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/MT enables multithreaded copying in Robocopy. Use /MT:n to request a specific number of copy threads, from 1 through 128; if you omit :n, Robocopy uses 8. More threads can improve throughput when bandwidth is the bottleneck, but a higher value is not automatically faster. Start with 8, test a few values on a representative workload, and keep the count that improves elapsed time without exhausting CPU, storage, network capacity, or capacity needed by other work.
The right setting also depends on whether you are performing an initial bulk transfer or a later differential or mirror pass. Microsoft reports balanced initial-copy performance at 8–20 threads in its Azure Files tests, with good performance up to 64 when processors can keep the threads active; those results are specific to Azure Files migration conditions, not a universal optimum.
What the Robocopy /MT option does
Robocopy normally processes copy operations with a single stream of work. Adding /MT asks it to perform copy work concurrently. The optional number sets the requested thread count:
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Threads control copy concurrency; they do not decide which files Robocopy selects, whether subdirectories are included, or which file attributes are copied. Those behaviors come from other switches. For example, /E includes subdirectories, including empty ones, independently of the thread count. Microsoft documents the syntax and limits in its Robocopy command reference.
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What a larger value can and cannot do
Several active copy operations can keep a fast network connection or storage system busier than one operation. That can reduce the time spent waiting for individual reads and writes. However, every additional thread also creates more concurrent work for the source, destination, network path, and processor. If any of those resources is already saturated, additional threads can provide no gain or can make the run less efficient.
Consequently, /MT:128 is not a promise of twelve times the performance of /MT:8, and it may be slower than the default on a constrained server. Microsoft’s Azure Files guidance explicitly cautions that a high thread count can saturate available bandwidth without making every migration faster.
Syntax, limits, and a safe starting point
| Command form | Meaning | Documented thread behavior |
|---|---|---|
/MT |
Enable multithreaded copying without specifying a count | Uses 8 threads |
/MT:1 |
Request one copy thread | Valid lower limit |
/MT:8 |
Request eight copy threads | Documented default when the number is omitted |
/MT:16 |
Request sixteen copy threads | Shown in Microsoft’s recursive-copy example |
/MT:128 |
Request 128 copy threads | Valid upper limit |
For an ordinary first test, use the documented default of 8. If measurements show that the network is underused and the computer and storage have spare capacity, compare it with a modestly higher value. Values such as 16 or 32 are useful test points, not universal recommendations. Keep the rest of the Robocopy command and the workload unchanged while comparing them.
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robocopy C:Source D:Destination /E /MT:16 /LOG:C:Logscopy.logThis example copies the source tree, includes all subdirectories and empty directories through /E, requests 16 threads, and writes output to the specified log. Replace the paths with locations that exist in your environment. The logging path must also be writable. The thread choice and the directory-selection behavior are separate settings.
How to choose a thread count
Use the following process rather than choosing a number from a generic “best settings” list.
1. Identify the kind of Robocopy run
An initial copy may need to move a large amount of data and many files. In that situation, enough concurrency can help keep available network bandwidth and storage queues active. A later differential or mirror pass may contain only a small amount of changed data. Its elapsed time can be dominated by scanning the namespace, comparing metadata, and processing many file entries rather than transferring bytes. The same count therefore need not be optimal for both runs.
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Record which phase you are tuning: initial seed, recurring differential backup, or a mirror synchronization. A count selected for a one-time bulk transfer should not automatically be imposed on a recurring production job.
2. Find the resource that limits the current run
Consider all of the resources that participate in the copy:
- Network bandwidth: If the link is not close to its practical limit while the copy is active, more concurrency may allow more outstanding work. If the link is already saturated, extra threads cannot create additional network capacity.
- Processor capacity: Thread management, directory enumeration, metadata work, and file-system activity consume CPU. A high count is useful only when processors can keep the requested threads active. Leave capacity for the operating system and other services.
- Storage I/O: Source and destination disks, arrays, and file servers may have their own throughput or queue limits. More simultaneous requests can increase contention rather than improve completion time.
