How to Fix BufferError: Memory Management and Large Output Handling in Paramiko

When using Paramiko’s Channel.recv() or exec_command() with commands producing large outputs, you may encounter BufferError due to excessive data buffering. This guide shows memory-efficient techniques—streaming, chunked reads, and pagination—to avoid BufferError and handle large SSH outputs robustly.

1. Understand BufferError Causes

• Accumulating entire stdout in memory via stdout.read() on large outputs.
• Paramiko’s internal buffer hitting capacity limits when data is unread.
• Blocking reads without emptying buffer regularly.

2. Stream Output Line by Line

Process each line as it arrives to avoid buffering the full output:

stdin, stdout, stderr = ssh.exec_command('your_large_command')
for line in stdout:
    process(line)  # handle each line immediately
    

3. Read in Fixed-Size Chunks

Read bytes in manageable chunks rather than entire output:

buffer_size = 1024 * 16  # 16 KB
stdin, stdout, stderr = ssh.exec_command('your_large_command')

while True:
    chunk = stdout.read(buffer_size)
    if not chunk:
        break
    process(chunk)
    

This frees each chunk after processing and prevents large buffer buildup.

4. Use Non-Blocking Mode and recv_ready()

Poll the channel for available data and read incrementally:

channel = ssh.get_transport().open_session()
channel.exec_command('your_large_command')
channel.settimeout(2.0)

while not channel.exit_status_ready():
    if channel.recv_ready():
        data = channel.recv(1024*16)
        process(data)
    else:
        time.sleep(0.1)
# Read any remaining data
while channel.recv_ready():
    process(channel.recv(1024*16))
    

5. Paginate Command Output

Use remote pagination tools (less, more) to limit chunk sizes server-side:

# Page through output 1000 lines at a time
command = 'your_command | split -l 1000 --numeric-suffixes=1 /tmp/chunk_'
ssh.exec_command(command)

# Retrieve each chunk file sequentially
for i in range(1, 100):
    filename = f"/tmp/chunk_{i:02d}"
    stdin, stdout, stderr = ssh.exec_command(f'cat {filename}')
    for line in stdout:
        process(line)
    ssh.exec_command(f'rm {filename}')
    

6. Clean Up and Close Channels

Always close channels and free resources:

stdout.channel.close()
stderr.channel.close()
stdin.channel.close()
ssh.close()
    

7. Summary Checklist

1. Stream line-by-line rather than reading all at once.
2. Read in fixed-size chunks to limit memory use.
3. Use non-blocking mode and recv_ready() to poll data.
4. Paginate large outputs server-side to break into smaller files.
5. Close channels explicitly to release buffers.