I want to display the total server read, and server write bytes from the
io_stats. However, this poses a challenge as the only way I can find to retrieve
these stats is from /proc/self/mountstats.
The issue I run into is that io_stats is per nfs_server record, while the output
is reported per mount. Multiple mounts can share the same nfs_server record.
This means while parsing the mountstats I need a way to reliably determine if
two mounts share the same nfs_server record.
From reading the source it appears that nfs_server and super_block are
correlated 1:1, and s_dev for NFS is allocated a unique value via
set_anon_super.
My plan is to run stat on the mount point to fetch the st_dev value. Thus is it
safe to assume (for NFS) if two mount points have the same st_dev value they
share the same io_stats, and if the st_dev value is different they have separate
io_stats, or is there some scenario that I've missed?
Longer term it would be nice to make this process simpler, I can see several
options on how to handle this:
1. Add a unique identifier in the mountstats (e.g. s_dev) that can be used as a
unique key.
The concept here is that anything scraping the stats does not need to
rely on the assumption that io_stats has a 1:1 correlation with super_block
or that s_dev is unique. If the implementation were to change in future the
key can be updated to match.
The only issue is if this uses an existing value such as s_dev consumers
might ignore the fact that the key should be treated as an opaque value and
only used to deduplicate io_stats and start relying on the key matching the
s_dev value. As such it might be necessary to allocate
a separate unique key to the io_stats.
2. Two separate files in procfs that only output each unique stat once.
* io_stats for each nfs_server excluding xprt: lines (multiple nfs_server
entries can share the same RPC transport).
* stats for RPC transports (xprt lines). These should be unique so that each
RPC connection is only displayed once.
3. Add a new file to procfs that outputs aggregated io_stats (including xprt
lines). These stats should be aggregated at the source server level. It is
still useful when a client has mounted exports from multiple servers to have
separate stats per server.
--
Chris