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Lecture 30 Multiple Replicas. Remote Replication. DR in practice.
Chapter Objectives After completing this chapter, you will be able to: • Explain remote replication technologies • Synchronous and asynchronous • Discuss host and array based remote replication • Functionality • Differences • Selecting the appropriate technology • Discuss network options for remote replication Remote Replication
What is Remote Replication? • Replica is created at remote site • Addresses risk associated with regionally driven outages • Could be a few miles away or half way around the globe • Modes of remote replication (based on RPO requirement) • Synchronous Replication • Asynchronous Replication Remote site Source site Remote Replication
1 4 2 3 Synchronous Replication • A write must be committed to the source and remote replica before it is acknowledged to the host • Ensures source and remote replica have identical data at all times • Write ordering is maintained • Replica receives writes in exactly the same order as the source • Synchronous replication provides the lowest RPO and RTO • Goal is zero RPO • RTO is as small as the time it takes to start application on the target site Source Host Data Write Data Acknowledgement Target Remote Replication
Required bandwidth Typical workload Synchronous Replication: Bandwidth Requirement • Response Time Extension • Application response time will be extended • Data must be transmitted to target site before write can be acknowledged • Time to transmit will depend on distance and bandwidth • Bandwidth • To minimize impact on response time, sufficient bandwidth must be provided at all times • Rarely deployed beyond 200 km Max Writes MB/s Time Remote Replication
1 2 3 4 Asynchronous Replication • Write is committed to the source and immediately acknowledged to the host • Data is buffered at the source and transmitted to the remote site later • Some vendors maintain write ordering • Other vendors do not maintain write ordering, but ensure that the replica will always be a consistent re-startable image • Finite RPO • Replica will be behind the source by a finite amount • Typically configurable Source Host Data Write Data Acknowledgement Target Remote Replication
Typical workload Asynchronous Replication: Bandwidth Requirement • Response time unaffected • Bandwidth • Need average bandwidth • Buffers • Need sufficient buffers • Can be deployed over long distances Required bandwidth Writes MB/s Average Time Remote Replication
Remote Replication Technologies • Host based • Logical Volume Manager (LVM) based • Support both synchronous and asynchronous mode • Log Shipping • Storage Array based • Support both synchronous and asynchronous mode • Disk Buffered - Consistent PITs • Combination of Local and Remote Replication Remote Replication
LVM Based • Duplicate Volume Groups at source and target sites • All writes to the source Volume Group are replicated to the target Volume Group by the LVM • Can be synchronous or asynchronous mode • In the event of a network failure • Writes are queued in the log file and sent to target when the issue is resolved • Size of the log file determines length of outage that can be withstood • Upon failure at source site, production can be transferred to target site IP Remote Replication
LVM Based – Advantages and Limitations • Advantages • Different storage arrays and RAID protection can be used at the source and target sites • Response time issue can be eliminated with asynchronous mode, with extended RPO • Limitations • Extended network outages require large log files • CPU overhead on host Remote Replication
Host Based Log Shipping Logs • Offered by most database Vendors • Advantages • Minimal CPU overhead • Low bandwidth requirement • Standby Database consistent to last applied log IP Original Logs Stand By Remote Replication
Storage Array Based Remote Replication • Replication performed by the array operating environment • Host CPU resources can be devoted to production operations instead of replication operations • Arrays communicate with each other via dedicated channels • ESCON, Fibre Channel or Gigabit Ethernet • Replicas are on different arrays • Primarily used for DR purposes • Can also be used for other business operations Source Array Target Array IP/FC Network Distance Source Replica Production Server DR Server Remote Replication
Write is received by the source array from host/server Write is transmitted by source array to the target array Target array sends acknowledgement to the source array Source array signals write complete to host/server Array Based – Synchronous Replication Network links Source Target Remote Replication
Write is received by the source array from host/server Source array signals write complete to host/server Target array sends acknowledgement to the source array Write is transmitted by source array to the target array Array Based – Asynchronous Replication • No impact on response time • Extended distances between arrays • Lower bandwidth as compared to Synchronous Network links Source Target Remote Replication
Asynchronous Replication: Ensuring Consistency • Maintain write ordering • Some vendors attach a time stamp and sequence number with each write, then send the writes to remote array • Apply these writes to the remote devices in exact order based on the time stamp and sequence numbers • Dependent write consistency • Some vendors buffer the writes in the cache of the source array for a period of time (between 5 and 30 seconds) • At the end of this time current buffer is closed in a consistent manner and the buffer is switched, new writes are received in the new buffer • Closed buffer is then transmitted to the remote array • Remote replica will contain a consistent, re-startable image on the application Remote Replication
