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Sun StorEdge™
Network Data Replicator 3.0
Configuration Guide
Sun Microsystems, Inc.
901 San Antonio Road
Palo Alto, CA 94303-4900 U.S.A.
650-960-1300
Part No. 806-7550-10
June 2001, Revision A
Send comments about this document to: docfeedback@sun.com
Copyright 2001 Sun Microsystems, Inc., 901 San Antonio Road • Palo Alto, CA 94303 USA. All rights reserved.
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Parts of the product may be derived from Berkeley BSD systems, licensed from the University of California. UNIX is a registered trademark in
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Contents
Preface
1.
v
Overview
1
Sun SNDR Software Description
1
Supported Hardware and Software
3
TCP/IP Connection Hardware Components
Network Multipathing
2.
Applications
5
Architecture
5
4
Configuration Considerations
Volumes Eligible for Replication
7
7
Choosing Volume Level Protection
Bitmap Volumes for Scoreboard Logs
8
8
Choosing a Connection Medium to Link My Sites
ATM Link Advantages
9
9
Configuring Redundant Links Between Sites
Link Security
3
10
10
Choosing Between Synchronous and Asynchronous Replication
When To Suspend Replication to the Secondary Site
11
12
iii
Configuring The Sun SNDR Software for Mutual Replication
Order-Dependent Writes and Volume Set Grouping
Recovery Considerations
12
13
14
Failing Over to the Secondary Site
14
Recovering the Primary or Secondary Site
14
Using The Sun SNDR and Sun StorEdge Instant Image Software
Sun SNDR Software and Sun StorEdge Fast Write Cache Software
One-to-Many and Multihop Volume Sets
18
Using The Sun SNDR Software in a Firewall Environment
iv
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
19
17
16
Preface
The Sun StorEdge Network Data Replicator 3.0 Configuration Guide describes the Sun
StorEdge™ Network Data Replicator (Sun SNDR) software and general information
about its use. This guide is intended for system administrators who have experience
with the Solaris™ operating environment and related disk storage systems.
Before You Read This Book
To fully use the information in this document, you must have thorough knowledge
of the topics discussed in these books:
■
Sun StorEdge Network Data Replicator 3.0 System Administrator’s Guide
■
Sun StorEdge Network Data Replicator 3.0 Installation Guide
■
Sun StorEdge Network Data Replicator 3.0 Release Notes
The Sun SNDR software must be installed as described in the installation and release
documentation.
v
Using UNIX Commands
This document may not contain information on basic UNIX® commands and
procedures such as shutting down the system, booting the system, and configuring
devices.
Refer to the software documentation that you received with your system.
Documentation Conventions
Typeface or
Symbol
vi
Meaning
Examples
AaBbCc123
The names of commands,
files, and directories;
on-screen computer output.
Edit your .login file.
Use ls -a to list all files.
% You have mail.
AaBbCc123
What you type, when
contrasted with on-screen
computer output.
% su
Password:
AaBbCc123
Book titles, new words or
terms, words to be
emphasized.
Command-line variable;
replace with a real name or
value.
Read Chapter 6 in the User’s Guide.
These are called class options.
You must be root to do this.
To delete a file, type rm filename.
[ ]
In syntax, brackets indicate
that an argument is optional.
scmadm [–d sec] [ –r n[:n][,n]...] [–z]
{ arg | arg}
In syntax, braces and pipes
indicate that one of the
arguments must be specified.
