WHITE PAPER. Header Title. Side Bar Copy. Real-Time Replication Is Better Than Periodic Replication WHITEPAPER. A Technical Overview

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1 Side Bar Copy Header Title Why Header Real-Time Title Replication Is Better Than Periodic Replication A Technical Overview WHITEPAPER

2 Table of Contents Introduction...1 Today s IT Landscape...2 What Replication Is: A Narrow Definition...2 What Replication Can Do...2 Replication Methods...3 Periodic Replication Not Well Suited to Multi-Tiered Computing Environment...3 Replicating in Real-Time with Double-Take...4 Summary...7 About Vision Solutions...7 1

3 Introduction Replication is a foundational construct in computing. In today s complex computing environments with geographically dispersed heterogeneous platforms, replication regardless of its form is an essential bridge. It can help you lower costs, increase efficiency and limit your exposure to risk. Today s IT Landscape Complicated. This word best describes the state of information technology these days. Pervasive, around-the-clock delivery of services to end users is heavily dependent on interdependencies between technologies that many IT organizations have only recently embraced. Today, most businesses have more than one location and one computer. Data is bigger and must be shared across several physical, virtual and Cloud servers that are often geographically dispersed. Today, you need to be able to transition to new platforms quickly, and deliver accurate and timely information for consumption by computer systems operated by your supply chain partners and clients. What Replication Is: A Narrow Definition In the context of computing, the term replication describes the process of sharing information between multiple computing or storage devices. In either case, the replication process must have built-in capabilities to ensure parity between writes on the publisher and any number of subscribers. Replication is particularly useful in distributed computing environments. It is a proven way to synchronize multiple decentralized servers. Replicating data across multiple servers maintains data in a synchronized state. In the interest of bandwidth optimization, replication engines designed for efficiency only copy data that has been changed since the data was previously replicated. What Replication Can Do Replication can be used to address several different types of information management challenges, and thus, improve efficiency and profitability. The ways in which replication can benefit your business are as follows: Migrations By using replication to move your entire computing environment to new systems you can cut a large block of time out of the migration process and lessen the expense through a reduction in sheer manpower. You also can virtually eliminate downtime from the migration process. Availability and Recovery When used to maintain an exact duplicate of your production system on a backup machine, replication can dramatically reduce the likelihood of a prolonged outage caused by a major system or environmental disaster. Availability and Recovery are the areas where replication technology is most commonly deployed. 2

4 Replication Methods Periodic Replication Periodic replication is a technological variant of replication that involves a source system, often referred to as a publisher batching up several transactions over the course of a defined interval and sharing that increment with other target, or subscriber at the end of the collection process. Periodic replication is relatively simplistic in its approach. Computing resources are consumed in the collection of data for each increment, and a considerable amount of network bandwidth is consumed when data is transmitted. Periodic replication is not suitable in environments were data is frequently changed because transactions committed on the publisher server but not yet transmitted to other subscriber servers can be lost if the publisher or connection fails. Therefore, periodic replication is suitable in environments where data rarely changes; when the publisher and subscriber do not need to remain synchronized, or where networks cannot tolerate large amounts of data shipped over short intervals. Real-Time Replication Real-time replication is a term that describes the process of copying data and system setting changes on the publisher to subscribers as soon as they occur. The term real-time is often used to describe two distinctly different technological approaches. The first is when writes occur on several platforms simultaneously. This is synchronous replication, where data is first replicated to the target, and once it is committed, it is also written to the production system. Synchronous replication may slow down your system if you don t have a large amount of low-latency bandwidth. The second is a scenario where a write occurs on the publisher and then immediately after on the subscribers but without holding up the publisher write operation. This is referred to as real-time asynchronous replication. Asynchronous replication also keeps both systems in sync with much less impact on performance. The difference is that asynchronous replication is tolerant of lower bandwidth connections and therefore can provide real-time replication over low bandwidth connections over any distance. Asynchronous replication commits data to the target system as quickly as possible over available bandwidth. Periodic Replication Not Well Suited to Multi-Tiered Server Environments While periodic replication has a place in the datacenter, it is better suited for lower-tier server environments versus multi-tiered ones. There are three reasons for this: First, periodic replication does not keep data continuously in sync which allows for stale data and data loss on the subscriber between replication intervals. Periodic replication is not a sufficient solution to protect systems that cannot tolerate data loss. Second, periodic replication has to be aware of the application it is protecting. Therefore, if the replication vendor has not written an interface for the application you need to protect, it may not be a suitable solution. 3

5 Third, periodic replication sends bursts of data. With periodic replication, you generally have data being sent in short periods of time which results in spiking of network traffic, a condition that you would not see with real-time replication. Spiking may impact the performance of other applications that are sharing the same network. Spiking in resource utilization also impacts disk I/O and other server resources such as CPU and memory. Replicating in Real-Time with Double-Take The Double-Take real-time replication engine is designed from the ground up to perform the necessary activities to quickly, safely, and efficiently identify I/O operations; determine what must be sent; transmit that information; and finally ensure it is written correctly on the other side. Double-Take s asynchronous replication architecture uses minimal processor and bandwidth so it can be used even in constrained network environments. A technical discussion of the Double-Take real-time replication engine follows. For the Double-Take replication engine to come to life in Microsoft Windows and Linux environments, it must first install a filter driver. This filter allows the Double-Take real-time replication engine to see all I/O operations that are destined for anything that Double-Take is monitoring. This includes any file systems supported by the Double-Take real-time replication engine for Linux and anything that is passed through NTFS on Windows even open, locked or binary files. The filters also allow the Double-Take real-time replication engine to see true byte-level operations, as opposed to block-level operations that occur further down the stack from the file system itself. Logically, if you only change 8k (a standard page size for Microsoft SQL Server ), the Double-Take real-time replication engine can identify the 8k of changes and not have to work with the 32k or 64k of a disk block. This dramatically minimizes the amount of data sent across the line. Read Operations To optimize performance, read operations are ignored as they pass through the filter. Write-and-change operations are copied as they pass through, so the Double-Take real-time replication engine provides real-time replication of I/O operations in the most efficient way possible. Or put a finer point on it, the Double-Take real-time replication engine will only duplicate write and change operations for those areas of the file systems that you define (through one of the workflows in the Console). This can be anything from single files up to an entire volume or set of volumes. Once again, the Double-Take real-time replication engine is minimizing what data has to be managed for transmission, both to speed up the overall process and to preserve bandwidth for other processes on the network. 4

