On Tue, Apr 16, 2019 at 10:48 AM Jamison, Kirk <k.jami...@jp.fujitsu.com> wrote: > > Hello Fujii-san, > > On April 18, 2018, Fujii Masao wrote: > > > On Fri, Mar 30, 2018 at 12:18 PM, Tsunakawa, Takayuki > > <tsunakawa.ta...@jp.fujitsu.com> wrote: > >> Furthermore, TRUNCATE has a similar and worse issue. While DROP TABLE > >> scans the shared buffers once for each table, TRUNCATE does that for > >> each fork, resulting in three scans per table. How about changing the > >> following functions > >> > >> smgrtruncate(SMgrRelation reln, ForkNumber forknum, BlockNumber > >> nblocks) DropRelFileNodeBuffers(RelFileNodeBackend rnode, ForkNumber > >> forkNum, > >> BlockNumber firstDelBlock) > >> > >> to > >> > >> smgrtruncate(SMgrRelation reln, ForkNumber *forknum, BlockNumber > >> *nblocks, int nforks) DropRelFileNodeBuffers(RelFileNodeBackend rnode, > >> ForkNumber *forkNum, > >> BlockNumber *firstDelBlock, > >> int nforks) > >> > >> to perform the scan only once per table? If there doesn't seem to be a > >> problem, > >> I think I'll submit the patch next month. I'd welcome if anyone could do > >> that. > > > > Yeah, it's worth working on this problem. To decrease the number of scans of > > shared_buffers, you would need to change the order of truncations of files > > and WAL > > logging. In RelationTruncate(), currently WAL is logged after FSM and VM > > are truncated. > > IOW, with the patch, FSM and VM would need to be truncated after WAL > > logging. You would > > need to check whether this reordering is valid. > > I know it's been almost a year since the previous message, but if you could > still > recall your suggestion above, I'd like to ask question. > Could you explain your idea a bit further on how would the reordering of WAL > writing and > file truncation possibly reduce the number of shared_buffers scan?
Sorry, I forgot the detail of that my comment. But anyway, you want to modify smgr_redo(info = XLOG_SMGR_TRUNCATE) so that the number of scans of shared_buffers to be decreased to one. Right? IMO it's worth thinking to change smgrtruncate(MAIN_FORK), FreeSpaceMapTruncateRel() and visibilitymap_truncate() so that they just mark the relation and blocks as to be deleted, and then so that they scan shared_buffers at once to invalidate the blocks at the end of smgr_redo(). Regards, -- Fujii Masao