sndradm -b {phost | shost}
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
Shell Prompts
Shell
Prompt
C shell
machine_name%
C shell superuser
machine_name#
Bourne shell and Korn shell
$
Bourne shell and Korn shell superuser
#
Related Documentation
Application
Title
Part Number
man pages
sndradm
scmadm
svadm
iiadm
N/A
Latest information
Sun StorEdge Network Data Replicator 3.0 Software
Notes
806-7513
Sun StorEdge Instant Image 3.0 Release Notes
806-7678
Sun StorEdge Network Data Replicator 3.0
Installation Guide
806-7514
Sun StorEdge Instant Image 3.0 Installation Guide
806-7675
SunATM 3.0 Installation and User’s Guide
Sun ATM 4.0 Installation and User’s Guide
805-0331
805-6552
Sun Gigabit Ethernet FC-AL/P Combination Adapter
Installation Guide
806-2385
Sun Gigabit Ethernet/S 2.0 Adapter Installation and
User’s Guide
Sun Gigabit Ethernet/P 2.0 Adapter Installation and
User’s Guide
805-2784
Sun Trunking 1.2.1 Installation and User's Guide
806-4207
Installation and User
805-2785
Preface
vii
Application
System administration
Configuration
viii
Title
Part Number
Platform Notes: The Sun Quad FastEthernet Device
Driver
806-3989
Sun Enterprise 10000 InterDomain Networks User
Guide
806-4131
Sun StorEdge Instant Image 3.0
System Administrator’s Guide
806-7677
Sun StorEdge Network Data Replicator 3.0
System Administrator’s Guide
806-7512
TCP/IP and Data Communications Administration
Guide
805-4003
SunScreen 3.1 Administration Guide
806-4127
Solaris Bandwidth Manager 1.6 System
Administration Guide
806-4704
Sun Enterprise 10000 InterDomain Network
Configuration Guide
806-5230
Sun StorEdge Instant Image 3.0 Configuration Guide
806-7676
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
Accessing Sun Documentation Online
A broad selection of Sun system documentation is located at:
http://www.sun.com/products-n-solutions/hardware/docs
A complete set of Solaris documentation and many other titles are located at:
http://docs.sun.com
For the latest version of released storage software documentation, go to:
http://www.sun.com/products-n-solutions/hardware/docs/Software/
Ordering Sun Documentation
For the latest version of storage software documentation, go to:
http://www.sun.com/products-n-solutions/hardware/docs/Software/
Fatbrain.com, an Internet professional bookstore, stocks select product
documentation from Sun Microsystems, Inc.
For a list of documents and how to order them, visit the Sun Documentation Center
on Fatbrain.com at:
http://www.fatbrain.com/documentation/sun
Sun Welcomes Your Comments
Sun is interested in improving its documentation and welcomes your comments and
suggestions. You can email your comments to Sun at:
docfeedback@sun.com
Please include Part No. 806-7550-10 in the subject line of your email.
Preface
ix
x
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
CHAPTER
1
Overview
Caution – Do not install or use the Sun StorEdge Version 3.0 Core and data services
software on servers in a Sun Cluster 3.0 environment. The Version 3.0 software is
not co-existent with the Sun Cluster 3.0 environment. The Version 3.0 software is
co-existent in the Sun Cluster 2.2 environment, where it will not interfere with
failover. The Version 3.0 core and data services software is cluster aware in the Sun
Cluster 3.0 Update 1 environment and provides high availability for the Sun
StorEdge software.
Sun SNDR Software Description
The Sun SDNR software is a remote replication facility for the Solaris™ operating
environment. It is intended for use as part of a disaster recovery and business
continuance plan to provide redundant storage of critical information across
physically separate sites. It is designed to be active during normal application access
to the data volumes, and it will continually replicate the data to the remote site.
The Sun SDNR software enables you to replicate disk volumes between physically
separate primary and secondary hosts in real time. To transport data, the Sun SDNR
software uses any Sun network adapter that supports TCP/IP.
The Sun SNDR software volume set consists of a primary volume residing on a local
host and a secondary volume residing on a remote host. The volume set also
includes a bitmap volume on each host to track write operations and differences
between the volumes.
You can update the data on the secondary volume by issuing a command to
resynchronize the volumes. You can also restore data from the secondary volume to
the primary volume by issuing a command to reverse resynchronize the volumes.
1
The replication mode can be synchronous or asynchronous based as a user-selectable
parameter for each Sun SNDR volume set. (Use the sndradm enable command and
select the sync or async parameter Use the sndradm -R m command to change
the replication mode thereafter.) The volumes can be updated synchronously in real
time or asynchronously using a store-and-forward technique.
