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.

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