Re: [PATCH v4 0/9] Generic per-sb io stats

["Theodore Ts'o" <tytso@mit.edu>]

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