On 2019/2/17 12:21 上午, Nix wrote: > On 16 Feb 2019, Andreas uttered the following: > >> Thanks for your reply. >> >> as far as I can tell bcache is caching BtrFS metadata for me, with or >> without my patch. Perhaps BtrFS correctly flags it as REQ_PRIO. In my > > I think that's probably it: XFS is different (and apparently REQ_PRIO is > the wrong thing to look for after all). > >> testing bcache was consistently (and repeatedly) bypassing file contents >> just because they were read as a result of a readahead operation, hence >> the need for my patch. I also found that, before my patch, >> sequential_cutoff and congestion thresholds had close to no effect on >> how much would be cached or bypassed. > > For what it's worth, on XFS that is definitely not true: push up the > sequential_cutoff and the amount cached goes up. (I'm pushing it up > periodically in RAID-chunk-size units until I find that enough is cached > for my metadata reads to disappear.) > >> The problem you linked to seems to be a separate issue where different >> filesystems flag their internal IO inconsistently leading to similar yet >> different problems. > > Yep. > Hi Andreas, >> a solution as I feel like my knowledge of bcache and the Linux kernel in >> general is far too limited to produce a patch of sufficient quality. > Don't worry about this, if you enjoy, please join :-) For me, reading your email and discuss with you is happy experience. > But that's true of everyone to start with! Our biggest problem is > impostor syndrome. Dive in! (But keep good backups.) > I can not agree more on this :-) BTW, it is good to know "impostor syndrome", I finally find a good word to express my awkward situation *^_^* Thanks. -- Coly Li
