bcache fork with HDD spin management and lifespan-first optimizations.

a-bcache extends the standard bcache-tools with a spin management daemon (a-bcached) that minimizes HDD mechanical wear through:

• I/O coalescing: accumulates dirty data on the SSD cache before flushing
• Spin-down awareness: spins down the HDD after idle periods, pauses writeback to prevent unwanted wake-ups
• Spin cycle budgeting: limits spin-up/spin-down transitions per hour to reduce motor/bearing wear
• Sequential-only writeback: leverages bcache's existing sequential writeback to avoid random HDD seeks

┌─────────────────────────────────────────┐
│  Applications                           │
├─────────────────────────────────────────┤
│  /dev/bcache0  (bcache block device)    │
├──────────────┬──────────────────────────┤
│  SSD Cache   │  a-bcached (this daemon) │
│  (fast I/O)  │  - monitors dirty data   │
│              │  - pauses/resumes wb     │
│              │  - manages HDD spin      │
├──────────────┴──────────────────────────┤
│  HDD (backing device)                   │
│  - spun down when idle                  │
│  - sequential writeback only            │
│  - spin cycles budgeted                 │
└─────────────────────────────────────────┘

# Format devices
make-bcache -C /dev/ssd_device -B /dev/hdd_device --writeback

# Register
echo /dev/ssd_device > /sys/fs/bcache/register
echo /dev/hdd_device > /sys/fs/bcache/register

make
sudo make install

# Foreground with verbose logging (for testing)
sudo a-bcached --foreground --verbose

# Or install as a service
sudo cp a-bcached.service /etc/systemd/system/
sudo systemctl enable --now a-bcached

You can test without real hardware using loop devices and dm-delay:

# Create mock SSD (128MB) and HDD (1GB with 10ms latency)
sudo ./test/setup-mock-devices.sh setup

# Check status
sudo ./test/setup-mock-devices.sh status

# Run integration tests
sudo ./test/test-spin-management.sh

# Test daemon in dry-run mode
sudo ./a-bcached -f -n -v

# Clean up
sudo ./test/setup-mock-devices.sh teardown

Note: hdparm -C (spin state detection) and hdparm -y (spin-down) won't work on loop devices. These features can only be tested on real hardware. The test suite skips those tests and marks them accordingly.

This repo includes all the original bcache-tools:

• make-bcache - Format devices for bcache
• bcache-super-show - Print bcache superblock info
• probe-bcache - Probe for bcache signatures
• bcache-register - Register bcache devices

See the bcache documentation for full details on bcache itself.

GPLv2 (same as bcache)