Technical Report Dell EqualLogic PS Series Array: Understanding Synchronous Replication (SyncRep) Abstract This Technical Report describes how EqualLogic Synchronous Replication works to protect your data and offers configuration recommendations when planning to use EqualLogic SyncRep. TR1085 V1.
Copyright © 2012 Dell Inc. All Rights Reserved. EqualLogic is a registered trademark of Dell Inc. Dell is a trademark of Dell Inc. All trademarks and registered trademarks mentioned herein are the property of their respective owners. Information in this document is subject to change without notice. Dell Inc. will not be held liable for technical or editorial errors or omissions contained herein. The information in this document is subject to change.
Preface PS Series arrays optimize resources by automating performance and network load balancing. Additionally, PS Series arrays offer all-inclusive array management software, host software, and free firmware updates. Audience The information in this guide is intended for administrators who wish to configure Synchronous Replication for real-time data protection.
Table of Contents Revision Information ............................................................................................................................. v Introduction ................................................................................................................................................1 How SyncRep Works .................................................................................................................... 2 Initial Synchronization .........................
Revision Information The following table describes the release history of this Technical Report. Report Date Document Revision 1.0 August 2012 Initial Release 1.1 April 2014 Updated with info on supported SyncRep configurations The following table shows the software and firmware used for the preparation of this Technical Report. Vendor Model Software Revision Dell™ PS Series Array Firmware Version 6.0.0 and above The following table lists the documents referred to in this Technical Report.
Introduction Synchronous Replication (SyncRep) is a new feature included in EqualLogic PS Series firmware v6.0. EqualLogic SyncRep is the simultaneous writing of volume data across two different storage pools in the same PS Series Group, resulting in two hardwareindependent copies of volume data. SyncRep will only acknowledge write completion when both the source and target volume has completed.
How SyncRep Works SyncRep is enabled on a per-volume basis. In volumes for which synchronous replication is not enabled, volume data and snapshots are located only in the pool to which the volume is assigned. In synchronous replication-enabled volumes, volume data exists simultaneous in two pools. Switched Gb Ethernet Pool 1 – SyncActive Vol 1 Pool 2 – SyncAlternate Vol 1 Group A SyncActive – The pool to which the iSCSI initiators (hosts) are connected to when reading and writing volume data.
Initial Synchronization The first time a volume is configured with SyncRep, a mirror volume is created in the SyncAlternate pool and the SyncActive volume will replicate all data to the SyncAlternate volume. This is a process that occurs in background and non-replicated changes will be tracked and sent to the SyncAlternate volumes. All new I/O will continue to both pools. 1. 2. 3. 4. Create SyncAlternate Volume. The SyncActive volume and SyncAlternate will receive all updates.
For SyncRep initial synchronization consider these factors: The initial volume size (SyncActive) o Large volumes when full of data will take time to completely synchronize. The network bandwidth between pools. o Bandwidth constraints may affect the time to complete initial synchronization of large volumes. For example, a 500 GB volume may take at least an hour over a dedicated 1 GigE network to initially synchronize. Other activity occurring on these pools during initial synchronization.
SyncRep States A volume that is configured for SyncRep can be in one of the following three states: In sync - Both the SyncActive and the SyncAlternate pools contain the same volume data. Switching roles is allowed. In sync process steps: 1. iSCSI initiator (host) sends a write to the group. 2. All writes are done to BOTH volumes; all reads are done from the SyncActive volume ONLY. 3. SyncActive and SyncAlternate volumes acknowledge to the group that the writes are complete. 4.
Paused – SyncRep can be paused through the Group GUI. While SyncRep is paused, the volume is still online, and initiators can connect to and write to the SyncActive volume. SyncRep can be paused and can later be resumed. For example, this could happen in a maintenance window during which the SyncAlternate pool is taken offline. If data is written to the volume while synchronous replication is paused, it is written only to the SyncActive pool, and the two pools are out of sync.
Note: If the SyncActive volume is out of sync, the option to switch the pools is replaced with the option to failover to the SyncAlternate pool. A Failover to the SyncAlternate option is available when a fault has already occurred. You can only failover to the SyncAlternate pool when the volume is already out of sync and the volume is taken offline. Note: When Synchronous Replication is paused by an Administrator, the volumes will go out of sync.
Synchronous Replication Configuration Requirements and considerations for using SyncRep PS Series Array firmware v6.0.0 or higher. Two pools, each containing at least one array member. Requires fast, reliable connectivity between pools. o The tolerance for latency that can be introduced to the network is determined by the application that is using the volume. o Latency of the link is the primary consideration and will be the gating factor for application write performance.
