So I just upgraded to 4.20 and revived my long-turned-off bcache now that the metadata corruption leading to mount failure on dirty close may have been identified (applying Tang Junhui's patch to do so)... and I spotted something a bit disturbing. It appears that XFS directory and metadata I/O is going more or less entirely uncached. Here's some bcache stats before and after a git status of a *huge* uncached tree (Chromium) on my no-writeback readaround cache. It takes many minutes and pounds the disk with massively seeky metadata I/O in the process: Before: stats_total/bypassed: 48.3G stats_total/cache_bypass_hits: 7942 stats_total/cache_bypass_misses: 861045 stats_total/cache_hit_ratio: 3 stats_total/cache_hits: 16286 stats_total/cache_miss_collisions: 25 stats_total/cache_misses: 411575 stats_total/cache_readaheads: 0 After: stats_total/bypassed: 49.3G stats_total/cache_bypass_hits: 7942 stats_total/cache_bypass_misses: 1154887 stats_total/cache_hit_ratio: 3 stats_total/cache_hits: 16291 stats_total/cache_miss_collisions: 25 stats_total/cache_misses: 411625 stats_total/cache_readaheads: 0 Huge increase in bypassed reads, essentially no new cached reads. This is... basically the optimum case for bcache, and it's not caching it! >From my reading of xfs_dir2_leaf_readbuf(), it looks like essentially all directory reads in XFS appear to bcache as a single non-readahead followed by a pile of readahead I/O: bcache bypasses readahead bios, so all directory reads (or perhaps all directory reads larger than a single block) are going to be bypassed out of hand. This seems... suboptimal, but so does filling up the cache with read-ahead blocks (particularly for non-metadata) that are never used. Anyone got any ideas, 'cos I'm currently at a loss: XFS doesn't appear to let us distinguish between "read-ahead just in case but almost certain to be accessed" (like directory blocks) and "read ahead on the offchance because someone did a single-block file read and what the hell let's suck in a bunch more". As it is, this seems to render bcache more or less useless with XFS, since bcache's primary raison d'etre is precisely to cache seeky stuff like metadata. :(
