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Bcache Cache Errors

src/health/guides/disks/bcache_cache_errors.md

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Understand the alert

This alert is triggered when the number of read races in the last minute on a bcache system has increased. A read race occurs when a bucket is reused and invalidated while it's being read from the cache. In this situation, the data is reread from the slower backing device.

What is bcache?

bcache is a cache within the block layer of the Linux kernel. It enables fast storage devices, such as SSDs (Solid State Drives), to act as a cache for slower storage devices like HDDs (Hard Disk Drives). This creates hybrid volumes with improved performance. A cache device is usually divided into buckets that match the physical disk's erase blocks.

Troubleshoot the alert

  1. Verify the current bcache cache errors:

    grep bcache_cache_errors /sys/fs/bcache/*/stats_total/*

    This command will show the total number of cache errors for all bcache devices.

  2. Identify the affected backing device:

    You can determine the affected backing device by checking the /sys/fs/bcache directory. Look for the symbolic link that points to the problematic device.

    ls -l /sys/fs/bcache

    This command will show the list of devices with corresponding names.

  3. Monitor the cache device's performance:

    Use iostat to check the cache device's I/O performance.

    iostat -x -h -p /dev/YOUR_CACHE_DEVICE

    Note that you should replace YOUR_CACHE_DEVICE with the actual cache device name.

  4. Check the utilization of the cache and backing devices:

    Use the following commands to check the utilization percentage of the cache and backing devices:

    # for the cache device (/dev/YOUR_CACHE_DEVICE)
cat /sys/block/YOUR_CACHE_DEVICE/bcache/utilization
 
# for the backing device (/dev/YOUR_BACKING_DEVICE)
cat /sys/block/YOUR_BACKING_DEVICE/bcache/utilization

    Replace YOUR_CACHE_DEVICE and YOUR_BACKING_DEVICE with the respective device names.

  5. Optimize the cache:

    • If the cache utilization is high, consider increasing the cache size or adding more cache devices.
    • If the cache device is heavily utilized, consider upgrading it to a faster SSD.
    • In case the read races persist, consider using a priority caching strategy.

    You may also need to review your system's overall I/O load and adjust your caching strategy accordingly.

Useful resources

  1. Bcache: Caching beyond just RAM
  2. Kernel Documentation - Bcache
  3. Arch Linux Wiki - Bcache
  4. Wikipedia - Bcache