PostgreSQL 9.3 beta: Federated databases and more

Posted May 15, 2013 23:17 UTC (Wed) by andresfreund (subscriber, #69562)
In reply to: PostgreSQL 9.3 beta: Federated databases and more by ncm
Parent article: PostgreSQL 9.3 beta: Federated databases and more

> Or use mmap() for everything except the journal?
The point is that writeout for file writes needs to have interlock with the writes for the journal. You can only writeout a modified page if its corresponding log entry has already been written out.

Writing out only the journal in an mmap()ed fashion would actually be far easier. But I don't see much benefit in that direction since only small amounts of data (up to maybe 64MB or so has been measured as being benefical) are held in memory for the log. And we frequently write to new files which would always requiring an mmap()/munmap() cycle (which actually sucks for concurrency).

PostgreSQL 9.3 beta: Federated databases and more

Posted May 16, 2013 5:19 UTC (Thu) by ncm (guest, #165) [Link] (2 responses)

Maybe I'm misunderstanding how PG does journaling these days. "Normally", on a no-overwrite store like PG's, a journal entry just says "block N containing metadata is now canon", which metadata is known to be on disk already, and identifies new data blocks also known to be on disk already, and other blocks that are now free. In this scenario, data and metadata blocks may be written out eagerly knowing they will all be ignored until the (tiny) journal entry that blesses the new metadata hits the disk.

You seem to be describing a process more like a traditional store and write-ahead log, where first you write in the log all the changes are planned for the main store, and then lazily update the main store, writing it all again, knowing that if you are interrupted somebody else can replay the rest of the log. But I thought the great advantage of the PG scheme is that you only have to write once.

Maybe only metadata goes to the journal and is then copied out, while bulk data goes directly into unused blocks?

PostgreSQL 9.3 beta: Federated databases and more

Posted May 16, 2013 7:17 UTC (Thu) by andresfreund (subscriber, #69562) [Link] (1 responses)

> Maybe I'm misunderstanding how PG does journaling these days. "Normally", on a no-overwrite store like PG's, a journal entry just says "block N containing metadata is now canon", which metadata is known to be on disk already, and identifies new data blocks also known to be on disk already, and other blocks that are now free. In this scenario, data and metadata blocks may be written out eagerly knowing they will all be ignored until the (tiny) journal entry that blesses the new metadata hits the disk.

> You seem to be describing a process more like a traditional store and write-ahead log, where first you write in the log all the changes are planned for the main store, and then lazily update the main store, writing it all again, knowing that if you are interrupted somebody else can replay the rest of the log.

Postgres' implementation is a pretty classical write ahead log scheme that is far more like the second scheme you describe than the first one. And afaik has been since the introduction of crash safety (in 7.0 or so).

> But I thought the great advantage of the PG scheme is that you only have to write once.

Hm. Not sure what that corresponds to then? Postgres' WAL doesn't write full pages (except in some circumstances, but let's leave them out for now), but only a description of the change like 'insert tuple at slot X of page YYY) so amount of data that has to be fsync()ed for commit is reasonably small. Perhaps that is what you were referring to?

PostgreSQL 9.3 beta: Federated databases and more

Posted May 16, 2013 8:10 UTC (Thu) by ncm (guest, #165) [Link]

So, more like the third paragraph, then. Still, disappointing. But fixable.