Re: Defaulting wal_sync_method to fdatasync on Linux for 9.1?

On 01/11/10 08:59, Greg Smith wrote:
> Marti Raudsepp wrote:
>> Unless fdatasync is unsafe, I'd very much want to see it as the
>> default for 9.1 on Linux (I don't know about other platforms). I
>> can't see any reasons why each write would need to be sync-ed if I
>> don't commit that often. Increasing wal_buffers probably has the same
>> effect wrt data safety.
>
> Writes only are sync'd out when you do a commit, or the database does
> a checkpoint.
>
> This issue is a performance difference introduced by a recent change
> to Linux. open_datasync support was just added to Linux itself very
> recently. It may be more safe than fdatasync on your platform. As
> new code it may have bugs so that it doesn't really work at all under
> heavy load. No one has really run those tests yet. See
> http://wiki.postgresql.org/wiki/Reliable_Writes for some background,
> and welcome to the fun of being an early adopter. The warnings in the
> tuning guide are there for a reason--you're in untested territory
> now. I haven't finished validating whether I consider 2.6.32 safe for
> production use or not yet, and 2.6.36 is a solid year away from being
> on my list for even considering it as a production database kernel.
> You should proceed presuming that all writes are unreliable until
> proven otherwise.
>

Greg,

Your reply is possibly a bit confusingly worded - Marti was suggesting
that fdatasync be the default - so he wouldn't be a new adopter, since
this call has been implemented in the kernel for ages. I guess you were
wanting to stress that *open_datasync* is the new kid, so watch out to
see if he bites...

Cheers

Mark