(Declaration of interest: I'm researching for a publication on OLTP system design) I have a question about file writes, particularly on POSIX. This arose while considering the extent to which cache memory and command queueing on disk drives can help improve performance. Is it correct that POSIX requires that the updates to a single file are serialised in the filesystem layer? So, if we have a number of dirty pages to write back to a single file in the database (whether a table or index) then we cannot pass these through the POSIX filesystem layer into the TCQ/NCQ system on the disk drive, so it can reorder them? I have seen suggestions that on Solaris this can be relaxed. I *assume* that PostgreSQL's lack of threads or AIO and the single bgwriter means that PostgreSQL 8.x does not normally attempt to make any use of such a relaxation but could do so if the bgwriter fails to keep up and other backends initiate flushes. Does anyone know (perhaps from other systems) whether it is valuable to attempt to take advantage of such a relaxation where it is available? Does the serialisation for file update apply in the case where the file contents have been memory-mapped and we try an msync (or equivalent)? -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
