Peter> Scale writeback cache per backing device, proportional to its Peter> writeout speed. By decoupling the BDI dirty thresholds a Peter> number of problems we currently have will go away, namely:
Ah, this clarifies my questions! Thanks! Peter> - mutual interference starvation (for any number of BDIs); Peter> - deadlocks with stacked BDIs (loop, FUSE and local NFS mounts). Peter> It might be that all dirty pages are for a single BDI while Peter> other BDIs are idling. By giving each BDI a 'fair' share of the Peter> dirty limit, each one can have dirty pages outstanding and make Peter> progress. Question, can you change (shrink) the limit on a BDI while it has IO in flight? And what will that do to the system? I.e. if you have one device doing IO, so that it has a majority of the dirty limit. Then another device starts IO, and it's a *faster* device, how quickly/slowly does the BDI dirty limits change for both the old and new device? Peter> A global threshold also creates a deadlock for stacked BDIs; Peter> when A writes to B, and A generates enough dirty pages to get Peter> throttled, B will never start writeback until the dirty pages Peter> go away. Again, by giving each BDI its own 'independent' dirty Peter> limit, this problem is avoided. Peter> So the problem is to determine how to distribute the total Peter> dirty limit across the BDIs fairly and efficiently. A DBI that You mean BDI here, not DBI. Peter> has a large dirty limit but does not have any dirty pages Peter> outstanding is a waste. Peter> What is done is to keep a floating proportion between the DBIs Peter> based on writeback completions. This way faster/more active Peter> devices get a larger share than slower/idle devices. Does a slower device get a BDI which is calculated to keep it's limit under a certain number of seconds of outstanding IO? This way no device can build up more than say 15 seconds of outstanding IO to flush at any one time. Thanks! John - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
