Thanks Tom and Thomas. Where do I find pg_controldata? I could not locate it on the file system.
pg_clog/ or pg_subtrans/ or pg_multixact/offsets/ are getting larger too but by only a few hundreds MBs. This is not a replicated system. How do I tell if a system is aggressively running "wraparound prevention" autovacuums? Sorry, I failed to follow the calculation. How did you get “~435 million more members can be created.”? What happens when no more members can be created? Does the database halt or shut down? Thanks. On Tue, May 22, 2018 at 7:20 PM Thomas Munro <thomas.mu...@enterprisedb.com> wrote: > On Wed, May 23, 2018 at 7:49 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: > > Tiffany Thang <tiffanyth...@gmail.com> writes: > >> Our pg_multixact/members directory has been growing to more than 18GB > over > >> the last couple of months. According to the documentation, the files in > >> there are used to support row locking by multiple transactions and when > all > >> tables in all databases are eventually scanned by VACUUM, the older > >> multixacts are removed. In our case, the files are not removed. > > > > Hmm. What does pg_controldata tell you about NextMultiXactId, > > NextMultiOffset, oldestMultiXid, oldestMulti's DB? > > Are pg_clog/ or pg_subtrans/ or pg_multixact/offsets/ getting large? > > Is there anything at all in pg_twophase/? Is this system a replication > > master, and if so are any of its slaves lagging behind? > > Some thoughts: > > There are MULTIXACT_MEMBERS_PER_PAGE = 1636 members for every 8KB > page. The reported directory size implies 18GB / 8KB * 1636 = > 3,859,808,256 members. Above MULTIXACT_MEMBER_SAFE_THRESHOLD = > 2,147,483,647 we should be triggering emergency autovacuums to try to > reclaim space. Only ~435 million more members can be created. > > Is this system now aggressively running "wraparound prevention" > autovacuums? > > There are MULTIXACT_OFFSETS_PER_PAGE = 2048 multixacts for every 8KB > page, so the default autovacuum_multixact_freeze_max_age should > soft-cap the size of pg_multixact/offsets at around 1.5GB ~= > 400,000,000 / 2048 * 8KB. > > Unfortunately autovacuum_multixact_freeze_max_age doesn't impose any > limit on the number of members. The totals can be quite explosive > with high numbers of backends, because when n backends share lock a > row we make O(n) multixacts and O(n^2) members. First we make a > multixact with 2 members, then a new one with 3 members, etc... so > that's n - 1 multixacts and (n * (n + 1)) / 2 - 1 members. > > -- > Thomas Munro > http://www.enterprisedb.com >