- File mix: A tree containing a few large files behaves differently from one containing millions of small files. Small-file workloads can spend a greater share of time opening, checking, and enumerating entries.
- Competing work: Backups, databases, virtual machines, antivirus scans, and other users may share the same processor, network, or storage. A setting that wins an isolated test can harm the production workload.
These factors interact. For example, a server can have an apparently fast network but still be limited by source-disk latency or CPU time spent walking a very large namespace.
3. Start conservatively
Run the representative job with /MT:8, or with plain /MT so the default is explicit in the command’s behavior. If the machine remains underused and the link is not saturated, test a higher value such as /MT:16. Test a still higher value only when the previous increase produced a measurable improvement without unacceptable resource contention.
Do not jump directly to 128 because it is the maximum. The documented range describes what Robocopy accepts, not what every workload benefits from.
4. Measure the real workload
Use a representative sample or a full run with the same source, destination, file-size distribution, permissions, and competing activity that matter in production. Keep the command switches constant except for /MT:n. Compare at least:
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- Elapsed time to complete the same work.
- Effective throughput, calculated from the amount copied and elapsed time.
- CPU utilization and whether processors remain available for other services.
- Network utilization and whether the link is actually saturated.
- Source and destination storage activity, latency, and queue pressure.
- Impact on users or applications sharing the server.
Use /LOG for each trial so the command output is retained consistently. Microsoft’s command reference recommends redirecting output with /LOG for better performance and demonstrates logging in its examples. A log also gives you a record of which command and result belong to each test.
5. Keep the value that improves the job, not the graph
Select the lowest tested value that delivers the required completion time while leaving acceptable headroom. If moving from 8 to 16 reduces elapsed time but increases storage latency enough to disrupt applications, 8 is the better production setting. If 16 and 32 finish within measurement noise, the lower value generally creates less contention and is easier to justify.
There is no Microsoft-prescribed benchmark duration or universal pass/fail threshold. The practical test method above follows Microsoft’s warnings that bandwidth, compute, storage, file mix, and other workloads determine the result.
6. Tune recurring runs independently
After the initial transfer, run a separate test for the recurring differential or mirror job. Microsoft advises matching later-run thread counts more closely to processor core count and threads per core, while reserving cores for other production work. A small-change run may gain little from a very high count because enumeration and comparison, rather than bulk transfer, dominate its time.
What Microsoft’s Azure Files results mean
Microsoft’s Azure Files migration guidance provides useful evidence, but it is tied to Azure Files migration conditions. It should not be presented as a universal Robocopy tuning table.
| Situation in the guidance | Reported observation | How to apply it |
|---|---|---|
| Initial Azure Files copy | Balanced performance at 8–20 threads in Microsoft’s tests | Use this as a source-backed range to test for an Azure Files initial migration, not as a guarantee for local disks or other shares. |
| Higher concurrency in those tests | Good performance up to 64 threads when processors can keep the threads active | Only consider such values when processor capacity and the rest of the environment support them. |
| Subsequent migration runs | Match the count more closely to processor core count and threads per core | Retune recurring passes and account for cores needed by production work. |
The page does not state a publication year for these figures. Their scope is Azure Files migration guidance, so they do not establish an optimum for a Windows workstation copying between local volumes, a different network share, or every server configuration.
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Designing a useful benchmark
Keep the trials comparable
Use the same source and destination for each trial when possible. Use the same set of files, Robocopy switches, authentication context, and network path. Avoid comparing an initial copy with a later run and calling the difference a thread-count improvement: the amount of changed data and the directory state are different variables.
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Change one variable at a time
Do not change /MT, compression, filtering, destination storage, and network route in the same experiment. If you change several conditions, you cannot tell which one affected the result. Run the same test with 8, then a modestly higher count, and record the exact command line for each.
Watch both averages and side effects
A short burst of high throughput can hide a later stall caused by storage queues or CPU pressure. Observe the complete run, not just the first minute. Note whether applications become slow, whether the destination reports sustained write latency, and whether network utilization remains stable. A setting that is slightly slower in isolation may be preferable if it leaves predictable capacity for business-critical work.
