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For thousands of small files copied over an SMB share, start with Robocopy multithreading and a log: robocopy "C:\Source" "D:\Destination" /E /COPY:DAT /MT:16 /R:2 /W:5 /LOG:"C:\Logs\copy.log". Then test a few /MT values on the actual source and destination. Eight threads is Robocopy’s default, 128 is the documented maximum, and more threads can reduce performance instead of improving it.
Small-file copies are often limited by the repeated work of opening, creating, and closing each file, SMB metadata operations, storage latency, and antivirus inspection. Low network utilization does not prove that the network is the bottleneck. The procedure below shows how to choose a safe command, measure it, interpret the log, and avoid destructive switches.
The fastest safe starting command
Open Command Prompt or PowerShell and run a command like this, replacing every path with your own:
robocopy "C:\Source" "D:\Destination" /E /COPY:DAT /MT:16 /R:2 /W:5 /LOG:"C:\Logs\copy.log"This is a measured starting point, not a guaranteed optimum. It recursively copies files, includes empty directories, preserves data, attributes, and timestamps, uses 16 worker threads, retries a failed item twice, waits five seconds between retries, and writes output to a log file instead of constantly updating the console.
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Create the log directory first if it does not exist. Make sure the destination has enough free space and that your account can read the source and write the destination. Do not put the log inside the source tree if another process may copy it during the run.
Why thousands of small files take so long
Microsoft describes a large small-file set as hundreds, thousands, or even millions of files generally under 1 MB. The bytes themselves may be modest, but each file requires separate SMB requests and file-system work. A single-threaded copy can spend more time creating directory entries and file objects than transferring file contents.
Antivirus software can also inspect every newly created file. On an SMB path, the source and destination each contribute latency, and the server may have to process metadata operations serially or in small batches. As a result, a transfer can show low network throughput while the disks, CPU, SMB server, or file-creation rate is the real limit.
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Multithreading keeps additional files in flight while other files are being opened or created. Redirecting output with /LOG avoids spending the copy’s foreground time rendering status lines. These two changes are Microsoft’s recommended first experiment for small-file SMB transfers.
Prepare a test that tells you something
Use a representative sample
Do not tune against one unusually large file. Use a directory with the same approximate file-size distribution, directory depth, number of files, and source and destination type as the real job. A local SSD-to-SSD copy, a mapped drive, a Windows file server, and a cloud-backed file share can have very different bottlenecks.
Keep the comparison fair
Record the start and finish time for each run, and use the same source, destination, command options, and workload when changing only the thread count. If a previous run has already populated the destination, a later run may do less work; note that when comparing results.
- Record whether the path is local storage or SMB/network storage.
- Note the approximate number of files, total bytes, and directory depth.
- Observe source and destination CPU, storage activity, and available network capacity.
- Confirm that the metadata you need is represented by the copy switches.
- Keep the log from every trial so errors and skipped files can be compared.
Choosing the /MT thread count
/MT:n enables multithreaded copying. The documented default when /MT is used without a number is eight threads. Robocopy accepts values from 1 through 128. Microsoft explicitly warns that too many threads may harm performance, so the highest number is a limit, not a target.
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| Setting | Meaning | When to try it |
|---|---|---|
/MT:8 |
Eight workers, also the documented default | Baseline for a conservative test |
/MT:16 |
Sixteen workers | Practical first test for many SMB small-file jobs |
/MT:20 |
Twenty workers | Reported as a balanced range in one Azure Files migration scenario, not a universal optimum |
/MT:32 or higher |
More concurrent work | Only if measured results improve without saturating CPU, storage, or the server |
/MT:128 |
Documented maximum | Stress-test value, not a default recommendation |
Run the same command at 8, 16, and one higher value such as 32, then keep the setting with the shortest successful elapsed time and an acceptable error count. If the destination becomes saturated, server latency rises, or elapsed time gets worse, reduce the thread count. A faster result on one directory does not establish a universal setting for every share.
Why more threads can be slower
Every worker consumes CPU, memory, file handles, and storage queue capacity. The SMB server may also have its own concurrency limits. With tiny files, workers can contend for the same directory metadata, disk, or antivirus scanner. Once those resources are saturated, additional threads add scheduling and queueing overhead instead of useful throughput.
