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Sagar Bhargava avatar image
Sagar Bhargava asked

Availability Group latency

Hi, I have a database which is a part of SQL 2012 availability group on a 3 node failover cluster. All 3 nodes are set for 'Synchronous Commit'. Node1 and Node2 are setup for Automatic failover and Node3 is set for Manual. Node2 and Node3 property 'Readable Secondary' is set to 'Read-intent only' Now what I see is that the "Redo Rate(KB/sec)" is about 17MB/sec for the Node2 and 5MB/sec for node3. Can anyone help explain why the difference? The log send rate is however similar on both nodes for the database. Regards,
availability groupslatency
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Is there anything different from the nodes in terms of hardware?
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@JohnM - These nodes are VM with same configuration. Only difference is that Node2 has IDE attached and Node3 has SSD's. Further checking, I found that the no. of VLF's was close to 19k for the database. I reduced it to less than 9k and looking to reduce it further. However, it has not helped in getting that Redo Rate up for Node3. There are 7 other small databases in this DAG and the Redo Rate is somewhat similar across Node2 and Node3 for them.
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sp_lock answered
If you dont have a benchmark to refer to, I would start by check the disc performance. If you are able to pause the sync on node 3 and the run [CrystalDiskMark][1] to check the through put on each of the volume. Start with 50MB chunks on the data and log drives. We have SSD's in a RAID setup and typlically see around 1000MB/s reads and 8000MB/s writes. For reference our redo rate is currently at 146,186 KB/sec and log send is 5,000KB [1]: https://crystalmark.info/software/CrystalDiskMark/index-e.html
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