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REP-4388: Add notes on performance problems. #18

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Jun 25, 2024
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REP-4388: Add notes on performance problems. #18

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add performance note
FGasper Jun 24, 2024
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FGasper Jun 24, 2024
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6 changes: 6 additions & 0 deletions README.md
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Expand Up @@ -229,6 +229,12 @@ Any collection metadata mismatches will occur in a task with the type '`verifyCo

In this case, '`failed_docs`' contains all the meta data mismatches, in this case an index named '`x_1`'.

# Performance

The migration-verifier optimizes for the case where a migration’s initial sync is completed **and** change events are relatively infrequent. If you start verification before initial sync finishes, or if the source cluster is too busy, the verification may freeze.

The migration-verifier is also rather resource-hungry. To mitigate this, try limiting its number of workers (i.e., `--numWorkers`), its partition size (`--partitionSizeMB`), and/or its process group’s resource limits (see the `ulimit` command in POSIX OSes).

# Known Issues

- The verifier may report missing documents on the destination that don’t actually appear to be missing (i.e., a nonexistent problem). This has been hard to reproduce. If missing documents are reported, it is good practice to check for false positives.
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