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__update_write_rate() uses a Proportion-Differentiation Controller algorithm to control writeback rate. A dirty target number is used in this PD controller to control writeback rate. A larger target number will make the writeback rate smaller, on the versus, a smaller target number will make the writeback rate larger. bcache uses the following steps to calculate the target number, 1) cache_sectors = all-buckets-of-cache-set * buckets-size 2) cache_dirty_target = cache_sectors * cached-device-writeback_percent 3) target = cache_dirty_target * (sectors-of-cached-device/sectors-of-all-cached-devices-of-this-cache-set) The calculation at step 1) for cache_sectors is incorrect, which does not consider dirty blocks occupied by flash only volume. A flash only volume can be took as a bcache device without cached device. All data sectors allocated for it are persistent on cache device and marked dirty, they are not touched by bcache writeback and garbage collection code. So data blocks of flash only volume should be ignore when calculating cache_sectors of cache set. Current code does not subtract dirty sectors of flash only volume, which results a larger target number from the above 3 steps. And in sequence the cache device's writeback rate is smaller then a correct value, writeback speed is slower on all cached devices. This patch fixes the incorrect slower writeback rate by subtracting dirty sectors of flash only volumes in __update_writeback_rate(). (Commit log composed by Coly Li to pass checkpatch.pl checking) Signed-off-by: Tang Junhui <tang.junhui@zte.com.cn> Reviewed-by: Coly Li <colyli@suse.de> Cc: stable@vger.kernel.org Signed-off-by: Jens Axboe <axboe@kernel.dk> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.