From c21c3b0befeb46a51b6bf3758ffa30813bea0ff0 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sat, 9 Mar 2024 14:19:22 +0100 Subject: Adding upstream version 1.44.3. Signed-off-by: Daniel Baumann --- health/guides/disks/bcache_cache_errors.md | 66 ++++++++++++++++++++++++++++++ 1 file changed, 66 insertions(+) create mode 100644 health/guides/disks/bcache_cache_errors.md (limited to 'health/guides/disks/bcache_cache_errors.md') diff --git a/health/guides/disks/bcache_cache_errors.md b/health/guides/disks/bcache_cache_errors.md new file mode 100644 index 000000000..5256c480f --- /dev/null +++ b/health/guides/disks/bcache_cache_errors.md @@ -0,0 +1,66 @@ +### 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](https://www.kernel.org/doc/html/latest/admin-guide/bcache.html#priority-caching). + + 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](https://lwn.net/Articles/394672/) +2. [Kernel Documentation - Bcache](https://www.kernel.org/doc/html/latest/admin-guide/bcache.html) +3. [Arch Linux Wiki - Bcache](https://wiki.archlinux.org/title/bcache) +4. [Wikipedia - Bcache](https://en.wikipedia.org/wiki/Bcache) -- cgit v1.2.3