What each switch in the starting command does
| Switch | Effect | Important qualification |
|---|---|---|
/E |
Copies subdirectories, including empty ones | It does not delete unrelated destination content |
/COPY:DAT |
Copies data, attributes, and timestamps | The example does not request security metadata |
/MT:16 |
Uses 16 copy threads | Change it only after measuring the real workload |
/R:2 |
Retries a failed file twice | Replace the value when your outage or locking policy requires something else |
/W:5 |
Waits five seconds between retries | Short waits prevent a failed run from appearing hung |
/LOG:'path' |
Writes command output to a log file | Inspect the log after completion; use a writable, persistent location |
The exact command uses double quotes around paths so spaces are handled correctly. In this article those quotes are displayed as HTML entities, but you should paste ordinary double quotes into the Windows command line.
Set retry and wait limits deliberately
Robocopy’s documented defaults are extremely generous: up to 1,000,000 retries with a 30-second wait between attempts. A locked file, disconnected share, or permission problem can therefore make a command continue for a very long time. Explicit /R and /W values make the run observable and easier to recover.
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Always create and inspect a log
Microsoft recommends creating a log and checking it after the command completes. A log lets you distinguish copied files from skipped files, retries, access failures, and other conditions without relying on a scrolling console window.
Use a unique log path for each trial when tuning. After the run:
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- Open the log and check the final summary for the number of files copied, skipped, failed, and mismatched.
- Search for terms such as
ERROR,Access is denied, or unavailable paths. - Confirm that the destination contains the expected top-level directories and representative files.
- If files failed, fix the underlying lock, permission, connectivity, or capacity issue and rerun with the same bounded retry policy.
Do not judge success solely by elapsed time. A fast command that skipped files or ended with errors is not a successful migration.
Protect the destination: understand /MIR
Do not add /MIR to a normal copy merely because it sounds faster. Microsoft defines /MIR as equivalent to /E plus /PURGE. It makes the destination match the source by deleting destination files and directories that are not present in the source.
Use /MIR only when deletion is an intentional part of the job and you have verified the source and destination paths. For a one-way population copy where existing destination files must remain, use /E without /MIR. Treat a mirror as a synchronization operation with destructive consequences, not as a multithreading switch.
Metadata and directory choices
The example’s /COPY:DAT preserves file data, attributes, and timestamps. That is appropriate when ordinary file content and those standard timestamps are required. It does not request security information, so do not assume that destination access-control lists, owner information, or auditing metadata have been reproduced.
/E creates the complete directory tree, including empty directories. If empty folders are irrelevant, a different directory-selection policy may reduce work, but change that only when you are certain the omitted structure is not needed by applications or users.
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When archiving can help—and when it cannot
Combining many small files into an archive can reduce the number of individual file operations during the network transfer. However, Microsoft cautions that the end-to-end result depends on compression and decompression speed. You must include the time to create the archive and extract it when comparing it with Robocopy.
Fast or no-compression modes reduce processing overhead, but they also provide less size reduction. Archiving is therefore an experiment for a specific workload, not a guaranteed optimization. Keep the original file hierarchy and metadata requirements in mind; an archive workflow may not preserve every property represented by a direct file copy.
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Local disks, SMB shares, and cloud-backed paths are different
Local-to-local copies
Local storage can be limited by the source disk, destination disk, filesystem metadata work, CPU, or antivirus scanning. The SMB-specific recommendation still provides a useful multithreaded test, but the best thread count may differ from a network copy. Measure on the actual disks and avoid assuming that network-oriented results transfer to a local path.
SMB file shares
The guidance is strongest for SMB small-file transfers. Network bandwidth is only one part of the path: each file also causes protocol and server work. Test the source and destination share directly, and watch for server-side load or intermittent connectivity. A low network graph can be normal when metadata and file creation dominate.
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Microsoft’s Azure Files migration guidance reports balanced initial-copy performance with roughly 8 to 20 threads for that particular service and scenario. It also demonstrates a command using 16 threads, low retry values, and a repeat run to copy changes after the first pass. Treat those values as Azure-specific guidance, not as a universal Windows setting; follow the migration procedure for the service and validate the result in your tenant.