6 How Data Is Transmitted Once Double-Take has an I/O copy that has to be replicated, it adds that copy to the send buffer. All of the buffers and queues used by the Double-Take real-time replication engine exist dynamically in RAM or on disk and you have control over how much of each of those resources the Double-Take real-time replication engine is allowed to use. If the buffer is empty, then it immediately places a copy of the I/O operation into an acknowledgment queue and transmits the original to the target device over TCP/IP. The next operation in the buffer is sequentially addressed, and so on. Since all data flows through the send buffer first, if a bottleneck occurs the Double-Take real-time replication engine simply buffers the changes in the correct write order and transmits them as quickly as bandwidth will allow. If you have chosen to enable throttling (either at a fixedrate or scheduled interval), then the Double-Take real-time replication engine will abide by those limits even if there is bandwidth available above that limit. Target Side Operations Once the I/O is received by the target system, the Double-Take real-time replication engine performs a series of integrity checks to make sure: The entire I/O operation was received without errors. The I/O operation is in the correct file-system write order (based on the source that sent it). After the validation check, the target system sends an acknowledgement back to the source device to confirm receipt, and places the I/O into the receive buffer for commit to disk. Without a bottleneck, the I/O will immediately leave that buffer and be written into the target s own file system. If there are disk bottlenecks, the buffer provides space to queue I/O operations and commits them as quickly as possible. In any case, the I/O operations are written in the same write order and to the same files to which the source wrote on its own file system. Transactional integrity is therefore maintained, even though the I/O in question is being written on a different device. Double-Take can re-direct I/O to a different directory/folder when necessary; but will always write the same I/O to the same files in the same write order without exception. Back on the source, the acknowledgment is received and Double-Take removes the copy of the now accepted I/O from the acknowledgment queue, freeing up space. This process happens continuously, so recognized I/O flows across the link as Double-Take continues to check new I/O and write transmitted I/O. The buffer systems handle any slowdowns and bottlenecks, and the acknowledgement queue provides a re-transmission mechanism to handle in-line errors caused by TCP anomalies and other networking issues. 5

7 Replication Services When starting out with a new job, Double-Take needs to verify the existence of a good copy of the existing data on the target. This process is accomplished through a synchronization event. Double-Take will either send all data to the target, or it will do an NTFS (or Linux FS) file-level comparison of the existing data between source and target. Any file segments that are CRC identical will be skipped, and those with differences will be copied from source to target using the same basic mechanics used for replication. This means you can pre-seed a target by placing a flat-file copy of the data on the volumes in question ahead of time. Replication begins and continues throughout the mirror process. Double-Take replicates new data as it compares the existing data both to shorten mirror times and to ensure there is not a large backlog of new data waiting to be transmitted while we check the existing information. If there is a major disruption between the two servers (short of a failover event), the Double-Take replication engine will automatically re-mirror using this same methodology. With this feature, non-failover emergencies (such as an unexpected reboot) can be handled without administrative intervention. 6

8 Summary With replication you can solve many computing challenges. Described as the process of sharing information between multiple computing or storage devices, replication is useful in today s complex, multi-platform, multi-tiered environments. Replication can be used for migrations and availability and recovery. Common replication methodologies fall into two classes, periodic and real-time. Periodic replication needs to be tightly coupled with the application it is monitoring and periodically bursts potentially large volumes of data across the communications line. Real-time replication continuously transmits data to other servers as it becomes available. It is generally application agnostic so it works with any application or database management system and it is bandwidth friendly making it a good choice for environments with bandwidth considerations. The Double-Take real-time replication engine is designed to quickly, safely, and efficiently identify I/O operations; determine what must be sent; transmit that information; and ensure it is written correctly on the other side. Double-Take s asynchronous replication architecture uses minimal processor and bandwidth so it can be used in situations where resources are limited. About Vision Solutions With more than 25,000 customers globally, Vision Solutions is one of the industry s largest providers of Business Continuity and Information Availability Solutions for Windows, IBM i (i5/os), AIX, Linux and Cloud environments. Vision s MIMIX, Double-Take and itera brands keep business-critical information continuously protected and available. With an emphasis on affordability and ease-of-use, Vision products and services help customers achieve their IT protection and recovery goals, which in-turn improves profitability, productivity, regulation compliance, customer satisfaction and quality of life. Vision Solutions oversees a global partner network that includes IBM, HP, Microsoft, VMware, Dell and hundreds of resellers and system integrators. Privately held by Thoma Bravo, Inc., Vision Solutions is headquartered in Irvine, California with development, support and sales offices worldwide. For more information call: (toll-free U.S. and Canada) or visit. Also find us on: Facebook: Twitter: YouTube: Vision Solutions Blog: Vision Solutions Inc, Barranca Parkway Irvine, CA Copyright 2012, Vision Solutions, Inc. All rights reserved. Windows is a trademark of Microsoft Corporation. Linux is a trademark of Linus Torvalds. WP_W_RealTimeRep_isBetter_E_