Josh Berkus wrote: > On 11/07/2014 04:43 PM, Alvaro Herrera wrote: > > This says that the live multixact range goes from 123 million to 162 > > million; roughly 40 million values. (The default value for > > vacuum_multixact_freeze_table_age is 150 million, which is what > > determines how many values are kept.) > > > > You gist.github paste tells us there are 4598 members files. Each file > > has 32 pages, and each page hosts 2045 members; so there are 32 * 2045 * > > 4598 members, or somewhat about 300 million. For 40 million > > multixacts, this means there are about 7 members per multixact, in > > average, which seems a reasonable number to me. > > So the basic problem is that multixact files are just huge, with an > average of 35 bytes per multixact?
The more members the multixacts have, the more space they occupy. I would have thought this was obvious enough. > > If you want to have vacuum truncate pg_multixact more aggresively, you > > need to decrease vacuum_multixact_freeze_table_age and > > vacuum_multixact_freeze_min_age. > > If that's the case, then we need to set the defaults more aggressively. > I suggest maybe 10 million. The alternative is allowing it to creep up > to 150million, which would be 5GB. I don't see adding 5GB to user > databases without warning them as good behavior. > > Of course, this will lead to LOTs of additional vacuuming ... There's a trade-off here: more vacuuming I/O usage for less disk space used. How stressed your customers really are about 1 GB of disk space? -- Álvaro Herrera http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
