On Thu, Sep 29, 2022 at 07:14:23PM +0300, Sagi Grimberg wrote: > > > > > 3. Do you have some performance numbers (we're touching the fast path > > > > here) ? > > > > > > This is pretty light-weight, accounting is per-cpu and only wrapped by > > > preemption disable. This is a very small price to pay for what we gain. > > > > It does add up, though, and some environments disable stats to skip the > > overhead. At a minimum, you need to add a check for blk_queue_io_stat() before > > assuming you need to account for stats. > > > > Instead of duplicating the accounting, could you just have the stats file report > > the sum of its hidden devices? > > Interesting... > > How do you suggest we do that? .collect_stats() callout in fops? Maybe, yeah. I think we'd need something to enumerate the HIDDEN disks that make up the multipath device. Only the low-level driver can do that right now, so perhaps either call into the driver to get all the block_device parts, or the gendisk needs to maintain a list of those parts itself.
