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For striping across devices, we maintain "clocks", and we advance them by the inverse of "how much free space this device has left", so that we round robin biased in favor of devices with more free space. This code was originally trying to do EWMA-ish stuff when originally written, ~10 years ago, and was never properly cleaned up when it was realized that an EWMA is not the right approach here. That left a bug, when we rescale to keep all the clocks in the correct range and prevent overflow. It was assumed that we'd always be allocated from the device with the smallest clock hand, but that's actually not correct: with the target options, allocations will be first tried from a subset of devices, and then the entire filesystem if that fails. Thus, the rescale from the first allocation - allocating from a subset of devices - can pick the wrong rescale value and cause the rest of the clocks to go to 0, losing information. This resuls in incorrect striping behaviour when the desired number of replicas doesn't fit on the foreground target. Link: https://www.reddit.com/r/bcachefs/comments/1jn3t26/replica_allocation_not_evenly_distributed_among/ Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.