systemd-swap

Smart dynamic swap management for Linux, written in Rust.

Features

• Auto-detection: Automatically chooses optimal swap strategy for your filesystem
• Zswap + SwapFC: Compressed RAM cache with dynamic btrfs swap files
• Zram + SwapFC: Alternative mode with zram as primary swap
• Zram writeback: Move idle pages from zram to disk (kernel 5.4+)
• Sparse files (thin provisioning): Swap files don't pre-allocate disk space
• Btrfs optimized: Optional compression mode for swap data
• Lightweight: ~250 KB binary vs ~10 MB Python version

Swap Modes

Mode	Description	Best For
auto	Auto-detect: btrfs → zswap+swapfc, other → zram	Most users
zswap+swapfc	Zswap cache + dynamic swap files	Desktop (btrfs)
zram+swapfc	Zram primary + swap files overflow	Memory-constrained
zram	Zram only (no disk swap)	Non-btrfs systems
manual	Use explicit settings	Advanced users

How Each Mode Works

zswap + swapfc (default for btrfs):

• Zswap compresses pages in RAM before writing to swap
• SwapFC creates swap files as sparse files (thin provisioning)
• Disk space is only used when zswap pool is full
• Best desktop performance with efficient disk usage

zram + swapfc:

• Zram creates compressed block device in RAM (highest priority)
• SwapFC provides overflow to disk when zram is full
• Optional: zram writeback moves idle pages to disk

zram only:

• All swap in compressed RAM
• No disk I/O, ideal for Live USB

Installation

# Build (requires Rust 1.70+)
cargo build --release

# Install
sudo make install

# Enable
sudo systemctl enable --now systemd-swap

Arch Linux / BigLinux

cd pkgbuild
makepkg -si

Usage

# Check status (works without root, extra stats when root)
systemd-swap status

# Show help
systemd-swap --help

# Restart after config changes
sudo systemctl restart systemd-swap

# View logs
journalctl -u systemd-swap -f

Status Output (as root)

Zswap:
  enabled: true
  compressor: zstd
  zpool: zsmalloc
  max_pool_percent: 45%

  === Pool Statistics (debugfs) ===
  pool_size: 234567890 (223.7 MiB)
  stored_pages: 58432 (228.2 MiB uncompressed)
  pool_utilization: 48%
  compress_ratio: 98%
  
  === Writeback Statistics ===
  written_back_pages: 1234 (4.8 MiB)
  pool_limit_hit: 0

  === Effective Swap Usage ===
  kernel_reported_used: 300.0 MiB
  in_zswap_pool (RAM): 228.2 MiB
  actual_disk_used: 71.8 MiB
  swap_in_ram: 76%

Configuration

Edit /etc/systemd/swap.conf:

################################################################################
# Swap Mode
################################################################################
swap_mode=auto

################################################################################
# Zswap (used in zswap+swapfc mode)
################################################################################
zswap_compressor=zstd            # lzo lz4 zstd lzo-rle lz4hc
zswap_max_pool_percent=45        # Max % of RAM for pool
zswap_zpool=zsmalloc             # Memory allocator
zswap_shrinker_enabled=1         # Move cold pages to disk
zswap_accept_threshold=80        # Accept threshold after pool full

################################################################################
# Zram (used in zram modes)
################################################################################
zram_size=50%                    # 50%, 1G, 512M, 100%
zram_alg=zstd                    # Compression algorithm
zram_prio=32767                  # Swap priority

# Zram writeback (requires CONFIG_ZRAM_WRITEBACK)
zram_writeback=0                 # 0=disabled, 1=enabled
zram_writeback_dev=              # Partition or empty for auto loop
zram_writeback_size=1G           # Auto backing file size

################################################################################
# SwapFC - Dynamic swap files
################################################################################
swapfc_chunk_size=512M           # Size: 512M, 1G, 10% (of RAM)
swapfc_max_count=32              # Maximum swap files
swapfc_free_ram_perc=35          # Create when free RAM < this %
swapfc_free_swap_perc=25         # Create more when free swap < this %
swapfc_path=/swapfc/swapfile     # Path for swap files

# Sparse files (thin provisioning) - ENABLED BY DEFAULT
# Swap files appear full size but only allocate disk space when written.
# Ideal with zswap: pages stay in RAM, disk is only used on writeback.
# swapfc_use_sparse_disable=1    # Uncomment to pre-allocate disk space

# Btrfs compression mode (experimental)
swapfc_use_btrfs_compression=0   # Double compression: zswap + btrfs

Custom Swap Location

You can place swap files on a different btrfs partition:

swapfc_path=/mnt/swap-drive/swapfile

Zram Writeback

Move idle/incompressible pages from zram to disk:

# Enable with auto loop device
zram_writeback=1
zram_writeback_size=2G

# Or use dedicated partition
zram_writeback=1
zram_writeback_dev=/dev/sda5

Requires kernel compiled with CONFIG_ZRAM_WRITEBACK.

Sparse Files (Thin Provisioning)

By default, swap files are created as sparse files:

• Files appear as 512M but start with 0 bytes on disk
• Disk blocks are allocated only when data is actually written
• With zswap, most pages stay compressed in RAM
• Disk is only used when zswap pool is full (writeback)

To disable and pre-allocate all disk space:

swapfc_use_sparse_disable=1

File Locations

Path	Description
/usr/bin/systemd-swap	Main binary
/etc/systemd/swap.conf	User configuration
/usr/share/systemd-swap/swap-default.conf	Default configuration
/run/systemd-swap/	Runtime data
/swapfc/	Swap files (default)

Requirements

• Linux kernel 5.0+
• Rust 1.70+ (build only)
• btrfs-progs (for btrfs features)
• util-linux (zramctl, losetup)

License

GPL-3.0-or-later