Network considerations The SAN network configured in an EqualLogic SyncRep solution follows the same network best practices as for setting up a PS Series group. Below is a short list of possible considerations which are documented and located at the Dell TechCenter or Dell EqualLogic support site: SyncRep requires a “flat” network infrastructure. Both pools should be in the same subnet (not routed) without compression, de-duplication or encryption devices; all which adds latency.
Configuring SyncRep on a Volume Configure SyncRep on an existing volume. 2 3 1 1 1. Select the Volumes button. 2. Select the volume for which you are configuring SyncRep (SyncRepVol1). 3. In the Activities panel, select Configure SyncRep to open SyncRep wizard.
4 5 4. Specify the SyncAlternate pool for the volume, (in this example there is only one pool to choose). 5. Click Next to go to the Summary step of the wizard.
6 7 6. Verify that the information contained in the summary is correct. 7. Click Finish to complete the wizard and enable synchronous replication.
Volume with SyncRep configured 1 2 3 4 1. SyncRep status (Status: in sync). 2. SyncRep icon. 3. SyncRep operations (Switch to SyncAlternate, Pause SyncRep, Disable SyncRep, and Disconnect SyncActive). 4. SyncRep pool information.
Configure Volume Collections with SyncRep Volume Collections allows multiple volumes to act as one object. A SyncRep collection: All volumes in the collection are paused at the same time. All volumes in the collection are switched over or failed over at the same time. Provides the ability to create crash consistent snapshots for multiple volumes at a single point in time. Note also: A volume must be in sync before it can be added to a SyncRep volume collection.
2 2. For each Volume in the Collection, Configure SyncRep. 3 3. Enable SyncRep for Collection will enable SyncRep on the collection of volumes.
Note: Collection Status changes to in sync only after all volumes are in sync.
Changing Pool Relationship Pool relationship change refers to the ability to change which pool is SyncActive and which is SyncAlternate. The reasons for a pool relationship change are SyncActive member failure, SyncActive pool failure or maintenance on SyncActive members. There are two ways pool relationships are changed: Switch to SyncAlternate and Failover to SyncAlternate. Switch to SyncAlternate This pool relationship change is done manually and is only available when the volume is in sync.
2 2. Click Yes 3 3.
Failover to SyncAlternate Failover is a pool relationship change that is required because of a fault occurring in the SyncActive pool. Failover to SyncAlternate is used to bring the SyncAlternate copy of data online and can only be run when the SyncRep status is out of sync. There are more steps to a manual failover than a manual switchover. There are three possible Failover situations which may occur: Out of sync when both pools are online, (one member down in a multimember pool for example).
The following outlines the Failover situation when the SyncActive pool is offline causing the volumes to become out of sync. 1 1 1. Click Failover to SyncAlternate, The Failover to SyncAlternate option will appear in the Activities list when the volume goes out of sync. 2 2.
3 3. Set the volume offline. 4 4.
5 5.
6 Note: If enabled, use the management port to log into a member of the pool containing the SyncActive volume. 6. Click Continue if the SyncActive pool has been verified offline or unreachable. If the SyncActive pool is still online, log directly into a member in the SyncActive pool and choose Disconnect SyncActive to properly failover to the SyncAlternate pool.
7 7. The Failover to SyncAlternate has completed. The original SyncAlternate has become the SyncActive. Note: After access to the original volume has been restored the system will resume synchronous replication, all changes will be tracked. When the volume returns to in sync you can switch back to the original pool configuration or choose to continue with the new configuration.
Snapshots Snapshots are valuable methods of data protection and are fully supported on SyncRep enabled volumes. SyncRep uses two types of snapshots: User snapshots – created by the user for data protection. System snapshots – created by the PS Series groups for state data of the volume. The following snapshot criteria are true of SyncRep enabled volumes: Scheduling or manual creation of a snapshot will locate that snapshot in the SyncActive pool only.
Snapshot pool residence during switch operations: 2 1 Original snapshots will remain in the default pool. 1. Originally, the SyncActive volume was in the default pool and snapshots were taken prior to switch. 2. Currently, the SSD-SAS pool is the SyncActive for this volume and any new snapshots will be contained here.
Switch back - New snapshots will be in the pool where the volume is SyncActive. 1 2 After switch back: 1. All snapshots created in the SSD-SAS pool will remain in the volumes SyncAlternate pool. 2. New snapshots for the SyncActive volume will now reside in the default pool along with the existing snapshots prior to the switch to the SSD-SAS pool. Note: The snapshots will remain with the pool where the volume was SyncActive at the time of snapshot or clone creation.
System snapshots– example if SyncActive volume is paused: 1 System snapshot process: 1.
View of Synchronous replication system snapshot. 2 2. Synchronous replication system snapshot created. 3 3. After volume is in sync the Synchronous replication system snapshot is deleted.
APPENDIX A: Technical Best Practices Several technical considerations should be evaluated to choose the appropriate volumes for SyncRep.
