FC4700-2?

Yes

No

Singleor multiple

array?

Single

Multiple

What is thepurpose of the

data copy?

Snapshot

BCV, data replication,online backup,

and data recoverywithin array

Online backup,decision support,and testing for

instantaneous copy

Clone

Tape or third-partysolutions for

data replication

Data replicationacross arrays; BCV

on remote site

Customeroperatingsystems

Microsoft Windows 2000 Server,IBM AIX, Linux, Sun Solaris,

Novell NetWare, HP-UX

Arraysto be

utilized

CX400, CX600,FC4700-2

Distancebetween mirrored

locations

Mainframe

CX200,Dell PowerVault 660F,

Dell PowerVault 650F

STOPMirrorView

not a solution

Over 500 km

60 km500 km

10 km60 km

Up to 10 km

Up to 500 m

Up to 300 mFibre Channel-1

Fibre Channel-2

Fibre Channel LW-GBIC

Dense wavelength divisionmultiplexing (DWDM) extender

MirrorView IPor third-party

solution

MirrorViewFibre Channel

or MirrorView IP

Figure 7. Decision tree for selecting snapshot, cloning, or mirroring

F O R M O R E I N F O R M A T I O N

EMC: http://www.emc.com

Dell|EMC: http://www.dell.com/emc