Typically, a primary volume is first explicitly copied to a designated secondary
volume to establish matching contents. As applications write to the primary volume,
the Sun SDNR software replicates changes to the secondary volume, keeping the two
volumes consistent.
■
In synchronous mode, a write operation is not confirmed as complete until the
remote volume has been updated. Synchronous mirroring forces the Sun SNDR
software to wait until an acknowledgement of the receipt of the data is received
from the secondary by the primary before returning to the application.
■
In asynchronous mode, a write operation is confirmed as complete before the
remote volume has been updated. Asynchronous mirroring allows the Sun SNDR
software to return to the host as soon as the write has been completed on the
primary volume and been placed on a per volume queue for the secondary site.
Subsequently, the secondary site receives the queued requests in the order that
they were queued. Once the I/O has been completed at the secondary site,
notification is sent to the primary.
In the event of planned or unplanned outages, the Sun SNDR software maintains
per-device scoreboard bitmap volumes that are marked to indicate changed blocks
with a granularity of 32 Kbytes per segment. This technique allows for optimized
resynchronization by allowing the Sun SNDR software to resynchronize only the
blocks that have changed.
Other features included as part of the Sun SNDR software are:
■
Grouping volume sets — The Sun SNDR software enables you to group volume
sets. Grouping volume sets guarantees write ordering. For example, in a
synchronization from the primary host to the secondary host, write operations to
the secondary volumes in the group occur in the same order as the write
operations to the primary volumes in the group.
You can also assign specific volume sets to a group to perform sndradm
commands on these volume sets and not on others you have configured. See
“Order-Dependent Writes and Volume Set Grouping” on page 13.
Note – You can also group volume sets according to their cluster or resource tag to
perform replication in a Sun Cluster 2.2 or 3.0 Update 1 clustered environment only.
2
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
■
One-to-many and multihop volume sets — The Sun SNDR software enables you
to create one-to-many and multihop volume sets. In a one-to-many volume set,
you can replicate data from one primary volume to many secondary volumes
residing on one or more hosts. In a multihop set, the secondary host volume of
one volume set can be the primary host volume of another volume set. See
“One-to-Many and Multihop Volume Sets” on page 18.
Supported Hardware and Software
Operating Environment
Solaris™ 7 or 8 operating environment or subsequent compatible version
Supporting Software
Network transport software such as SunATM™ or Gigabit Ethernet transports
Optional
Software
Sun StorEdge Instant Image software Version 3.0; install this package for
additional point-in-time capability
Supported Volume Manager
Software
Sun Logical Volume Manager
Sun Solstice DiskSuite™
Veritas Volume Manager
Host Types
Server hosts using the Solaris operating environment and any Sun-supported
network interface card. Hosts include:
• Sun Enterprise™ Server models 2x0 through 4x0
• Sun Enterprise Server models 3x00 through 10000
Supported Attached Storage
The Sun SNDR software is storage-hardware independent
TCP/IP Connection Hardware Components
The Sun SNDR software requires a TCP/IP connection between the primary and
secondary server. Each server must have the proper ATM or Ethernet hardware
installed to support the TCP/IP link.
The Sun SNDR software operates over any TCP/IP networking technology but has
been qualified only on 10, 100, and 1000 Mbit Ethernet and ATM 166 and 622
technologies.
A dedicated TCP/IP link is not required.
Chapter 1
Overview
3
Network Multipathing
Network multipathing with failover is supported on hosts running the Solaris 8,
Update 3 operating environment. This feature is provided as part of the standard
Solaris operating environment and is transparent to the Sun SNDR software.
The Sun SNDR software does not natively support the functionality listed in TABLE 1 .
However, other Sun products can be used with the Sun SNDR software to provide
these facilities.
TABLE 1
4
Sun SNDR Software Related Functionality
Functionality
Sun Product
Network interface aggregation and
network alternate pathing
Sun Trunking™ software and the native
facilities provided in a Solaris operating
environment update.
