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Suggestions on speeding up initial sync?

I have a 5TB, 2 collection instance that I need to move from zlib to zstd. The initial sync is painfully slow right now on a very beefy secondary instance (m5.12xlarge, 192GB of RAM, 48vCPUs). The initial sync is scheduled to be completed in 8 days.

Primary instance is on an even beefier machine with no load. I tried diagnosing the slowdown and determined

  • disk is not saturated with iostat

  • cpu is not saturated with htop.

  • changing instance type to increase/decrease RAM

  • played around with maxIndexBuildMemoryUsageMegabytes with setParameter

  • played around with replWriterThreadCount with setParameter

It seems like the instance can do a lot more with disk + CPU not saturated on both the primary and the secondary. Is there anything else I can try?

Thanks!
Oliver

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