CentOS Swap最佳实践

Creating Swap Space
Swap can be configured as a dedicated partition or a swap file, depending on system requirements and disk layout. For partition-based setup, use fdisk or parted to create a new partition (e.g., /dev/sdb2), then format it with mkswap /dev/sdb2 and enable it via swapon /dev/sdb2. To make it persistent, add an entry to /etc/fstab (e.g., /dev/sdb2 swap swap defaults 0 0). For file-based setup, use fallocate -l 4G /swapfile (or dd if=/dev/zero of=/swapfile bs=1M count=4096 for older systems) to create a 4GB file, set strict permissions with chmod 600 /swapfile, format it with mkswap /swapfile, and enable it with swapon /swapfile. Add /swapfile none swap sw 0 0 to /etc/fstab for automatic mounting at boot.

Determining Optimal Swap Size
The ideal swap size depends on physical memory (RAM) and workload characteristics. Common recommendations include:

• 4GB or less RAM: 2x RAM (e.g., 4GB RAM → 8GB swap).
• 4–8GB RAM: Equal to RAM (e.g., 8GB RAM → 8GB swap).
• 8–64GB RAM: Fixed at 8GB (sufficient for most moderate workloads).
• 64–256GB RAM: Fixed at 16GB (avoids over-provisioning for large memory systems).
  For memory-intensive servers (e.g., databases), avoid exceeding 4GB of swap to prevent performance degradation from excessive disk I/O. These guidelines balance memory overflow handling and disk space efficiency.

Tuning Swappiness Parameter
The vm.swappiness parameter controls the kernel’s tendency to use swap (range: 0–100). A lower value reduces swap usage (preferring to keep data in RAM), while a higher value increases it (useful for freeing RAM quickly). For most CentOS servers (especially production), set swappiness between 10–30 to minimize swap-induced latency. To adjust:

• Temporarily: sudo sysctl vm.swappiness=10 (takes effect immediately).
• Permanently: Add vm.swappiness=10 to /etc/sysctl.conf, then run sudo sysctl -p to apply changes. This setting is critical for maintaining performance on systems with sufficient RAM.

Performance Optimization Techniques

• Use SSDs (With Caution): Placing swap on an SSD significantly improves read/write speeds compared to HDDs, but avoid enabling swap on aging SSDs (due to limited write cycles). For cloud-hosted CentOS instances, disable swap unless absolutely necessary (SSD wear can impact long-term reliability).
• Enable Zram: Zram creates a compressed block device in RAM, reducing reliance on physical disk swap. Install it via sudo yum install zram-config, start the service with sudo systemctl start zram-config, and configure the size (e.g., ZRAM_DEVICE_SIZE=4G in /etc/zram-config.conf). Zram is ideal for systems with limited disk space or high I/O demands.
• Adjust Kernel Dirty Parameters: Tune dirty page handling to reduce disk writes. Add these lines to /etc/sysctl.conf:
  • vm.dirty_ratio=10 (max percentage of system memory for dirty pages before syncing to disk).
  • vm.dirty_background_ratio=5 (percentage of memory for background writeback).
    Run sudo sysctl -p to apply. These settings improve performance for write-heavy workloads by delaying non-critical disk writes.

Monitoring and Maintenance
Regularly monitor swap usage to identify potential issues (e.g., excessive swapping leading to performance bottlenecks). Key commands include:

• free -h: Shows total, used, and free swap space in a human-readable format.
• swapon --show: Lists active swap devices/files and their sizes.
• vmstat 1: Displays real-time system statistics (including swap activity under “si” (swap in) and “so” (swap out)).
• top/htop: Monitors processes consuming excessive memory (potential triggers for swap usage).
  Review these metrics weekly or after system updates to ensure swap usage aligns with expectations (e.g., minimal swap activity on systems with ample RAM).

Important Considerations

• Avoid Over-Reliance on Swap: While swap prevents out-of-memory (OOM) errors, excessive use (e.g., >50% of swap capacity) slows performance due to slow disk I/O. Regularly review applications with high memory usage and optimize or upgrade RAM if needed.
• SSD Wear Awareness: Frequent swap writes on SSDs reduce their lifespan. Use swap sparingly on SSD-based systems, and prioritize Zram or increasing RAM over adding swap.
• HDD Placement for Swap: If using HDDs, place swap on a separate disk (not the same as the root partition) to avoid I/O contention. For example, use /dev/sdb1 for swap instead of /dev/sda1 (root disk).
• Backup Critical Data: Before modifying swap configurations (e.g., resizing partitions, changing swappiness), back up important data to prevent accidental loss during disk operations.