Network encryption
SunScreen™ Software
Network bandwidth allocation
Solaris Bandwidth Manager
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
Applications
The Sun SNDR software provides protection of critical enterprise information. The
following information types are candidates for remote replication:
■
Database partitions and logs
■
Data that is important for business continuance in a disaster situation
The Sun SNDR software can be leveraged for additional applications, such as:
■
Data migration
■
Remote backup
■
Remote data analysis
■
Test and development
Architecture
The Sun SNDR software operates on logical volumes that can be linear, striped, or
RAID devices. You can create the logical volumes by using the Sun Solstice
DiskSuite™ or Veritas Volume Manager software.
The core Sun SNDR software code is a kernel module that interfaces to the network
storage control module (NSCTL) data service framework. The Sun SNDR software is
configurable on any device that is accessible through the NSCTL data service
framework. The sndradm CLI is the external user interface used to manage the Sun
SNDR software.
FIGURE 1 shows the relationship between SNDR and the rest of the NSCTL data
service framework.
Chapter 1
Overview
5
Primary host
Application
Sun STE or SV
Sun SNDR
NSCTL Network storage
control module
User
User
Sun STE or SV
Kernel
IP
Kernel
Sun SNDR
Sun StorEdge
Instant Image
Sun StorEdge
Instant Image
SDBC
SDBC
Device driver
Device driver
Hardware
Hardware
FIGURE 1
6
Secondary host
The Sun SNDR Software Architecture
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
NSCTL Network storage
control module
CHAPTER
2
Configuration Considerations
Volumes Eligible for Replication
Generally, include the following critical volumes in the Sun SNDR configuration:
■
Database and database management system (DBMS) logs (the total database or
online DBMS log)
■
Access control files
You can exclude volumes from the Sun SNDR software configuration if they can be
reconstructed at the recovery site or if they seldom change:
■
Temporary volumes (such as those used in sort operations)
■
Spool files
■
Paging volumes
When selecting a volume to be used in the Sun SNDR software volume set, ensure
that volume does not contain disk label private areas (for example, slice 2 on a
Solaris operating environment-formatted volume). The disk label region is contained
in the first sectors of cylinder 0 of a disk. The safest method is to ensure that cylinder
0 is not part of any logical volume that is replicated (except for volumes under
Veritas Volume Manager control, where cylinder 0 can be part of a logical volume
that is replicated).
7
Choosing Volume Level Protection
In general, customers should protect their data with some level of RAID. With the
Sun SNDR software, the primary volumes can be protected by any software or
hardware RAID level desired. The protection level of the secondary volumes does
not have to match that of the primary volumes. For example, if the primary volumes
are RAID1, the secondary volumes could be RAID5.
Bitmap Volumes for Scoreboard Logs
Note – After editing the /usr/kernel/drv/rdc.conf file, reboot your server.
The Sun SNDR Version 3.0 software does not support bitmap files. The Sun SNDR
software uses regular raw devices to store bitmaps, used to maintain the Sun SNDR
scoreboard logs. For example, you can specify a bitmap volume as
/dev/rdsk/c6t0d0s4.
These raw devices should be stored on a disk separate from the disk that contains
the data. Configure RAID (such as mirrored partitions) for these bitmap devices and
ensure that the mirrored members are not stored on the same disk as the data.
In a clustered environment, a bitmap must reside only on a volume. The bitmap
volume in this case must be part of the same disk group or cluster resource group as
the corresponding primary or secondary data volume.
A bitmap maintained on disk can persist across a system crash, depending on the
setting of rdc_bitmap_mode in /usr/kernel/drv/rdc.conf. For example:
# rdc_bitmap_mode
# - Sets the mode of the RDC bitmap operation, acceptable values are:
#
0 - autodetect bitmap mode depending on the state of SDBC (default).
#
1 - force bitmap writes for every write operation, so an update resync
#
can be performed after a crash or reboot.
#
2 - only write the bitmap on shutdown, so a full resync is
#
required after a crash, but an update resync is required after
#
a reboot.
