Garrett: ext4, application expectations and power management

I agree with your points. And putting fsyncs all over the place in applications is not very helpful.

My mythtv backend (which does a lot of other stuff as well) used to have lots of problems with ivtv buffer overruns. It turns out that mythtv users a fairly small cache, and when it writes to disk it does an fsync on every write. That means that the disk write cache is almost constantly getting flushed and the ability of the kernel to re-order writes is compromised, which then causes io waiting when the system is busy with other stuff as well.

When I increased the buffer moderately and got rid of the fsync everything worked great. So, if I lose power maybe I might lose an extra 10 seconds of the TV show I was recording. However, before the fix I was getting glitches in the video all the time due to overruns.

The role of the OS should be to allow applications to indicate the sensitivity of data and then the OS should figure out how to balance contention for the disk taking into account this kind of weighting. Applications should not be micro-managing the disk cache - that defeats the ability of the kernel to optimize the cache.