centos swapper官方文档

Note: There is no dedicated “official documentation” for “centos swapper” from CentOS/RHEL. The term “swapper” typically refers to the Linux kernel’s swap subsystem (swap partition or swap file), which is managed using standard system tools. Below are the official and community-accepted best practices for configuring and managing swap space on CentOS systems.

1. Check Existing Swap Space

Before configuring swap, verify if your system already has swap enabled:

free -h

Look for the “Swap” line in the output. If “Swap” shows 0B, no swap is configured.

2. Create a Swap File (Recommended for Most Users)

A swap file is flexible and does not require modifying disk partitions.

Steps:

1. Create a swap file (e.g., 2GB):

   sudo dd if=/dev/zero of=/swapfile bs=1M count=2048
   

   • if=/dev/zero: Input source (infinite zero stream).
   • of=/swapfile: Output file path.
   • bs=1M: Block size (1MB).
   • count=2048: Number of blocks (2048 × 1MB = 2GB).

2. Set strict permissions (only root can read/write):

   sudo chmod 600 /swapfile
   

3. Format the file as swap:

   sudo mkswap /swapfile
   

4. Activate the swap file:

   sudo swapon /swapfile
   

5. Enable auto-mount on boot:
   Edit /etc/fstab and append:

   echo "/swapfile swap swap defaults 0 0" | sudo tee -a /etc/fstab
   

   This ensures swap is re-enabled after a reboot.

3. Create a Swap Partition (Alternative for Server Environments)

For production servers, a dedicated swap partition is often preferred for stability.

Steps:

1. Identify unused disk space:

   sudo fdisk -l
   

   Look for a disk with unallocated space (e.g., /dev/sdb).

2. Create a new partition:
   Use fdisk to create a partition (replace /dev/sdb with your disk):

   sudo fdisk /dev/sdb
   

   • Press n (new partition) → p (primary) → enter partition number → press Enter (default start sector) → +2G (set size, e.g., 2GB) → t (change partition type) → 82 (Linux swap type) → w (write changes).

3. Format the partition as swap:

   sudo mkswap /dev/sdb1
   

4. Activate the partition:

   sudo swapon /dev/sdb1
   

5. Enable auto-mount on boot:
   Edit /etc/fstab and append:

   echo "/dev/sdb1 swap swap defaults 0 0" | sudo tee -a /etc/fstab
   

6. Verify:
   Use sudo swapon --show or free -h to confirm.

4. Adjust Swappiness (Optional but Recommended)

The swappiness parameter controls how aggressively the system uses swap (range: 0–100).

• 0: Avoids swapping unless absolutely necessary (ideal for systems with lots of RAM).
• 60: Default value (balanced for general use).
• 100: Aggressively swaps to disk (suitable for memory-intensive workloads).

Steps:

1. Check current value:

   cat /proc/sys/vm/swappiness
   

2. Temporarily modify (until reboot):

   sudo sysctl vm.swappiness=10
   

3. Permanently modify:
   Edit /etc/sysctl.conf and add:

   vm.swappiness=10
   

   Apply changes:

   sudo sysctl -p
   

4. Reboot (optional, to ensure persistence):

   sudo reboot
   

5. Monitor Swap Usage

Regularly check swap usage to ensure optimal performance:

• View total/used swap:

  free -h
  

• List active swap devices:

  sudo swapon --show
  

• Detailed swap statistics:

  cat /proc/swaps
  

• Real-time monitoring (install sysstat first):

  sar -W 1 5
  

Key Notes

• Swap size recommendations:
  • For systems with < 4GB RAM, allocate 2–4x RAM size.
  • For systems with 4–16GB RAM, allocate 1–2x RAM size.
  • For systems with > 16GB RAM, 2–4GB is usually sufficient.
• Performance impact: Swap is much slower than RAM. Avoid over-reliance on swap for production systems with high memory demands.
• CentOS version compatibility: The steps above work for CentOS 7/8/Stream. For RHEL-based systems, commands are identical.

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