Diagnose SMB Performance and Multichannel Bottlenecks

Admin

October 11, 2026

12 min read

Slow file copies can come from client storage, server storage, CPU, network paths, or the workload’s file layout. This guide investigates SMB on Windows Server 2019, 2022, and 2025 using measured transfers and negotiated connection state. It treats Multichannel as one part of the path rather than assuming every slow transfer needs another NIC.

Define the Workload and Record Both Endpoints

Baseline

Record the client and server builds, share name, file sizes, file count, direction, concurrency, and whether the application opens many small files or transfers a few large ones. Use a direct server path for the initial test so DFS referrals or name aliases do not obscure the endpoint. A single large-file copy and a small-file workload have different limits; keep the data set unchanged when comparing runs.

# Run locally on client and server.
Get-CimInstance Win32_OperatingSystem |
  Select-Object CSName, Caption, Version, BuildNumber
Get-NetAdapter | Select-Object Name, InterfaceDescription, Status, LinkSpeed
Get-Volume | Select-Object DriveLetter, HealthStatus, SizeRemaining, Size
# On the client:
Resolve-DnsName FILE01.contoso.com
Test-NetConnection FILE01.contoso.com -Port 445

Inspect the Negotiated SMB Connection

Protocol

Open the approved share from the client, then inspect the actual connection. Check the negotiated dialect, encryption state, and authenticated user. Inspect the configuration on the respective endpoints, but distinguish a configured preference from a negotiated connection property. An empty connection list can simply mean there is no open SMB session yet; do not infer a protocol failure from that result.

# Client: access the approved share first.
Get-ChildItem '\\FILE01.contoso.com\Bench' | Select-Object -First 5
Get-SmbConnection | Where-Object ServerName -Match '^FILE01' |
  Format-List *
Get-SmbClientConfiguration |
  Select-Object EnableMultiChannel, RequireSecuritySignature

# Server:
Get-SmbServerConfiguration |
  Select-Object EnableMultiChannel, RequireSecuritySignature, EncryptData
Get-SmbShare -Name Bench | Select-Object Name, Path, EncryptData

Find the Paths Multichannel Actually Selected

Network Selection

SMB Multichannel can use multiple network connections where endpoint capabilities and policy allow it. Inspect selected and unselected interface pairs, their addresses, and RSS/RDMA capabilities. Compare these with the intended client/server networks. Multiple channels do not guarantee proportional speed: a single RSS-capable adapter can use multiple channels, and slow storage or CPU can cap throughput. An RDMA-capable NIC also needs a correctly configured, supported end-to-end fabric for SMB Direct.

# Client, while an SMB session/transfer is active:
Get-SmbMultichannelConnection -ServerName FILE01.contoso.com `
  -IncludeNotSelected | Format-List *
Get-SmbClientNetworkInterface | Format-Table -AutoSize

# Server:
Get-SmbServerNetworkInterface | Format-Table -AutoSize

Correlate NIC Errors, Routes, and Interface Configuration

Path Health

Read NIC statistics before and after the transfer. Rising discard/error counts are more useful than lifetime totals. Inspect RSS and RDMA support without changing them, and verify the route to each selected server address. Resolve unexpected path choices before changing adapter settings. Driver counter meanings vary, so compare suspicious values with the hardware vendor’s documentation and switch-port telemetry.

Get-NetAdapterStatistics | Format-List *
Get-NetAdapterRss | Format-List *
Get-NetAdapterRdma | Format-List *
# Replace with a server address selected in Step 3.
Find-NetRoute -RemoteIPAddress '10.20.0.20'
Get-NetIPConfiguration
# Repeat NIC statistics after the same bounded workload.

Measure CPU, Storage, and SMB Counters Together

Bottleneck

Collect counters on both endpoints while reproducing the slow transfer. Discover SMB counter sets locally because availability, instance names, and language differ. Compare CPU use and disk latency with network bytes and application throughput. For a server that is slow only on small files, look at metadata operations, per-file latency, and application concurrency; link speed alone is a poor predictor of that workload.

Get-Counter -ListSet '*SMB*' | Select-Object CounterSetName, PathsWithInstances

# English Windows counter paths; select equivalents on localized systems.
Get-Counter -Counter @(
  '\Processor(_Total)\% Processor Time'
  '\PhysicalDisk(*)\Avg. Disk sec/Read'
  '\PhysicalDisk(*)\Avg. Disk sec/Write'
  '\Network Interface(*)\Bytes Total/sec'
) -SampleInterval 1 -MaxSamples 15

Run a Repeatable Copy Without Deletion Flags

Controlled Test

For one approved large file, use unbuffered Robocopy I/O and disable retries so a failed connection does not become a long test. First list the operation with /L. Use a new destination directory for each run; otherwise a skipped file can look like a fast copy. Save the exit code immediately: values 8 or higher indicate copy failures, while lower values encode outcomes such as copied or extra files. Measure from the copy summary and correlate with Step 5.

$source = 'C:\BenchSource'
$destination = '\\FILE01.contoso.com\Bench\Run01' # New approved directory
if (-not (Test-Path -LiteralPath "$source\test-data.bin" -PathType Leaf)) {
  throw 'The approved source file is missing.'
}
if (Test-Path -LiteralPath $destination) {
  throw 'Choose a new destination directory for this test.'
}

robocopy $source $destination test-data.bin /L /J /R:0 /W:0 /NP
# After reviewing the listing and capacity, perform the approved copy:
robocopy $source $destination test-data.bin /J /R:0 /W:0 /NP
$copyExitCode = $LASTEXITCODE
"Robocopy exit code: $copyExitCode"
if ($copyExitCode -ge 8) { Write-Warning 'Copy failed; inspect the summary.' }

Choose a Repair Based on the Limiting Resource

Remediation

Use the evidence to assign the repair: stale or unreachable channel addresses to networking; rising NIC errors to the driver/fabric owner; high storage latency to the storage owner; CPU saturation under signing/encryption to capacity and supported-version review. If Multichannel is configured off, investigate why before enabling it. Make one canary change with a recorded baseline and rollback. Do not disconnect every SMB session or update every NIC during a live incident.

# Client: retain events around the test window.
Get-WinEvent -FilterHashtable @{
  LogName = 'Microsoft-Windows-SMBClient/Connectivity'
  StartTime = (Get-Date).AddMinutes(-30)
} -MaxEvents 50 | Select-Object TimeCreated, Id, Message

# Server: inspect current sessions instead of terminating them.
Get-SmbSession | Select-Object ClientComputerName, ClientUserName, NumOpens

Validate Performance and Application Behavior

Closeout

Repeat the same test with a new destination and the same concurrency. Reinspect negotiated security, selected channels, NIC deltas, storage latency, and CPU. Then test the original application; a faster bulk copy does not establish that small-file or metadata operations improved. Remove only the test directory through the share owner’s approved cleanup process, and retain enough evidence to reproduce the comparison after the next driver or OS update.

Get-SmbConnection | Where-Object ServerName -Match '^FILE01' | Format-List *
Get-SmbMultichannelConnection -ServerName FILE01.contoso.com | Format-List *
Get-NetAdapterStatistics | Format-List *
# Repeat Steps 5-6 with a new destination and the original workload.