#
rdc_bitmap_mode=0;
8
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
If your server is configured in a clustered environment, set the bitmap mode to 1. If
your server is not configured in a clustered environment, you can also choose the
bitmap mode setting of 1 to improve error or disaster recovery.
Choosing a Connection Medium to Link
My Sites
The primary and secondary sites must be connected by a standard medium that
supports TCP/IP. For example, you can use any medium such as ATM, ISDN,
Ethernet, Gigabit-Ethernet, or T1 and T3 lines to replicate data. Consider cost,
bandwidth, and latency when choosing a connection medium.
The Sun SNDR software uses the maximum network bandwidth available to it. If the
Sun SNDR software must share the network connection with other applications, you
can use a bandwidth allocator to restrict the bandwidth available to an application.
ATM Link Advantages
ATM is a standards-based high-performance WAN protocol that establishes routes at
circuit initialization. Its performance, combined with SONET/SDH’s ring-healing
resiliency, have made it the choice of carriers in their core backbone networks.
ATM Quality of Service parameters allow customers to choose the circuit
characteristics that best suit their needs and budget. They define how different WAN
ports competing for bandwidth should be prioritized relative to each other, each
having “virtual paths” through the network fabric. Voice, video, and LAN traffic can
move over the same connection as critical remote mirroring connections between
primary and recovery sites. ATM switches make these varied protocol adaptations
possible.
The Sun SNDR software will operate over any TCP/IP networking technology but
has only been qualified on 10, 100, and 1000 Mbit Ethernet and ATM technologies.
Chapter 2
Configuration Considerations
9
Configuring Redundant Links Between Sites
The Solaris 8 operating environment offers IP multipathing. This feature permits a
Solaris host to group multiple physical network interfaces into a single logical
interface. Redundancy between paths permits transparent operation in the event of a
failure anywhere on the path including failures of network interface cards, links,
switches and routers.
This feature also provides the Sun SNDR software with the transparent means for
surviving link failures. It provides the ability to aggregate bandwidth across the
physical interfaces permitting a pair of ATM 622 Mbit links to operate as one logical
1244 Mbit link with resiliency.
Link Security
Solaris 8 operating environment offers IPsec, the industry standard form of
integrating encryption and authentication mechanisms into standard IP
internetworking. A Solaris 8 host can authenticate and encrypt traffic on a
conversation-by-conversation basis.
Additionally, Sun offers SunScreen TM software technology that permits both
encryption and authentication of IP traffic. Coupled with the Sun SNDR software,
SunScreen software can provide an additional level of security to the data being
transmitted between sites. SunScreen software uses public key encryption
technology to guarantee that traffic is strongly encrypted. The public key mechanism
is also used to authenticate that the sender of the traffic is the correct entity.
SunScreen software is normally configured at the edge of the data center network.
Of course, cooperating SunScreen software is required at each data center.
10
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
Choosing Between Synchronous and
Asynchronous Replication
In synchronous mode, a write operation is not confirmed as complete until the
remote volume has been updated. Generally, synchronous mirroring is limited to
relatively short distances (that is, tens of kilometers) because of the detrimental
effect of round-trip propagation delay on I/O response times. However, there are
applications such as web server replication that can tolerate relatively slow remote
updates and operate in a synchronous mode over long stretches.
With synchronous mirroring, the remote copy always matches the local host’s view
of committed writes. Should the local primary site be rendered inoperative, the
remote secondary copy can be used to continue operations after the user community
and the applications are switched to the alternate site.
What if the link goes down while running in synchronous mode? Should the local
I/O remain incomplete until the link comes back up? Some applications require that
the primary and secondary sites be exactly in sync and therefore would desire that
the I/O not complete. However, many mission-critical customers decide not to block
local access to the data upon encountering intersite transmission errors. Instead,
software keeps track of these writes at the primary site and then updates the
recovery site when the remote service is reliably restored. If configured this way,
disaster recovery procedures must take into account loss of in-flight and logged data.
That is, data might have been committed to the local application but never had a
chance to arrive at the backup site.
