Double-Take, GeoCluster, and GeoCluster+
NSI Software offers three high-availability products. Double-Take supports data replication and failover from one node to another that has the same application installed. GeoCluster enhances Microsoft Cluster service with support for replicated-data configurations, letting customers create Microsoft Cluster service stretch clusters that extend over geographically dispersed sites. GeoCluster+ enables remote availability of the data outside of the initial cluster on any other storage resources, including to non-Microsoft Cluster service nodes.
Double-Take combines a replication engine with automatic or manual application failover support. Double-Take supports one-to-one active-active and active-standby configurations, as well as many-to-one configurations in which one server is configured to stand in for applications on several primary servers.
A Double-Take replication set, described in more detail below, defines a set of data to be replicated. A Double-Take connection links a replication set to a target machine and defines the location to receive replicated data. A connection can specify the NIC on the target server to receive the data and cause the initial mirroring of data to begin when the network connection is first established. Alternatively, you can schedule mirroring or start it manually.
At the target machine, Double-Take initiates monitor requests (Internet Control Message Protocol—ICMP—pings) to the monitored IP addresses on the source machine and declares failure when a user-specified timeout period passes with no response. You can configure failover to occur automatically or initiate it manually. When failover occurs, Double-Take executes an optional, administrator-supplied prefailover script on the stand-in machine. Double-Take then adds the name and IP address of the source machine to the identity of the stand-in machine, creates any file shares that are within the scope of the replication set, and updates the Address Resolution Protocol (ARP) cache of local routers to ensure that client requests are sent to the new machine. An optional administrator-defined post-failover script completes the failover process.
You manually initiate failback on the now-active target machine after removing the original source machine from the network. The failback process removes the name, IP address, and shares added to the identity of the target at failover and optionally runs prefailback and post-failback scripts. You then place the repaired source machine back on the network and remirror replication set data before letting users connect.
The Double-Take Management Console, which Figure 7 shows, is a Win32 GUI that you can use to manage and monitor servers running Double-Take in attached networks. Within the console, the Connection Wizard lets you quickly configure basic one-to-one mirroring, replication, and failover. The Connection Manager component lets you further configure replication tasks. You use another Win32 GUI, the Failover Control Center, to configure and monitor failover activity. You can also choose to manage or script Double-Take operations by using the fully functional DTCL command-line utility, or you can use the interactive DTTEXT command-line interface. Double-Take also includes SNMP support, so it can relay statistics, data, and SNMP traps to an SNMP-based management console.
Double-Take implements asynchronous file replication by intercepting and queuing file-system writes in a manner that preserves the order of I/O operations. Supporting NTFS, FAT, and FAT32 file systems, Double-Take replicates changes to file attributes and permissions in addition to data. You define data to be replicated by creating a replication set, which specifies some combination of volumes, directories, and files. The directory structure of the target volume need not be the same as the source volume—you can specify a directory on the target server to which Double-Take will replicate the data. Thus, you can replicate from several source servers to one target server, if desired. You can also create several replication sets for one source server and send the sets to different locations at different times.
Double-Take also supports chained replication, in which Server A replicates to Server B, and Server B replicates to Server C. You can use this configuration to create both local and offsite copies of data while limiting replication traffic from Server A to one target. Double-Take also supports single-server replication, which you can use, for example, to create a backup copy of application data on another SAN or SCSI volume.
Double-Take lets you transmit queued data as file changes occur, according to a preset schedule or manually. You can specify bandwidth limitations to restrict the amount of network bandwidth used for Double-Take data transmissions. You can also schedule a periodic replication verification that compares the source and target copies of the replication set and reports on differences.
Double-Take, GeoCluster, and GeoCluster+ pricing is based on the OS edition on which the software is installed. At the lower end, Double-Take, server edition, is $2495; GeoCluster, advanced edition, is $4495; and GeoCluster+, advanced edition, is $7495. At the high end, the datacenter edition of Double-Take and GeoCluster is $39,995, and the datacenter edition of GeoCluster+ is $59,995.
Summing Up
Table 1 lists these high-availability and fault-tolerant products and the technologies they use. Although the products have features in common, each is unique.
BrightStor provides replication-based failover. The product incorporates file-system replication, rapid failback replication, and process-monitoring technology from CA's Unicenter enterprise-management solution.
Co-StandbyServer AAdvanced supports two-node shared-storage and replicated-data configurations and includes an application availability reporting feature. AAM supports 100 nodes and a system of rules and triggers that allows automated responses to a variety of situations.
Endurance offers the only true fault-tolerant solution described here. It turns two identically configured servers into one fault-tolerant entity. Endurance supports almost any off-the-shelf Windows application.
Double-Take has a file-oriented replication engine that can send less data across the replication network than a comparable block-oriented replication engine. Replication options include many-to-one, one-to-many, chained, and same-server configurations. GeoCluster enhances Microsoft Cluster service by adding support for replicated-data configurations to Microsoft Cluster services' native support for only shared-data configurations. GeoCluster+ enables remote availability of data on any storage resource, including non-Microsoft Cluster service nodes.
Matrix Server is a shared-storage cluster server that supports up to 16 nodes. The PolyServe SAN File System distinguishes this product from others by giving multiple cluster nodes concurrent read/write access to one copy of shared data.
LifeKeeper supports both shared-disk and replicated-data cluster configurations. With its wizard-based configuration procedures and the automated application setup that its application-specific recovery kits provide, SteelEye touts ease of use and broad application support as key features.
Storage Foundation HA 4.1 for Windows supports a broad range of cluster configurations from basic replicated-data and shared-storage configurations to systems that include high-end storage management products. Flexible security options support granular delegation of administrative authority.
This comparison of high-availability solutions and visits to the vendor Web sites should help you determine which solution best suits your needs.
End of Article
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