On Sun, Jan 15, 2012 at 10:28 PM, Greg Smith <g...@2ndquadrant.com> wrote:
> I would like to be able to tell people they need never turn on > log_checkpoints if they monitor pg_stat_bgwriter instead, because that sets > a good precedent for what direction the database is going. It would be nice > for pg_stat_bgwriter's format to settle down for a long period too. While on the topic of the format of pg_stat_bgwriter and it converging on stability: I'm finding the backend_writes column pretty unfortunate. The only use I know of for it is to determine if the bgwriter is lagging behind. Yet it doesn't serve even this purpose because it lumps together the backend writes due to lagging background writes, and the backend writes "by design" due to the use buffer access strategy during bulk inserts. If I'm running a tightly controlled benchmark on an otherwise unused server and I know that no BAS is being used, then I can meaningfully use backend_writes. That is a pretty limiting limit. I think we should either create a separate column to segregate BAS backend_writes, or just don't report them at all and report only the non-BAS ones into pg_stat_bgwriter. I know you've argued against this before (http://archives.postgresql.org/pgsql-performance/2011-03/msg00340.php), but I think the assumption that all writes are absorbed by the OS without blocking is assuming an awful lot. If the server is not behaving well, then it very well may be blocking on writes. If we are confident that the OS can always efficiently cache writes, why bother with a background writer at all? I think the argument that it is not important to know which strategy motivated a backend write could just as validly be reformulated as an argument that we don't need to know how many backend writes there were in the first place. Knowing only an aggregation of BAS-motivated backend writes and straggling-bgwriter-motivated backend writes just doesn't seem useful to me. Am I missing a use for it? Not only is it probably not useful, it seems to be actively harmful as I've seen several times where it leads people down false paths (including me several times, as about once a year I forget why it is useless and spend time rediscovering it). If we are not willing to separate the two types of backend writes, or just omit the BAS ones altogether, then I would suggest we drop the column entirely. Would a patch to separate the backend writes into two columns be entertained for 9.3? Cheers, Jeff -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
