James Bottomley <james.bottom...@hansenpartnership.com> wrote: > you mean the order of write out, if we have to do it, is > important. In the rest of the kernel, we do this with barriers > which causes ordered grouping of I/O chunks. If we could force a > similar ordering in the writeout code, is that enough?
Unless it can be between particular pairs of pages, I don't think performance could be at all acceptable. Each data page has an associated Log Sequence Number reflecting the last Write-Ahead Log record which records a change to that page, and the referenced WAL record must be safely persisted before the data page is allowed to be written. Currently, when we need to write a dirty page to the OS, we must ensure that the WAL record is written and fsync'd first. We also write a WAL record for transaction command and fsync it at each COMMIT, before telling the client that the COMMIT request was successful. (Well, at least by default; they can choose to set synchronous_commit to off for some or all transactions.) If a write barrier to control this applied to everything on the filesystem, performance would be horrible. -- Kevin Grittner EDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
