Alasdair, You mentioned this morning there might be some old code laying around related to IO statistics and device mapper/multipath? Could you email me a pointer, if there is such? Otherwise, I'll try to start by just throwing out some ideas and going from there. I'm guessing we'd like some way to collect statistics through the /sys file system. But we don't want the numbers to be reported in the same way as for real disks, because we don't want to confuse tools like iostat. So we want some other sysfs file for these counters. But then we have to worry about tools for reporting those statistics. In addition, it would be nice to have the statistics implementation be modular in some way, either as a library within the kernel dm code, or as a "io statistics layer" that can be pushed onto the device mapper stack. Does this summarize where things stand right now? Thanks! Dave
