On Sun, Oct 1, 2017 at 10:23 AM, Coly Li <i@xxxxxxx> wrote: > Hi Mike, > > Your data set is too small. Normally bcache users I talk with, they use > bcache for distributed storage cluster or commercial data base, their > catch device is large and fast. It is possible we see different I/O > behaviors because we use different configurations. A small dataset is sufficient to tell whether the I/O subsystem is successfully aggregating sequential writes or not. :P It doesn't matter whether the test is 10 minutes or 10 hours... The writeback stuff walks the data in order. :P ***We are measuring whether the cache and I/O scheduler can correctly order up-to-64-outstanding writebacks from a chunk of 500 dirty extents-- we do not need to do 12 hours of writes first to measure this.*** It's important that there be actual contiguous data, though, or the difference will be less significant. If you write too much, there will be a lot more holes in the data from writeback during the test and from writes bypassing the cache. Having all the data to writeback be sequential is an artificial/synthetic condition that allows the difference to be measured more easily. It's about a 2x difference under these conditions in my test environment. I expect with real data that is not purely sequential it's more like a few percent. Mike