Asynchronous mirroring affirms primary I/O completion to the originating host
before updating the remote image. This type of mirroring is often used when the
distance and relatively low bandwidth telecommunications lines of 45 Mbps or less
between primary and secondary sites would introduce prohibitive latencies if
performed synchronously. Here, the long-distance pipe becomes the bottleneck,
forcing local writes to be queued for later transmission. Consequently, there is a
higher possibility of losing buffered and in-flight data if the primary system fails.
Chapter 2
Configuration Considerations
11
When To Suspend Replication to the Secondary
Site
For many customers, the Sun SNDR software is used to mirror the primary site’s
information onto the alternate location as quickly as possible under the constraints
of distance and technology. Operator errors, software bugs, and hardware data
corruption are solid arguments for taking more deliberate, albeit less real-time,
approaches to remote copies.
A deferred-write policy under such conditions delays remote updates until the local
data has been confirmed to be good. The data integrity checks are performed
periodically, followed by the remote copy. Some IT organizations reason that it is
better to have slightly stale, good data at the remote site than to risk corrupting the
remote copy with the lock-step updates. Consciously scrutinized database redo logs
facilitate the recovery and update process, while minimizing the consequences of the
errors.
Another method is to ship the data remotely in real time, take a snapshot of the data
at the remote site with Sun StorEdge Instant Image software at a known point in
time, and verify its integrity in the background before setting it aside for recovery.
Telco costs might similarly dictate a deferred copy policy. Some customers must
timeshare narrow pipes between sites during the peak processing hours, leaving
only off-hours for bulk intersite copies. Such prospects look for incremental update
capabilities to reduce the transmission load and copy time. In other words, they
prefer to send only what has changed since the previous remote image was
generated.
Configuring The Sun SNDR Software for Mutual
Replication
Sometimes the distinction between primary (A) and secondary (B) sites is blurred.
As applications are geographically distributed, a secondary storage system at Site B
might function as a remote volume replication to Site A and as a direct storage
resource for applications on Host B. Under these circumstances, you might also elect
to have critical applications running on Host B to have their volumes replicated to
Site A. This reciprocal replication arrangement supported by the Sun SNDR software
is known as mutual replication.
Each server can concurrently transmit and receive writes to and from its remote
counterpart. Each system contains primary disks in a Sun SNDR software set that
are accessible by local hosts, as well as remote mirrors secondary to remote hosts.
Mutual replication might be used where critical applications and storage are split
across sites, and both sites require remote replicated volumes.
12
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
Order-Dependent Writes and Volume Set
Grouping
For the Sun SNDR Version 2.0 software, write ordering is only maintained by the
Sun SNDR software within a logical volume. In Version 3.0, write ordering is also
maintained for groups of asynchronously replicating volume sets. (The general
definition of write ordering here is that write operations directed to the target
volume occur in the same order as write operations to the source.) The group of
target volumes is a point-in-time copy of the group of source volumes.
This feature is especially valuable in those cases where you can avoid application
requirements that limit operations. For example, a database application might limit
partition sizes to no greater than 2 Gbytes. In this case, you might group volume sets
to create a virtual large “volume” that preserves write operations. Otherwise, you
might risk having inconsistent data by trying to update volume sets individually
instead of as a group.
When an application has multiple logical volumes assigned, application data
integrity can be maintained by one of the following:
■
Specifying all Sun SNDR software volumes associated with that application as
SYNC
■
Using Sun StorEdge Instant Image software to take periodic recoverable
point-in-time copies
If you use Instant Image software, the remote point-in-time is taken while the
application is in the recoverable state. For example, most database applications allow
for a hot backup. If a remote point-in-time copy were made of the entire replicated
database while the primary was in hot backup mode, then a consistent remote
database is available by using the point-in-time copy and the log files taken while
the database was in hot backup mode.