Azure Data Box Disk
For Azure Data Box Disk, Microsoft gives a separate scenario-specific recommendation for mostly small files under 512 KB: four Robocopy sessions with 16 threads per session. The same guidance states a per-session ceiling of 7,000 directories and 150 million files. Those figures describe the Data Box Disk workflow and constraints; they are not general Robocopy limits and should not be applied to an ordinary disk or SMB share.
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The command appears to run forever
Check whether you left the defaults in place. One million retries with a 30-second wait can create extremely long pauses. Stop the run if appropriate, correct the command to include bounded /R and /W values, and inspect the log for the first recurring failure rather than repeatedly waiting on the same item.
Throughput is low even though the network is idle
That pattern is consistent with small-file overhead. Examine source and destination storage activity, CPU, antivirus activity, and server responsiveness. Test a higher thread count, but stop increasing it when elapsed time stops improving or resource contention appears.
More threads made the copy slower
Reduce /MT and retest. Microsoft warns that too many threads may harm performance. Contention for disk queues, directory metadata, CPU, or server resources can outweigh the benefit of parallelism.
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Files are missing from the destination
Read the log summary and error lines. Check for access-denied messages, disconnected shares, source files locked during the run, and destination capacity. A bounded retry policy exposes these conditions sooner; it does not silently make them successful.
The destination lost files
Check whether /MIR or another purge behavior was used. A mirror intentionally deletes destination-only content. For a non-destructive copy, remove /MIR and use /E when empty directories are required.
The command fails before copying
Verify that both quoted paths exist, the log directory is writable, the source account has read access, and the destination account has create and write access. Test the share in File Explorer or with a small, noncritical directory before launching a large run.
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A repeatable tuning procedure
- Choose a representative source tree and confirm the destination is safe to modify.
- Create a writable log directory outside the source tree.
- Run the starting command with
/MT:8, bounded retries, and logging. - Record elapsed time, copied and failed counts, and resource behavior.
- Repeat with
/MT:16, changing no other option. - If the result improves and resources remain healthy, test one higher value such as 32; if it worsens, return to the faster lower value.
- Open every log, resolve errors, and rerun until the final summary matches the required file set.
- For a second pass after the source changes, rerun the same non-destructive command and verify the log again.
This procedure gives you a workload-specific setting rather than an unsupported promise of a particular speedup. The optimum depends on file count, file-size distribution, directory depth, storage, server behavior, and whether antivirus or other software is inspecting each file.
Practical command variants
Conservative baseline
robocopy "C:\Source" "D:\Destination" /E /COPY:DAT /MT:8 /R:2 /W:5 /LOG:"C:\Logs\copy-8.log"Use this to establish the documented eight-thread baseline.
Moderate parallel copy
robocopy "C:\Source" "D:\Destination" /E /COPY:DAT /MT:16 /R:2 /W:5 /LOG:"C:\Logs\copy-16.log"This is the practical first experiment for many SMB small-file jobs, not a benchmark-backed guarantee.
High-concurrency test
robocopy "C:\Source" "D:\Destination" /E /COPY:DAT /MT:32 /R:2 /W:5 /LOG:"C:\Logs\copy-32.log"Use only as a comparison trial. Keep it if it measurably reduces successful elapsed time without increasing failures or destabilizing the source, destination, or server.
Final checklist before you start
- Confirm whether the transfer is local, SMB, Azure Files, or Data Box Disk.
- Count the approximate files and identify whether most are below 1 MB or, for the Data Box scenario, below 512 KB.
- Choose a representative test set.
- Start with
/MT:8or/MT:16; do not jump straight to 128. - Set explicit
/Rand/Wvalues. - Use
/LOGand inspect the completed log. - Use
/Efor a normal recursive copy and avoid/MIRunless destination deletion is intentional. - Verify that the selected copy flags preserve the data, attributes, timestamps, and any other metadata your application requires.
- Compare complete end-to-end time if you try archiving.
- Do not treat a thread count from a cloud or appliance migration guide as a universal Windows rule.
For most Windows SMB jobs involving thousands of small files, the reliable path is simple: use Robocopy with a moderate /MT value, redirect output to a log, bound retries, test on the real workload, and select the fastest setting that completes without errors. That approach addresses the file-creation bottleneck without turning a routine copy into an accidental destructive mirror.
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