Array Based – Disk Buffered Replication • Local and Remote replication technologies can be combined to create consistent PIT copies of data on target arrays • RPO usually in the order of hours • Lower Bandwidth requirements • Extended distance solution Source Data Local Replica Source Remote Replica Local Replica Host Remote Replication Source Storage Array Target Storage Array
Remote Replicas – Tracking Changes • Remote replicas can be used for BC operations • Typically remote replication operations will be suspended when the remote replicas are used for BC operations • During business operations changes will/could happen to both the source and remote replicas • Most remote replication technologies have the ability to track changes made to the source and remote replicas to allow for incremental re-synchronization • Resuming remote replication operations will require re-synchronization between the source and replica Remote Replication
Array Based – Which Technology? • Synchronous • Is a must if zero RPO is required • Need sufficient bandwidth at all times • Rarely above 125 miles • Asynchronous • Extended distance solutions with minimal RPO (order of minutes) • No Response time elongation • Generally requires lower Bandwidth than synchronous • Must design with adequate cache/buffer capacity • Disk buffered • Extended distance solution with RPO in the order of hours • Require lower bandwidth than synchronous or asynchronous Remote Replication
Three Site Replication • Eliminates disadvantages of two site replication • Single site disaster leads to a window when there is no DR protection • Data replicated to two remote sites • Implemented in two ways • Three Site Cascade/Multi-hop • Three Site Triangle/Multi-target Remote Replication
Three Site Replication – Cascade/Multi-hop • Synchronous + Disk Buffered • Synchronous + Asynchronous Source Data Remote Replica Local Replica Synchronous Disk Buffered Remote Replica Local Replica Bunker Site Source Site Remote Site Source Data Remote Replica Local Replica Synchronous Asynchronous Remote Replica Local Replica Bunker Site Source Site Remote Site Remote Replication
Asynch with Differential Resynch Three Site Replication – Triangle/Multi-target BUNKER Sync SAN SOURCE SAN Async SAN REMOTE Remote Replication
Replicate from one storage array to any other storage array over the SAN/WAN Implement tiered storage Data migration Remote vaulting Heterogeneous arrays support No impact to servers or the LAN SAN Based Remote Replication EMC Symmetrix EMC CLARiiON SAN/WAN Hitachi HP IBM Remote Replication
SAN Based Replication: Terminologies • Control Array: Array responsible for the replication operations • Control Device: Device on controlling array to/from which data is being replicated • Remote Array: Array to/from which data is being replicated • Remote Device: Device on remote array to/from which data is being replicated • Operation • Push: Data is pushed from control array to remote array • Pull: Data is pulled to the control array from remote array Remote Array Control Array SAN/WAN C C PUSH Remote Device Control Device PULL Remote Replication
Network Options for Remote Replication • A dedicated or a shared network must be in place for remote replication • Use ESCON or FC for shorter distance • For extended distances, an optical or IP network must be used • Example of optical network: DWDM and SONET • Protocol converters may require to connect ESCON or FC adapters from the arrays to these networks • Native GigE adapters allows array to be connected directly to IP Networks Remote Replication
Optical Channels ESCON Fibre Channel Gigabit Ethernet Optical Lambda λ Electrical Optical Dense Wavelength Division Multiplexing (DWDM) • DWDM is a technology that puts data from different sources together on an optical fiber with each signal carried on its own separate light wavelength • Up to 32 protected and 64 unprotected separate wavelengths of data can be multiplexed into a light stream transmitted on a single optical fiber Remote Replication
Synchronous Optical Network (SONET) • SONET is Time Division Multiplexing (TDM) technology • Traffic from multiple subscribers is multiplexed together and sent out onto SONET ring as an optical signal • Synchronous Digital Hierarchy (SDH) similar to SONET but is the European standard • SONET/SDH, offers the ability to service multiple locations, its reliability/availability, automatic protection switching, and restoration OC48 OC3 SONET STM-16 STM-1 SDH Remote Replication
Chapter Summary Key points covered in this chapter: • Modes of remote replication • Synchronous and asynchronous mode • Host based remote replication • LVM based and log shipping • Array based remote replication • Synchronous, asynchronous and disk buffered • Three site replication • SAN based remote replication • Network options for remote replication Remote Replication
Concept in Practice – EMC Remote Replication • EMC Symmetrix Arrays • EMC SRDF/Synchronous • EMC SRDF/Asynchronous • EMC SRDF/Automated Replication • EMC CLARiiON Arrays • EMC MirrorView/Synchronous • EMC MirrorView/Asynchronous • EMC SAN Copy • SAN based remote replication solution for EMC CLARiiON Remote Replication
Check Your Knowledge • What is the difference between Synchronous and Asynchronous mode? • Discuss one host based remote replication technology? • Discuss one array based remote replication technology? • What are differences in the bandwidth requirements between the array remote replication technologies discussed in this chapter? • Discuss the effects of a bunker failure in a three-site replication for the following implementation: • Multihop—synchronous + disk buffered • Multihop—synchronous + asynchronous • Multitarget Remote Replication