On 03/22/2015 06:45 AM, Andres Freund wrote: > On 2015-03-21 13:53:47 -0700, Josh Berkus wrote: >> Coincidentally, I am just at this moment performance testing "running >> with scissors mode" for PostgreSQL on AWS. When intentional, this mode >> is useful for spinning up lots of read-only replicas which are intended >> mainly as cache support, something I've done at various dot-coms. > > Which is where fsync=off barely has any effect?
Well, I'll admit that I'm not testing each setting independantly. I have enough tests to run already. >> So, -1 on removing the setting; it is useful to some users. > > Agreed on that. > >> Further, full_page_writes=off is supposedly safe on any copy-on-write >> filesystem, such as ZFS. > > FWIW, I think that's a myth. One I heard various versions of by now. As > long as the OSs page size (4kb nearly everywhere) is different from > postgres' (8kb) you can have torn pages. Even if individual filesystem > page writes are atomic. ZFS's block size is larger than Linux's memory page size. That is, ZFS on Linux uses a 8kB to 128kB block size depending on which blocks you're looking at and how you have it configured. Does that make a difference at all, given that Linux's memory page size is still 4kB? FYI, the BTRFS folks are also claiming to be torn-page-proof, so it would be really nice to settle this. Not sure how to force the issue through testing though. -- Josh Berkus PostgreSQL Experts Inc. http://pgexperts.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
