On Sat, Mar 05, 2022 at 06:04:15PM +0200, Amir Goldstein wrote: > > Dave Chinner asked why the io stats should not be enabled for all > filesystems. That change seems too bold for me so instead, I included > an extra patch to auto-enable per-sb io stats for blockdev filesystems. Perhaps something to consider is allowing users to be able to enable or disable I/O stats on per mount basis? Consider if a potential future user of this feature has servers with one or two 256-core AMD Epyc chip, and suppose that they have a several thousand iSCSI mounted file systems containing various software packages for use by Kubernetes jobs. (Or even several thousand mounted overlay file systems.....) The size of the percpu counter is going to be *big* on a large CPU count machine, and the iostats structure has 5 of these per-cpu counters, so if you have one for every single mounted file system, even if the CPU slowdown isn't significant, the non-swappable kernel memory overhead might be quite large. So maybe a VFS-level mount option, say, "iostats" and "noiostats", and some kind of global option indicating whether the default should be iostats being enabled or disabled? Bonus points if iostats can be enabled or disabled after the initial mount via remount operation. I could imagine some people only being interested to enable iostats on certain file systems, or certain classes of block devices --- so they might want it enabled on some ext4 file systems which are attached to physical devices, but not on the N thousand iSCSI or nbd mounts that are also happen to be using ext4. Cheers, - Ted