Chapter 2
Configuration Considerations
13
Recovery Considerations
The Sun StorEdge Network Data Replicator 3.0 System Administrator’s Guide contains
information about various recovery scenarios. This section describes the following
topics:
■
Failing Over to the Secondary Site
■
Recovering the Primary or Secondary Site
■
Using The Sun SNDR and Sun StorEdge Instant Image Software
Failing Over to the Secondary Site
Should the primary site fail, the secondary copy can be used to continue operations
after the user community and the applications are switched to the alternate site. You
should implement application level recovery procedures at the secondary site to help
ensure application recovery to a well-known state.
Recovering the Primary or Secondary Site
Occasionally, remote mirroring operations are interrupted either intentionally or by
unplanned outages. Normally, these interruptions are handled by the Sun SNDR
software because whenever a member of a Sun SNDR software volume pair becomes
unavailable, the write activity is scoreboarded in the volume’s bitmap. When the
service is restored, a fast update or resynchronization is performed.
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Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
Choosing the Resynchronization Type: Update or Full
Tip – If a synchronization initiated by the sndradm -m or
sndradm -m -r commands is interrupted, use the sndradm -m -u update
command to complete the synchronization. Use
/usr/opt/SUNWscm/sbin/scmadm -S to view the synchronization progress.
An update resynchronization applies changes that occurred at the primary data to
the secondary site during the time replication was suspended. The primary site can
also be updated from the secondary, if desired.
A full synchronization is a complete disk-to-disk copy. This operation is the most
time consuming of the synchronization operations. A full synchronization is only
done when the Sun SNDR software volume set is:
■
Initially established
■
Damaged due to a disaster
■
Data integrity is questions
If the integrity of the Sun SNDR software volume data is questioned, then the only
way to get the volume back into a synchronized set is to perform a full volume copy.
The Sun SNDR software volume set data might be in question if, for example, there
was a double disk failure on a RAID 5 set or if the Sun SNDR software is shutdown
manually and writes occurred to either the primary or secondary volumes without
scoreboarding being active. The full copy can either be performed from the primary
to the secondary or if appropriate, from the secondary to the primary.
Choosing Automatic or Manual Resynchronization
The Sun SNDR software provides a choice of automatic or manual resynchronization
after a disaster. Automatic resynchronization is discouraged if the interruption is the
warning of a larger rolling disaster. It is best to maintain the secondary site in a
dated but consistent state, rather than risk a disastrous interruption that leaves the
secondary inconsistent and difficult to recover from. The autosynchronization option
is disabled by default for this reason.
See also “Using The Sun SNDR and Sun StorEdge Instant Image Software” on
page 16. Before you start a resynchronization operation, ensure that you have an
appropriate Instant Image software point-in-time copy of the Sun SNDR software
target volume.
Chapter 2
Configuration Considerations
15
Using The Sun SNDR and Sun StorEdge Instant
Image Software
To help ensure the highest level of data integrity on both sites during normal
operations or during data recovery, use the Sun SNDR software with the Sun
StorEdge Instant Image software.
The Instant Image software is best used just before you perform a resynchronization,
to help ensure that a consistent copy of data exists. In this case, if the
resynchronization is interrupted, you at least have a copy of known good data that is
usable.
During the resynchronization process of updating the local and remote sites, the
data on a secondary Sun SNDR volume is temporarily inconsistent with the primary
volume. The secondary volume cannot be relied on for data recovery. Consistency is
restored when the resynchronization is complete. To help ensure data integrity, use
Instant Image software regularly to create a point-in-time copy of data at both sites.
See the Instant Image documentation listed in “Related Documentation” on page vii.
When to Use The Sun StorEdge Instant Image Software at the
Remote Site
In most cases, customers should also install Instant Image software at the secondary
site to protect against a dual failure. Before you resynchronize the primary site with
the secondary, take a point-in-time snapshot copy of the data. If failure the occurs
during the resynchronization, you have the point-in-time data copy to roll back to
and you can start again when its safe to do so.
Whenever a fast resynchronization is being performed, the I/O transfer is in block
order, not in I/O order. Therefore the volume being resynchronized will not be
consistent until the fast resynchronization has been completed and the volume is
back in replicating mode. If a point-in-time copy is not available at the remote site,
then the remote site recovery plan should cover the exposure that will exist during
the time required to perform the fast resynchronization.
