centos swapper官方文档 - 问答

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:

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).
Set strict permissions (only root can read/write):
```
sudo chmod 600 /swapfile
```
Format the file as swap:
```
sudo mkswap /swapfile
```
Activate the swap file:
```
sudo swapon /swapfile
```
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:

Identify unused disk space:
```
sudo fdisk -l
```
Look for a disk with unallocated space (e.g., /dev/sdb).
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).
Format the partition as swap:
```
sudo mkswap /dev/sdb1
```
Activate the partition:
```
sudo swapon /dev/sdb1
```

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

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

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:

Check current value:
```
cat /proc/sys/vm/swappiness
```
Temporarily modify (until reboot):
```
sudo sysctl vm.swappiness=10
```
Permanently modify:
Edit /etc/sysctl.conf and add:
```
vm.swappiness=10
```
Apply changes:
```
sudo sysctl -p
```
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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