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This alert presents the number of bcache read races in the last minute. The bucket was reused and invalidated while reading from the cache. When this occurs, the data is reread from the backing device.

bcache is a cache in the block layer of the Linux Kernel. It allows fast storage devices, as SSDs
(Solid State Drives), to act as a cache for slower storage devices, such as HDDs (Hard Disk Drives). As a result, hybrid volumes are made with performance improvements. Generally, a cache device is divided up into buckets, matching the physical disk’s erase blocks.1 2

There is a mechanism where bcache can keep the cache disk full (typically your SSD), and when it needs to write more data, it selects a bucket, invalidates it, and removes all pointers from it. The alert got triggered, because while there was a reading operation from the cache (meaning the data is stored inside a bucket) that bucket got invalidated so the read operation couldn’t be completed. Following up, the data is reread from the backing device (normally your HDD).

References and Sources

[1] Wikipedia
[2] Bcache: Caching beyond just RAM