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Presently we start garbage collection late - when we start running out of free zones to backfill max_open_zones. This is a reasonable default as it minimizes write amplification. The longer we wait, the more blocks are invalidated and reclaim cost less in terms of blocks to relocate. Starting this late however introduces a risk of GC being outcompeted by user writes. If GC can't keep up, user writes will be forced to wait for free zones with high tail latencies as a result. This is not a problem under normal circumstances, but if fragmentation is bad and user write pressure is high (multiple full-throttle writers) we will "bottom out" of free zones. To mitigate this, introduce a zonegc_low_space tunable that lets the user specify a percentage of how much of the unused space that GC should keep available for writing. A high value will reclaim more of the space occupied by unused blocks, creating a larger buffer against write bursts. This comes at a cost as write amplification is increased. To illustrate this using a sample workload, setting zonegc_low_space to 60% avoids high (500ms) max latencies while increasing write amplification by 15%. Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Carlos Maiolino <cem@kernel.org>
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.