Re: wait events for disk I/O

On Mon, Jan 30, 2017 at 10:01 PM, Rushabh Lathia
<rushabh(dot)lathia(at)gmail(dot)com> wrote:
> Attached is the patch, which extend the existing wait event infrastructure
> to implement the wait events for the disk I/O. Basically pg_stat_activity's
> wait event information to show data about disk I/O as well as IPC primitives.
>
> Implementation details:
>
> - Added PG_WAIT_IO to pgstat.h and a new enum WaitEventIO
> - Added a wait_event_info argument to FileRead, FileWrite, FilePrefetch,
> FileWriteback, FileSync, and FileTruncate. Set this wait event just before
> performing the file system operation and clear it just after.
> - Pass down an appropriate wait event from caller of any of those
> functions.
> - Also set and clear a wait event around standalone calls to read(),
> write(), fsync() in other parts of the system.
> - Added documentation for all newly added wait event.

Looks neat, those are unlikely to overlap with other wait events.

> Open issue:
> - Might missed few standalone calls to read(), write(), etc which need
> to pass the wait_event_info.

It may be an idea to use LD_PRELOAD with custom versions of read(),
write() and fsync(), and look at the paths where no flags are set in
MyProc->wait_event_info, and log information when that happens.

> Thanks to my colleague Robert Haas for his help in design.
> Please let me know your thought, and thanks for reading.

Did you consider a wrapper of the type pg_read_event() or
pg_write_event(), etc.?
--
Michael