Business Purpose for SyncRep The design goal of EqualLogic SyncRep is to enable customers to elevate the availability of key volumes by protecting a pool within a PS Series Group. Below are guidelines to determining the appropriate business use cases for SyncRep. Example Use Cases: Datacenter protection: single member pools same rack or floor. Normal operations: Pools are in sync Pools are out of sync During failover to Pool1 changes are tracked to allow for re-sync when Pool2 is available.
Extended Datacenter protection: pools located in different racks or floors. For instance a group that spans two buildings or floors. Note: Network Switches, NICs and other components should follow EqualLogic Network Reference Architecture recommendations for the Group. See network section for more information. Multi Member Pools separated by floors within same Group. Note: All members within a pool should be in close proximity to each other. For instance Pool1 members are all on the First Floor.
Planned Maintenance. Switch to SyncAlternate for continued availability during scheduled outages. Steps for maintenance: 1. 2. 3. 4. Switch between SyncActive and SyncAlternate Pools. Connect to the New SyncActive Pool – (no interruption to host I/O). Perform the desired maintenance to the SyncAlternate (Pool1). Changes are tracked and will update the SyncAlternate once returned to in sync.
Distance Considerations Short distances are required to maintain an acceptable level of latency tolerance. Latency is dominated by two factors, the distance between the two pools that have SyncRep enabled and the algebra associated with the round trip time in TCP. Regardless of the data that needs to be transferred or the bandwidth that is available, it takes a finite amount of time for the data to travel from source to destination.
Volume I/O and application profile Considerations SyncRep will have the best performance with applications that have random read and write profiles. SAN HeadQuarters should be installed and used to monitor and analyze the workloads of the potential candidates for SyncRep. Some large sequential write profiles may degrade performance by 50% or greater on SyncRep volumes. Therefore, choosing the appropriate volumes for SyncRep is important.
Snapshot space Considerations SyncRep allocates the same percentage of snapshot reserve in both the SyncActive and the SyncAlternate pools. For example, if you are creating a 20 GB volume and setting the snapshot reserve to 100%, you need to have a total of 40GB in each pool.
APPENDIX B: SyncRep and Dell EqualLogic Host Integration Tools Host Integration tools and applications such as SAN HeadQuarters, Auto-Snapshot Manager and VMware Integration tools are available to all Dell PS Series customers from the Dell EqualLogic support web site. Auto-Snapshot Manager/Microsoft Edition (ASM/ME) ASM/ME 4.5 provides application-consistent Smart Copies of SQL, SharePoint and Exchange databases. ASM/ME 4.5 will create Smart Copies on the SyncActive pool.
SyncRep properties on the volume in ASM/ME 1 2 1. Synchronous Replication State 2. Features indicate the Volume is enabled for SyncRep.
Available ASM/ME Smart Copy operations on the SyncAlternate Note: Smart Copies that reside in the SyncAlternate pool will only provide Delete or View Backup Document operations.
ASM/ME Smart Copies in the SyncActive Pool: Note: The Current SyncActive Pool will provide all management capabilities.
VMware integration with SyncRep Dell EqualLogic Multi-pathing Extension Module (MEM) Support: Customers utilizing the Dell EqualLogic MEM module will need to update to version 1.1.1 for SyncRep support. VAAI/UNMAP – Dell recommends that UnMap operations are not performed on SyncRep volumes. Please see the PS Series Storage Release Notes Version 6.0 for more information. When the primary SyncRep volume goes offline, the naa value of the volume will change but the VM registration will still be valid.
APPENDIX C: SAN HeadQuarters Management with SyncRep SAN HeadQuarters is an advanced performance and health monitoring tool for PS Series Arrays. SAN HeadQuarters v2.5 and greater provides views into SyncRep activity. SAN HeadQuarters v2.5 includes: Progress of initial or subsequent change synchronizations. Synchronization status over time. The ability to identify volumes participating in SyncRep. Details of SyncRep volumes at the Group, Pool or Volume level.
The SAN HeadQuarters view below can show at the object level the activity over time for SyncRep. If the volume is undergoing replication during any point in the time selected “remaining changes” will be indicated. Also note that change tracking is provided using this view. 1 2 3 1. Remaining Changes for the interval – y axis will show amount of GB/MB of changes while the x axis will indicate the time. 2. Summary of SyncRep statistics for the selected time. 3.
Technical Support and Customer Service Dell support service is available to answer your questions about PS Series SAN arrays. Contacting Dell 1. If you have an Express Service Code, have it ready. The code helps the Dell automated support telephone system direct your call more efficiently. 2. If you are a customer in the United States or Canada in need of technical support, call 1-800-945-3355. If not, go to Step 3. 3. Visit support.equallogic.com. 4.
