I'm using bcache with about 1 TByte of SSD caching for about 40 TByte of data. With writeback and 10% configured for bcache, about 100 GByte of dirty data exist only on the caching devices.
I like bcache to keep using the configured percentage of the cache for delayed writes, but I'd prefer bcache to clear it over time. In other words, sudden write bursts should go to the SSD, and this piled up "debt" should be cleared over the next few seconds/minutes/hours.
Is this possible?
Related, how exactly does the writeback strategy look like? Can it happen that old data is kept on the SSD forever, while only younger data is written back?