Use logs consistently
Redirect each run to its own log file, for example:
robocopy C:Source D:Destination /E /MT:8 /LOG:C:Logsmt8.logThen repeat with another count while changing only the /MT:n value. Keep the logs with the measured elapsed time and resource observations. The log records command output; it does not replace monitoring of CPU, network, and storage utilization.
Initial copies and later differential or mirror passes
Initial copy
An initial seed normally transfers the largest data volume. If the network is the limiting resource and the source and destination can service concurrent requests, increasing from the default may shorten the transfer. The Azure Files evidence of 8–20 balanced threads applies specifically to that service and migration scenario.
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Subsequent copy
A later run may transfer only a few changed files while still examining a very large directory tree. In that case, raising the count can have little effect because the dominant work is namespace processing and comparison. CPU availability, file count, and the cost of opening and checking entries become more important than raw link capacity.
Mirror jobs
A mirror or differential schedule should be benchmarked at the frequency and change rate at which it will actually run. A setting chosen during a quiet weekend seed can be too aggressive during business hours, when the same server is handling users or applications.
Compatibility and command-line details
Options that cannot be combined
Microsoft documents that /MT cannot be combined with /IPG or /EFSRAW. If a command contains either of those switches, remove the incompatible option or redesign the command rather than trying to solve the error by changing the thread count.
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Logging and performance
The Robocopy command reference recommends redirecting output with /LOG for better performance. Use a log path on a location that is available throughout the run and has enough free space. Keep logging consistent across benchmark trials so output handling does not become an uncontrolled difference.
Selection and copy behavior remain separate
Changing /MT:n does not add recursion, delete destination files, preserve additional metadata, or filter the file set. Review the other switches independently. In the example command, /E is responsible for including subdirectories and empty directories; it is not a consequence of multithreading.
Symptoms that indicate the count is wrong
| Observation | Likely interpretation | Adjustment to test |
|---|---|---|
| Throughput and elapsed time barely change as the count rises | Another resource, such as storage, CPU, or the network, is already limiting the run, or namespace work dominates. | Keep the lower count and verify which resource is saturated before testing again. |
| Network utilization is low while CPU and storage have headroom | The copy may have insufficient concurrency to keep the link busy. | Compare the default with a modestly higher count on the same workload. |
| CPU is continuously busy and applications slow down | The requested concurrency is competing with production work. | Reduce the count or schedule the job when more processor capacity is available. |
| Storage latency or queue depth rises sharply | The source or destination cannot efficiently service the additional concurrent requests. | Return to the previous lower value and protect storage capacity. |
| The initial transfer improves but recurring passes do not | The two runs have different bottlenecks; later namespace processing may dominate. | Benchmark the recurring pass separately and use a count aligned with processor capacity. |
| Robocopy reports an option conflict | /MT is being used with /IPG or /EFSRAW. |
Remove or replace the incompatible switch, then rerun a controlled test. |
A practical decision checklist
- Confirm that the job is an initial copy, differential pass, or mirror run.
- Start at 8 threads unless an existing, measured baseline justifies another value.
- Check available network bandwidth, CPU cores and hardware threads, source and destination storage, file count and size, and competing workloads.
- Test one or two higher values only if the initial run leaves relevant resources underused.
- Use the same command, files, paths, and operating conditions for each comparison, changing only
/MT:n. - Record elapsed time, effective throughput, CPU, network, storage activity, and impact on other services.
- Choose the value that improves the real job while preserving operational headroom.
- Retune later differential or mirror runs instead of inheriting the initial-copy setting.
- Keep
/IPGand/EFSRAWout of commands that use/MT, and retain logs with/LOG.
The central rule is simple: /MT supplies concurrency, not guaranteed speed. The documented 1–128 range and default of 8 define the control; your source, destination, network, processors, storage, file mix, and competing work determine the useful value. Measure those conditions and select the smallest count that meets the job’s completion requirement.
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