Additionally, a point-in-time snapshot at the secondary site can be used for
applications such as remote backup, remote data analysis, or other functions.
16
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
Sun SNDR Software and Sun StorEdge
Fast Write Cache Software
Note – You cannot use the Sun StorEdge Fast Write Cache (FWC) product, including
the SUNWnvm Version 3.0 software, in a Sun Cluster environment because cached
data is inaccessible from other machines in a cluster. To compensate, you can use a
caching array, such as the Sun StorEdge A3500 disk array.
The Version 2.0 and Version 3.0 Sun StorEdge data services are binary incompatible.
If your system includes Version 2.0 of the Sun StorEdge Instant Image (including
Instant Image 2.0.1 with STE 1.2), Sun SNDR software, or Sun StorEdge Fast Write
Cache, you must remove them before installation. For example, you cannot use Sun
StorEdge Instant Image software Version 2.0 with Sun SNDR software Version 3.0.
When you plan to install or upgrade to a Version 3.0 data service, you must remove
all Version 2.0 and 2.01 data services.
However, the Sun StorEdge Core Services Version 3.0 Software CD contains the Sun
StorEdge SUNWnvm Version 3.0 software package for those users whose systems
include Version 2.0 of the Sun FWC hardware and software product and who wish to
continue using the Sun FWC product.
For optimum performance in a noncluster environment, you can use the Sun SNDR
software with the Sun StorEdge Fast Write Cache SUNWnvm Version 3.0 product on
the primary and secondary sites for the volumes containing bitmaps and any other
volumes for which write performance is critical (for example, database log files).
Using the FWC product allows the scoreboard to be preserved across a system crash
without adversely affecting performance during replication.
Chapter 2
Configuration Considerations
17
One-to-Many and Multihop Volume Sets
The Sun SNDR software enables you to create one-to-many and multihop volume
sets.
In a one-to-many volume set, you can replicate data from one primary volume to
many secondary volumes residing on one or more hosts. One primary and each
secondary host volume is a single volume set (each secondary volume requires its
own unique secondary bitmap volume). When you perform a forward
resynchronization, you can synchronize one volume set or all volume sets; in this
case, issue a separate command for each set. You can also update the primary
volume by using a specific secondary volume. FIGURE 1 shows one primary and
three secondary host volumes and therefore three volume sets: A and B1, A and B2,
and A and B3.
Note – You can group one-to-many sets that share a common primary volume in a
single I/O group to operate on all sets simultaneously instead of issuing a separate
command for each set. See also “Order-Dependent Writes and Volume Set
Grouping” on page 13.
In a multihop set, the secondary host volume of one volume set can be the primary
host volume of another volume set. FIGURE 1 shows one primary and one secondary
host volume; the secondary host volume B becomes the primary host volume A1 to
the secondary host volume B1.
A
One-To-Many
Secondary host volume
/dev/rdsk/c0t0d0s5
B1
Primary host volume
/dev/rdsk/c0t0d0s5
Secondary host volume
/dev/rdsk/c0t0d0s7
B2
Secondary host volume
/dev/rdsk/c0t0d0s9
B3
Multihop
Primary host volume
/dev/rdsk/c0t0d0s5
A
One volume set
Secondary host volume
/dev/rdsk/c0t0d0s5
B
A1
Secondary host volume
/dev/rdsk/c0t0d0s5
B1
Another volume set
A = Primary host volume
B = Secondary host volume
FIGURE 1
18
One-to-Many and Multihop Volume Sets
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
Using The Sun SNDR Software in a
Firewall Environment
You can use the Sun SNDR software with any enhanced network product that Sun
offers. Port 121 must be available for use by the Sun SNDR sndrd daemon.
You can use IP masquerading. SNDR uses connection-oriented service (COTS) based
remote procedure calls (RPC; that is, TCP). The RPC part of the packet does not
contain any address information.
Chapter 2
Configuration Considerations
19
20
Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
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