What /MT robocopy multithread actually does

Robocopy can copy multiple files at once using a thread pool. You control the pool size with /MT:n where n = 1–128 (default is 8 if you don’t specify a number). Microsoft notes that multithreading improves throughput and recommends redirecting output to a log for best performance; /MT is incompatible with /IPG and /EFSRAW.

Quick-start recipes (copy & tweak)

1) Fast local or LAN folder sync (keeps permissions & times)

robocopy "D:\Source" "\\Server\Share\Dest" /MIR /COPY:DATSO /DCOPY:DAT /MT:32 /R:5 /W:3 /XJ /NFL /NDL /NP /LOG:C:\logs\job1.log

• /MIR mirrors (add + delete). Use with care.
• /COPY:DATSO = Data, Attributes, Timestamps, Security (ACLs), Owner.
• /DCOPY:DAT preserves dir attributes & times.
• /XJ skips junctions/symlinks (avoids loops).
• /NFL /NDL /NP trims console spam; logging helps speed with /MT.

2) Lots of huge files (VMs, ISOs)

robocopy "E:\VMs" "\\NAS\VMs" *.vhdx *.iso /E /J /MT:16 /R:3 /W:2 /TEE /LOG+:C:\logs\vm_copy.log

• /J = unbuffered I/O (often faster for big files).

3) Millions of tiny files (code, photos)

robocopy "D:\Photos" "Z:\Photos" /E /MT:64 /R:2 /W:1 /FFT /XJ /NP /LOG:C:\logs\photos.log

• High /MT helps small-file workloads; /FFT relaxes timestamp granularity for cross-platform shares. (General /MT behavior & limits: MS docs.)

4) “Resumeable” copy across flaky links

robocopy "C:\Data" "\\Remote\Backup" /E /ZB /MT:16 /R:5 /W:5 /LOG:C:\logs\resume.log

• /ZB uses restartable mode, falling back to backup mode if needed. (Tip: /Z can slow some links; tune case by case.)

5) Throttle (don’t saturate the pipe)

• You can’t use /IPG with /MT. If you must throttle, drop /MT or rate-limit at the network layer/QoS.

How many threads should I use?

• Start at 16–32 for most LAN jobs; 64–96 can help with mountains of small files. Going past that can cause diminishing returns or contention on slow storage/NAS CPUs. (Default 8; allowed 1–128.)

Performance switches that matter

• /MT:n — thread count (default 8; max 128).
• /J — unbuffered I/O; great for very large files.
• /NOOFFLOAD — disable Windows copy offload (use if your storage offload behaves poorly).
• /R:n /W:n — lower retries/waits (defaults are huge; set like /R:5 /W:3).
• /LOG[:file] — redirect output; recommended with /MT for speed and clean logs.

What to avoid / common pitfalls

• Using /IPG with /MT (not supported).
• Mindless /MIR on the wrong path—deletes at destination. Dry-run first with /L. (General guidance.)
• Huge /R & /W defaults; they can stall jobs for hours on one bad file. Tune them.
• Expecting linear speedup with threads; disk/NAS/CPU and SMB limits cap gains. (Empirical testing shows diminishing returns.)
• Console spam hurting speed with /MT; use /LOG + /NFL /NDL /NP.

Verifying success (exit codes)

Robocopy’s exit code is a bitmask (0–7 = success states; ≥8 indicates failure). Many tools treat “non-zero = error”, which is wrong for Robocopy. Adjust your scripts accordingly.

Example batch snippet:

robocopy "C:\A" "D:\B" /E /MT:32
set RC=%ERRORLEVEL%
if %RC% GEQ 8 (
  echo Robocopy failed with code %RC%
  exit /b %RC%
) else (
  echo Robocopy completed (code %RC%)
)

Logging & monitoring

• Use /LOG:c:\logs\job.log or /LOG+ to append; pair with /TEE to also echo to screen. (Logging improves /MT performance, per docs.)

Review: When Robocopy (/MT) is the right tool

Pros

• Rich flags for ACLs, timestamps, mirroring, retries, resume, logs.
• Built-in, scriptable, fast for big trees (especially small files) thanks to /MT.

Cons

• No native bandwidth cap with /MT (since /IPG conflicts).
• Diminishing returns at high thread counts; can overwhelm slow NAS/SMB links. (Empirical tests.)
• Syntax quirks; exit code bitmask confuses CI/backup wrappers.

Good alternatives

• For one-shot local copies of very large files: plain Explorer or Copy-Item -ToSession (PowerShell Remoting) sometimes ties.
• For scheduled backups with versioning: use backup software; Robocopy doesn’t version files.
• For WAN/Cloud: use tools that handle latency/compression/delta-sync.

Handy cheatsheet

• Mirror + ACLs (safe-ish defaults)
  /MIR /COPY:DATSO /DCOPY:DAT /R:5 /W:3 /XJ /MT:32 /LOG:job.log
• Big files
  /E /J /MT:16 /R:3 /W:2 /LOG:big.log
• Many tiny files
  /E /MT:64 /NFL /NDL /NP /LOG:tiny.log
• Dry-run
  ... /L /UNILOG:c:\logs\whatwouldhappen.log (no changes; show actions)

Further reading (solid, up-to-date sources)

• Microsoft Learn: full Robocopy reference and /MT caveats (defaults, incompatibilities, logging tip).
• PDQ’s “Hitchhiker’s Guide”: modern switch coverage with examples.
• Petri’s “Complete Guide to Robocopy” (updated 2025): broader overview & scenarios.
• SS64 exit codes table (indicates 0–7 = success).
• Benchmarks & thread scaling discussion (why more threads ≠ always faster).

FAQs

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