Re: RFC: Detailed reorder buffer stats dumps

Hi,

On 2021-05-05 18:33:27 +0800, Craig Ringer wrote:
> I'm thinking of piggy-backing on the approach used in the "Get memory
> contexts of an arbitrary backend process" patch in order to provide access
> to detailed reorder buffer content statistics from walsenders on request.
>
> Right now the reorder buffer is mostly a black-box. I mostly rely on gdb or
> on dynamic probes (perf, systemtap) to see what it's doing. I intend a
> patch soon to add a couple of fields to struct WalSnd to report some very
> coarse reorder buffer stats - at least oldest buffered xid, number of
> buffered txns, total bytes of buffered txns in memory, total bytes of
> buffered txns spilled to disk.
>
> But sometimes what I really want is details on the txns that're in the
> reorder buffer, and that's not feasible to export via always-enabled
> reporting like struct WalSnd. So I'm thinking that the same approach used
> for the memory context stats patch might work well for asking the walsender
> for a detailed dump of reorder buffer contents. Something like a
> per-buffered-txn table of txn topxid, start-lsn, most recent change lsn,
> number of changes, number of subxids, number of invalidations, number of
> catalog changes, buffer size in memory, buffer size spilled to disk.
>
> Anyone drastically opposed to the idea?

I am doubtful. The likelihood of ending with effectively unused code
seems very substantial here.

Greetings,

Andres Freund